Privacy Policy

Last updated: October 14, 2026

Underbook helps you find a lower hotel rate. We collect as little as possible: the extension does something on its own only on a hotel page and, right after you book on Booking.com, Expedia, Hotels.com or Agoda, on that site's booking confirmation page, where it offers to watch the booking and sends nothing unless you ask it to. If you turn on its price-drop mark (it is off unless you do), it may also look at the prices of the stays you asked for again, once a day for up to 14 days, to mark its icon if the going rate falls. It sends the hotel name (and the town the page lists it in), your dates and your party size to our price service, and we never see your payment information. The only personal data we store is the email you give us if you choose to set a price-drop alert, and, if you tell us you already booked the stay, the price you paid and your free-cancellation date.

When the extension is active

The extension can run on the websites you visit, but on its own it shows something on the page only in three places: a hotel page (a page where it can identify a specific hotel, from the page's title or metadata, and your check-in and check-out dates), a small tip on a booking site it reads, and a booking confirmation page (described below). On a booking site it reads (Booking.com, Expedia and its sister sites, Hotels.com, Agoda, Trip.com, Priceline, KAYAK, and the hotel chains' own sites it supports), the Underbook icon in your toolbar shows "ON", and on a hotel page it shows a mark while a check runs and when it is done. That happens inside your browser: the extension tells its own icon what to show and sends nothing to us about it. On such a site's home page or hotel search results, it may show a small tip saying it works there: the first time a new install is on a booking site's hotel search results for real dates, this is a one-time tip saying to open a hotel from the list. It shows at most one tip a day, at most once every 14 days on each site, never on a site where you compared a hotel in the last 14 days, and never again once you press "Don't show tips again". It counts that a tip was shown, and that tips were turned off, as anonymous daily totals (the event name only: no URL, site, destination or search). Third, on the booking confirmation page that Booking.com, Expedia, Hotels.com or Agoda shows right after you book, it may show one card offering to watch that booking (see "Watching a booking from its confirmation page" below). It shows that card at most once for each booking, and it sends nothing about the page unless you type your email and press the card's button. On every other page (search engines, email, banking, anything that isn't a hotel or a booking site it reads) it shows nothing and sends nothing about the page on its own. When you first install it on Chrome or Edge, it also runs once in the tabs you already have open, exactly as it would if you reloaded them, so a hotel page you were already looking at gets its comparison without a reload; it sends nothing more than a reload of that page would, and it never does this when the extension updates. (Firefox and Safari do the same by themselves.) The other automatic request on such pages is the once-a-day anonymous usage ping described below, which carries no URL, page title or page content. On a hotel page that shows no dates, it shows a small card asking for them, with two date fields and a party size, and, if this browser checked a stay in the last 21 days whose check-in has not passed, a button naming that stay's dates and party; nothing about the page is sent unless you press one of the card's buttons, which sends the hotel's name with the dates and party size it names, like any hotel check. The dates are not kept for later pages. If you click the Underbook toolbar button on a page, that is you asking it to check that page: it reads the page's title or main heading as a possible hotel name, sends that name and any dates it finds to our price service like any hotel check, and counts the click as an anonymous daily total.

The price-drop mark on the icon (off unless you turn it on). Under a result with prices, on a computer, a line "Mark the toolbar if this stay's price drops" lets you turn on a mark on the Underbook icon. It is off until you tick it, and the choice is remembered on this browser. If you never tick it, nothing in this paragraph happens: the extension never looks at a stay again by itself, and the stays you checked never leave your device on their own. Once you tick it, for that stay and for each stay you check after it, for up to 14 days after each check and while its check-in is still ahead, the extension may look at the stay's prices again by itself, while its icon is on your browser's toolbar (where the mark can be seen), at most once a day and one stay a day, when you open your first web page of the day (nothing about that page is sent). The look sends our price service what your own check of that stay sent: the hotel's name and town as the page showed them, the dates, the number of adults, the market and the currency. It is not counted as a price check by you. If the going rate is 5% or more under the one you saw when you checked, on two looks in a row, the icon shows a small blue mark; its tooltip names the stay and both figures, and a click on it opens the page where you checked that stay, which checks it again there. The mark goes away when you click it, after 3 days, at check-in, or when you check that stay again. It appears at most once for a stay and at most once a week, never while another one shows, and not on phones. Untick the line, or press "Stop these marks" on a page a mark opened, and it stops: no more looks or marks, and the details kept for it are deleted from your device. We count, as anonymous daily totals (the event name only, never the stay), that the line was shown, that it was ticked, that a look was made, that the icon was marked, that a mark was clicked, and that it was turned off.

What it reads, and what we send

On a hotel page, it reads the hotel name, your selected dates and the number of adults from the page, and the hotel's town when the page shows it next to the name.
It sends exactly these to Underbook's price service (our API at underbook-backend.underbook.workers.dev) to look up and compare current rates across booking sites: the hotel name, the hotel's town as the page shows it (when it does; it tells apart hotels with the same name), your check-in and check-out dates, the number of adults (so the rates are for the right occupancy), the booking site's market and currency (so prices appear in the currency you were already looking at; the currency comes from the page, and the market from the site's country or, where the site does not say one, your browser's language setting, never from your location), and the version number of the extension. We do not send your identity, your browsing history, the other pages you visit, or any payment information.
To find prices, our price service passes the hotel name (with its town when the page shows one), your dates, the number of adults and the market and currency to the price-data services it uses: SerpApi, which reads Google Hotels, and MakCorps, which it asks only when Google Hotels cannot be reached. Neither receives your identity, your email or anything about your device.
On our service, a priced result is cached for 30 minutes under the hotel, dates, party size and market, so repeat lookups are fast; that cache holds no identity. When a lookup returns no rates or fails, we keep the hotel name, its town if the page showed one, dates, market and the time it happened in a short rolling list (the most recent 50, kept for at most 30 days) so we can find and fix coverage gaps. That list records nothing about who made the lookup. When no hotel matched the name exactly and we picked one from a list of close matches (Google Hotels' list, or MakCorps' when MakCorps priced the stay), we keep the name you typed, the name we picked and the date in a second list with the same limits (the most recent 50, at most 30 days), so we can check that the pick was the right hotel. Our service also keeps daily totals of how long it took to answer each price check (in ranges: under 1 second, 1 to 2, 2 to 4, 4 to 8, 8 to 15, 15 to 25, or more than 25 seconds), whether the answer came from that 30-minute cache, and how many price searches it took (1, 2, 3, or 4 or more), split only by whether the check came from the extension, the web price checker or neither. These totals hold no hotel, dates, price or anything about who checked.
When a check's results show that the listing the banner speaks about has no free-cancellation label on Google Hotels and another booking site lists free cancellation for the same price or less, the banner says so, with both prices and a button to that site. It uses only the results of the same check and sends nothing more.
We don't use cookies or tracking pixels, and we don't build an advertising profile of you.

Anonymous usage statistics

To understand where people stop getting value (and fix it), the extension sends a small number of anonymous usage events to our service. Here is exactly what an event contains — and all it can ever contain:

A random install id — a random string generated on your device the first time the extension runs. It is not derived from you, your Google account, or your browser, and it is not stored on our servers; it exists only so "days since install" is honest.
Once a day, with the usage ping, whether the Underbook icon is pinned to your browser's toolbar, sent only when it is. Your browser tells the extension this; nothing else about your toolbar or your other extensions is read.
Which banner event happened (for example: a comparison banner was shown, a button was clicked, a price watch was offered, started or set (and whether it was offered with the email field on the banner, or on a stay you came back to, and whether it was for a stay you already booked), a note that another booking site lists free cancellation for the same price or less was shown, or its button tapped, the banner was dismissed, you came back to a stay you checked before, you clicked the Underbook toolbar button and whether it found a hotel on the page, or a new install reached its first hotel page and whether that first banner (or, on older versions, the one-time first-run card) was tapped or closed, or the one-time search-results tip was shown, or a booking site's tip was shown, or tips were turned off, or a hotel page that shows no dates showed the card asking for them (and whether it offered the last stay you checked), a check was started from that card (with that stay, or with dates picked on the card), or a booking site's button was pressed on the answer to such a check (the card being shown and the checks started from it are also counted by days since install, and counted apart when the card was on a booking site's hotel page that Underbook recognizes by its address alone, such as Expedia's, Hotels.com's or Priceline's), or you pressed Share on a banner, or the request to rate Underbook in its store was shown, at which of its moments (after you opened a cheaper listing, and whether it came on that banner or, because it could not ask there, on a later one; versions before 0.5.50 also asked on a result with prices on a later visit, or on a result with prices once you had checked a second stay), and whether you tapped its link or "Not now", or that one of those moments came while the request could still be made, once per result, whether or not it could be seen then, or a banner with no price for your stay showed its searches on other booking sites, or one of those searches was opened, or the link beside a lower price to our page about that booking site ("Is <site> legit?") was opened, at most once per banner, or one of the price-drop mark's events described above: its line shown or ticked, a look at a stay you asked for, a mark on the icon, a click on it, or the mark turned off).
The verdict type the banner showed (e.g. "cheaper elsewhere", "no site 5% lower", "no result") — a category label, nothing about the hotel itself.
How many days since the extension was installed, as a whole number, and on the once-a-day usage ping also as the number of calendar days (UTC) since the install date.
Whether this copy of the extension was loaded unpacked by a developer (a yes/no flag). It is "yes" only for our own test and development builds, never for a Chrome Web Store install; it keeps our own runs out of the statistics.

What these events never contain:

No browsing history — no URLs, no page titles, nothing about the sites you visit.
No hotel names and no dates of your stays.
No email address or any other identity — even if you've set a price watch, your email is never attached to these events.
No IP addresses stored — our service keeps only per-day totals ("how many banners were shown today"), never who sent an event.

When you first install the extension it opens a welcome page on underbook.co. That page counts, as daily totals with no identifier and no cookie: that it was opened by a new install, and whether you tried it, split by which way (an example hotel link or the check-a-hotel form). It also counts, as a daily total, whether the real-hotel link at the top of that page was clicked, at most once each time the page is opened. If you use its "Where are you staying next?" box, the place and dates you type go only to the booking site you pick (Booking.com or Expedia), which opens with that search in a new tab; the page counts, as a daily total, that the box opened a search, at most once each time the page is opened, and never what you typed. It also runs one live price check of its example hotel (the Bellagio, fixed dates, two adults) so you see a result at once; that check sends nothing about you and is counted as a daily total. That the welcome page was opened by a new install, that a new install reached its first hotel page, and, from the extension, its once-a-day usage ping (with the install's age in whole days, which the ping already carries) and each comparison banner it showed, are also counted, as daily totals, by the first language your browser sends with every request: English, Spanish, German, French, Portuguese (Brazil), Italian, Japanese, Korean, Dutch, Polish, Turkish, Swedish, Indonesian, Vietnamese, Chinese (Simplified), Chinese (Traditional), Portuguese (Portugal and every other country but Brazil), or another language. Only that one word is kept, never the rest of your browser's language settings. The welcome page and the web price checker are shown in your browser's language when it is one of the languages named above: your browser loads the translation as a file from underbook.co, and nothing about you is sent to choose it.

These statistics exist for one purpose: to see where users drop off in the experience so we can fix it. They are not used for advertising, not shared, and not sold.

Sharing a check from the banner (optional)

If you press "Share this check" on a banner that shows prices, the extension copies to your clipboard a short note of what that banner shows (the hotel, your dates and party size, the prices and the room-name line) and a link to the web price checker on underbook.co. The link carries only the hotel name, the check-in and check-out dates, the number of adults, the currency and a marker that it is a shared link, shared from the extension; never your email, the price you paid, the address of the page you were on, or anything else you typed. Copying sends nothing to us: the press itself is counted like the other banner events above, with no hotel, dates or link, and you decide whether to paste the note anywhere. Whoever opens the link gets the same check run again with that day's prices. Their visit is counted as described under the web price checker below, with the visits, checks and booking-site clicks from links shared from the extension counted apart, as daily totals with no identifier.

Price-drop alerts (optional, opt-in)

Only if you type your email into a price-drop alert (on the extension's banner or on the web price checker) and press its button, or tap the alert button after you have set one before, we store your email address together with the hotel (and its town, if the page showed one), dates and number of adults you asked us to watch. We use it only to email you about that stay: a confirmation, a note when a rate first appears if there was none yet, and at most one alert if the going rate is at least 5% lower on two daily checks in a row. That alert ends with one line asking, if you use the extension, for an honest rating in your browser's add-on store, with a link to underbook.co/rate. We stop checking after your check-out date. We don't use it for marketing and we don't sell or share it. Every email has a one-click unsubscribe link, and you can ask us at hi@underbook.co to stop a watch or delete your email at any time.

Watching a stay you already booked (optional, opt-in)

If you tick "I already booked this stay" (on the web price checker, or on the extension's banner where it offers it) and press the button, a booked watch keeps only: your email, the hotel (and its town, if the page showed one), dates and number of adults, the price you paid per night and its currency, the market we price it in (for example "us" or "gb"), your free-cancellation date if you give one, and, if it is sent, the name of the site you booked on. We use them only to compare your price with the rates listed for the same hotel and dates once a day, and to email you about that stay: a confirmation, at most one alert if a listed rate is at least 5% lower (and at least 5 a night lower in dollars, euros or pounds, or a similar amount in your currency) on two daily checks in a row, and, for a watch set from a booking confirmation page, two reminders before your free cancellation ends: one about a week before and one the day before, sent even when no lower rate shows up (each only if the watch was set before that day began; none after a price alert or after free cancellation has ended). The price alert ends with one line asking, if you use the extension, for an honest rating in your browser's add-on store, with a link to underbook.co/rate. When an alert goes out, our own inbox gets a note of the stay and the two prices, without your email, so we can check the alert was right. We stop checking after your free-cancellation date, or at check-in if you did not give one. The record is deleted automatically the day after your check-out. Every email has a one-click unsubscribe link that stops the watch and deletes the record at once. Our service counts, as daily totals with no identifier, how many watches were set from a booking confirmation page, how many reminders were sent, and how many watches that asked for reminders were stopped.

A booked watch never needs your booking or confirmation number, your name, your phone number or address, or any payment details, and we do not store them. Please don't type them into the form. We don't see your booking and we don't contact the hotel or the booking site: if you rebook, you book and cancel yourself.

Watching a booking from its confirmation page (optional, opt-in)

After you book a stay on Booking.com, Expedia, Hotels.com or Agoda, the extension looks on that site's booking confirmation page for these details only: the hotel's name and town, your check-in and check-out dates, the number of adults, the total or nightly price and its currency, and the date your free cancellation ends. To find them it compares short pieces of text on the page with a fixed list of labels (such as "Check-in" or "Free cancellation until") and keeps only the value next to a matching label, and it reads the page's standard booking markup for the same details. It does not keep, store or send anything else from the page. It never reads your booking or confirmation number, PIN, name, email address, phone number, postal address or payment details. It looks at the site and the page's path only to recognize a confirmation page, never at the rest of the page's address, and stores or sends nothing of that address.

If it finds the hotel, the dates and a free-cancellation date at least two days away, it shows one card with those details filled in, so you can check them and change the price, the free-cancellation date, and the number of adults if the page did not show it. Nothing from the page or about the booking leaves your device while the card is shown, and closing it or pressing "No thanks" sends nothing. Only if you type your email and press "Watch this booking" does the extension send, as for any watch on a stay you already booked: your email, the hotel and its town, the dates, the number of adults, the price per night you confirmed (the total on the page divided by the number of nights, unless you change it), its currency, the market we price it in, the free-cancellation date and the name of the booking site, with a marker that the watch was set from a confirmation page and asks for deadline reminders.

The card is offered once per booking. To remember that, the extension keeps on your device a short code computed from the site, the hotel name and the dates, with the check-out date; it holds no booking number, name or price, and it is deleted the first day the extension runs after the check-out date.

What we store on your device

Local flags recording that you've seen the one-time first-run note (on older versions, the welcome card) and the one-time search-results tip, and for the booking-site tips: when a tip last showed, and for each booking site, when its tip last showed and when you last compared a hotel there (the site's name and a time, no address, hotel or search), and whether you turned tips off.
The random install id and install date used for the anonymous usage statistics above, plus a small flag that keeps usage pings to at most one per day.
The last 25 stays you checked, other than the welcome page's example (hotel name, dates, party size, currency, the going rate we found and when), so the banner can say what the price did when you come back to the same stay, and so that on a hotel page that shows no dates it can offer to check that hotel for the dates and party size of the last stay you checked. They never leave your device on their own: only if you press that button are those dates and that party size sent, with the name of the hotel on the page, as a hotel check; and, only if you turned on the price-drop mark described above, the stays you asked it for are sent again for its daily look (the fields your own check sent, for up to 14 days).
Only if you turn on the price-drop mark: that it is on; with each stay it may look at (the one where you turned it on, and those you check after), the hotel's town, the market and the address of the page where you checked it (when that address carries your dates); which of those stays it looked at and when, and whether the last look was under the going rate you saw; the mark while it shows (the stay, both figures and when); and, for 5 minutes after you click a mark, which stay it opened, so that page can say why it opened. It never leaves your device, and turning the mark off deletes the stays' details.
A small counter of recent no-result lookups that resets after 6 quiet hours, used to word a no-result message. It never leaves your device. (Versions before October 2026 also counted how many comparisons worked, to decide when to ask for a review; that count is no longer used.)
A small record for the request to rate Underbook: how many result banners you have seen (so the first, which introduces Underbook, never asks), when you last opened a cheaper listing from a banner that did not ask (so a later banner can ask instead, from 30 minutes to 14 days later), whether the request was shown, and what you chose (its link, or "Not now" and until when). It is used so the request comes only after you opened a cheaper listing Underbook found, and at most once, or once more no sooner than 90 days after "Not now". Versions before 0.5.50 also kept when your first banner was, and how many different stays had prices with a short code for the last of them, not its name; your next banner deletes them. It never leaves your device.
For each booking whose confirmation page offered a watch: a short code computed from the site, the hotel name and the dates, and the check-out date, so the offer is not shown again for that booking. No booking number, name, price or address. An entry is deleted the first day the extension runs after its check-out date, and at most 50 are kept.
If you set a watch, your email is also kept locally (via chrome.storage.local) so you don't have to retype it: the next alert is one tap. It stays in the extension's storage; the banner does not write it into the web page you are on unless you open the form to change it or to retry. It is sent to our price service only when you set a watch.

How Underbook makes money

When you open a booking site through Underbook and complete a booking, we may earn an affiliate commission from that site — at no extra cost to you (you pay the same price you'd pay going there directly). This never changes which rates we show you or the order we show them: we always surface the genuinely lowest rate we find, and we'll link you to a cheaper option even when it earns us nothing. We never take payment from you, and we don't sell your data.

When you click through to a booking site from Underbook, the link passes through our service (/go), which adds one to a daily click total (for the web checker, split by phone or computer, whether the visit started from a shared check link and whether that link was shared from the extension, whether the check started from one of our hotel or city pages, and whether the click was on one of the two booking-site searches the checker offers when it finds no price; for the web checker and the extension, whether the site you opened is one of the lesser-known booking sites we have an "Is <site> legit?" page about, never which one), noting only whether the browser's standard request headers mark it as a tap or click by a person, so crawlers following links are not counted as clicks, turns the link into a partner link where the booking site has one, and forwards you. We do not record who clicked. A converted partner link may be kept for up to 6 hours so the next click on the same link is fast; it contains nothing about you. One exception: when the banner has no price for your stay, its searches on other booking sites (Booking.com, Expedia, Hotels.com or Orbitz) open that site's own search directly, for the hotel's name, your dates and number of adults: they do not pass through our service, are not partner links and do not tell the site which page you came from, and the site receives the search as if you had typed it there.

What we never do

We never see, store, or transmit your payment details — you always book directly on the site you choose.
We don't sell or share your data.
We don't track you across the web or record your browsing.

Permissions, and why

Access to the sites you visit: so the comparison can appear on any hotel page, on major booking sites and individual hotels' own websites. The UI only activates when a hotel and dates are detected, or on the booking confirmation pages of Booking.com, Expedia, Hotels.com and Agoda as described above; nothing happens on other pages, apart from the toolbar icon's state on booking sites and the tips described above. On Chrome and Edge the extension also holds host permission for the same web pages (http and https), so that right after you install it, it can run in the tabs you already had open; the browser's install warning is the same either way.
activeTab & scripting: so that when you click the Underbook icon, it can check the hotel page you're on.
Storage: to remember the one-time first-run note, the tips and when they last showed, the request to rate Underbook, which bookings were already offered a watch on their confirmation page, the stays you checked, the price-drop mark if you turn it on, and, if you set a watch, your email.
Connection to our price service (underbook-backend.underbook.workers.dev): where the rate comparison is performed.

The website (separate from the extension)

Page views. Pages on underbook.co record one anonymous page view per visit: the referring site or campaign tag and the time. Clicks on the store buttons and on the welcome page's examples are kept only as daily totals. Our hotel and city price pages (underbook.co/hotels) also count, as daily totals: that the page was opened, that its check form was used (and whether with its "Watch your booking" button), and, for each, whether your visit began on a search engine. The watch-your-booking page counts the same way that it was opened, and it, our homepage and our rebooking guides count clicks on their "Watch my booking" links as daily totals. Our homepage also counts, as daily totals split only by whether the browser says it is a phone or a computer, that it was opened, that a price check was started from it, and clicks on its store buttons. Only that kind is noted, never which search engine or what you searched for. On a phone or tablet, where the extension cannot be added, our homepage and the web price checker offer to send you the link to open on your computer (underbook.co/get): the button opens your phone's own share menu or copies the link, which carries only campaign tags saying where the link came from (such as our homepage or a video), never anything about you, a hotel or your dates. We count, as daily totals with no identifier, how often that button was pressed and how often the share finished or the link was copied; the app or person you send it to is never known to us. When such a link is opened in a browser that can add the extension, underbook.co/get counts that as a daily total before it sends you to your browser's add-on store. When underbook.co/get sends a phone, a tablet or Safari to the price checker, it leaves the link's campaign tags in that tab's own storage (sessionStorage), which the checker reads and deletes within two minutes; they never leave your browser, and on a phone or tablet the checker counts that arrival as a daily total with no identifier. Once our iPhone app is listed (it is not in the App Store yet), these places also link to it on an iPhone, and clicks on that link are counted as a daily total. When you arrive on underbook.co from a link in an AI assistant (ChatGPT, Perplexity, Claude, Copilot, Gemini, You.com, Phind, Meta AI, DeepSeek, Grok or Le Chat), known from the link's campaign tag or the site or app that sent you, the first page you open counts that arrival as a daily total per assistant, with only the assistant's name, and as a separate daily total per kind of our page you arrived on, one word from a fixed list (our homepage, the price checker, the extension page, How Underbook works, the price study, an "is it legit" guide, a comparison of two sites, another guide, our hotel and city pages, the help center, or other); the two totals are not linked to each other, and the link's full address, its tags, the site that sent you and what you asked the assistant are never sent. If you remove the extension, your browser opens underbook.co/uninstall; that page counts, as daily totals only, that it was opened and which reason you tapped, if any. underbook.co/rate, the page that line links to, counts as a daily total with no identifier and no cookie that it was opened, then sends you to your browser's add-on store; which store, and whether you rate Underbook, is never sent to us. No cookie, no identifier and no IP address are stored. To keep these daily totals to people, our service also looks at what each request already carries (whether it comes from a cloud or hosting network, as reported by our host Cloudflare; whether it has the standard headers your browser sends, such as its language; and whether many identical events arrive within a few seconds) and adds requests that look automated to a separate daily total instead. None of this is stored (apart from the language counts described above), and a price check is answered the same either way.

The web price checker. The price checker sends the hotel name, dates and number of adults you type, the currency you pick and a market taken from your browser's language setting (for example "us" or "gb") to the same price service the extension uses, and counts each check as a daily total, split only by whether the browser says it is a phone or a computer, whether the check started from one of our hotel pages, and the kind of page that sent you to the checker (a search engine, our own site, a social site, another site, or none). The address of that page never leaves your browser; only the kind is sent. It also counts, as daily totals with no identifier, how often a result showed the price-drop alert box, how often someone started typing in it and how often it was submitted; your email is never part of those counts. On a computer, it also counts, as daily totals with no identifier, how often a result showed the offer of the extension and how often its button was clicked, with the clicks also split by whether the check started from one of our hotel or city pages. Once you open a booking site from a result, that offer names the site you opened, and a click on its button after that is also counted as a daily total with no identifier; the site is never part of that count. When a check finds no price, the checker shows two links that search Booking.com and Expedia for the hotel name you typed, with your dates and number of adults; if you click one, that name, those dates and the number of adults go to the booking site you chose, through our service like any booking-site link. We count, as a daily total with no identifier, how often those links were shown; the name and the dates are never part of that count. We also keep daily totals of what each price check showed: whether a lower price was shown, how far below the going rate it was (5 to 10%, 10 to 20%, 20 to 40%, or 40% or more) and whether the room name matched, or that no site was lower or that there was no price, and whether the lower price was on one of the lesser-known booking sites we have an "Is <site> legit?" page about (never which one), split only by whether the check came from the extension or the web price checker. When a lower price is on such a site, the extension's banner and the checker's result link to that page, and when the site is one that may ask you to sign in before it shows the price, they say so. We count, as one daily total with no identifier for the banner and the checker together, how often that link was opened (at most once per result); the site, the hotel and the dates are never part of that count. These totals hold no hotel, dates, price or anything about who checked. When a result shows that another booking site lists free cancellation for the same price or less, we count, as daily totals with no identifier, how often that note was shown and how often its button was clicked; the hotel, the dates and the sites are never part of those counts. If you tick "I already booked this stay", the checker also sends the price you paid per night, its currency, the market and the free-cancellation date you typed, and counts, as daily totals with no identifier, how often that box was shown, ticked and used to set a watch, how often the "Already booked this stay?" link under a result was pressed, and how many booked watches were set after arriving from a hotel page's or another of our pages' watch-your-booking link or after pressing that link; the price and the date are never part of those counts. If you press "Share this check" (on a result that shows a lower price, the button reads "Send them this check"), the link you share carries only the hotel name, the dates, the number of adults, the currency and a marker that it is a shared link (and, when the check used it, the hotel's public Google listing ID), never your email, the price you paid or anything else you typed. We count, as daily totals with no identifier, how often Share was pressed (and, of those presses, how many were on a result that showed a lower price), how often the share finished or the link was copied, how often a shared link was opened (and whether its dates had passed), and the checks and booking-site clicks made after opening one, each also split by whether the link was shared from the extension's banner; the link itself, the hotel and the dates are never part of those counts. A link that already carries a hotel and dates runs the check as soon as it opens only when a person asked for it: when you press a check button or link on one of our pages, your browser keeps the time of that press in its own storage for that tab (sessionStorage), and the checker reads and deletes it when it opens, within two minutes; it never leaves your browser. Shared links and the welcome page's form run at once too. Opened any other way, the checker shows the hotel and dates with a Check prices button, and counts that, as a daily total with no identifier. Browsers that identify themselves as automated, as a crawler or as a link preview are not checked. To stop automated misuse, each visitor can run up to 40 checks a day. We count them under a one-way hash of your IP address, salted and changed every day and deleted after about 26 hours; the address itself is never stored.

Email you gave us earlier. Emails left in the homepage's old waitlist or "watch this booking" forms (both removed in September 2026) are used only to contact you about that request or about Underbook. We don't sell them. Ask at hi@underbook.co to have yours deleted.

Contact

Questions, or want your data deleted? hi@underbook.co.

We'll post any material change to this policy on this page with a new "last updated" date.