What is consent mode?

Consent mode is not a banner or a notice layer. It is the protocol by which the decision the user makes in the banner reaches Google tags in a format they understand. The tool that displays the notice and stores the choice is the consent management platform; consent mode is the pipe between that choice and the measurement and advertising products.

The practical distinction lies in who decides what. The site owner defines the legal basis and the scope of processing with legal support. The notice collects the answer. Consent mode merely communicates it. If the signal indicates that advertising is not authorised, tags stop writing advertising cookies and stop sending identifiers, even though the code remains loaded on the page.

It should also be separated from plain tag blocking. Blocking means the script does not run and nothing happens. With consent mode the script can run in a restricted state, without storage and without persistent identifiers. That difference explains why two implementation versions exist and why they produce different data.

The mechanism has two moments. First a default state is declared, before the user answers, normally with everything denied for European traffic. Then, when the person chooses, an update with the actual state is sent. That order matters: the update must be registered on the same page, before the visitor navigates to another one.

Four signals are relevant. ad_storage governs the storage of cookies and identifiers for advertising purposes. analytics_storage does the same for analytics measurement. ad_user_data authorises sending personal data to Google for advertising purposes. ad_personalization authorises personalised advertising, including remarketing. The last two were added in November 2023 with the second version.

On that basis there are two ways to implement it. In the basic version Google tags do not load until the user interacts with the notice, so before that interaction no data at all goes to Google, not even the consent state. In the advanced version tags load immediately in a denied state and send cookieless signals: the consent state itself, timestamps, user agent and referrer, without identifiers that would allow the person to be recognised.

That is where the difference in modelled conversions comes from. The advanced version allows advertiser-specific modelling, because Google observes the behaviour of those who accept on that same site and projects it onto the non-consented segment. The basic version only has access to general modelling.

What depends on this is whether ad personalisation, remarketing and Google measurement can still be used with traffic from the European Economic Area. Google requires collecting consent from those users and transmitting the corresponding signals in order to keep those features, and its documentation places the start of that requirement for ad personalisation in March 2024. Data from European users without consent is not processed for those purposes.

The effect does not arrive as a penalty or as a notice in the account. It is silent: audience lists stop growing, remarketing loses reach in that market, and recorded conversions drift away from actual sales. Many teams only notice weeks later, as a drop in performance with no apparent cause.

There is also an implementation decision. The basic version is more conservative about the data that leaves the site and usually fits when the internal rule is to communicate nothing before an explicit answer. The advanced version recovers more signal and improves modelling, at the cost of sending cookieless requests before the choice is made. Which of the two is defensible in a specific case depends on the legal review of the site and not on a technical preference.

• Declare the default state as denied for traffic from the European Economic Area and update it only once the user has answered.
• Load that declaration before any measurement tag, so that none runs with an undefined state.
• Verify the four signals in the browser using the tag manager preview, instead of taking the vendor's configuration at face value.
• Check that accepting, rejecting and revoking later produce the three corresponding states, including persistence across pages.
• Document which version is in place and why, with a date, and review it whenever the notice or the tag manager changes.
• Compare observed and modelled conversions before and after every change to the notice, to separate a measurement problem from a real drop in business.

• Installing the notice and assuming the signals travel, without checking even once what the tag receives.
• Leaving the consent update behind a redirect, so the choice is lost before it is registered.
• Keeping an old implementation with only ad_storage and analytics_storage, without the two signals added in 2023.
• Blocking tags entirely and enabling consent mode as well, which loses the signal without gaining any additional protection.
• Reading a rise in modelled conversions as real business growth and carrying it into sales reports.