The overlooked impact of consent management on your data & marketing performance

 Lotte Vranckx

Introduction

By now, most organizations understand the importance of managing user consent on their websites, not as a nice-to-have, but as a legal obligation. Privacy regulations such as the GDPR and the ePrivacy Directive have made consent management a fundamental requirement for any business operating online. Getting it wrong can have serious legal and reputational consequences, from regulatory fines to erosion of user trust.

But compliance is only one side of the story. The way consent is implemented also has a direct impact on your data quality and measurement infrastructure. In fact, a poorly configured consent setup can silently undermine the integrity of your analytics, disrupt conversion tracking, and even compromise marketing performance.

Too often, teams approach consent with a “compliance-only” mindset: the banner is visible, the buttons function, and legal has signed off so the job is considered done. Yet, behind the scenes, subtle technical missteps can lead to significant data blind spots. A delayed consent signal, an incorrect default state, or a tag firing before consent is registered can easily distort your KPIs. Even something as small as a consent banner loading 300 ms too late can prevent Google Consent Mode from receiving the user’s choice in time, causing conversions to lose attribution without anyone noticing. Or consider a CMP that only exposes the consent state after the page’s JavaScript has fully loaded: in that brief window, tags may fire in an undefined state, inflating pageview counts or triggering unwanted data collection. The consequences are tangible, from GA4 conversions dropping, to remarketing audiences no longer populating, or attribution models becoming less reliable.

These issues are not rare exceptions, they’re common symptoms of treating privacy and measurement as separate conversations. In this article, we’ll focus on bridging that gap. We’ll explore how to design a consent setup that is both legally compliant and technically robust, share best practices, highlight common pitfalls, and provide actionable solutions to ensure that privacy protection and data accuracy go hand in hand.

Choosing Your CMP: Legal First… But Never Legal Only

When organizations select a consent management platform today, the evaluation is usually driven by the same set of criteria. Most teams, often led by legal, procurement, or compliance, compare CMPs based on:

These criteria are all very valid and necessary, but they also create a selection process heavily weighted toward compliance, legal safety, and operational convenience.What is often missing is a thorough assessment of how a CMP impacts your measurement.

Despite being the backbone of your analytics, marketing attribution, and consent mode setup, the technical behavior of the CMP is rarely analyzed with the same rigor. Teams seldom evaluate:

These measurement-related factors determine whether your tracking framework will operate accurately and consistently, yet they are rarely part of the CMP selection checklist.

As a result, many websites end up with a CMP that is legally compliant but technically restrictive, or one that unintentionally disrupts analytics, breaks conversion tracking, or delays consent signals in ways that distort KPIs.

The following sections will dive deeper into these key technical considerations that are often overlooked when choosing or implementing a CMP. We’ll unpack how storage methods, banner deployment choices and consent-signal timing directly influence the accuracy of your measurement setup. By understanding these elements, not just from a legal perspective, but from a tracking and data-quality perspective, you can design a consent framework that protects user privacy and preserves the integrity of your analytics

Your cookie banner implementation: Hardcoded vs GTM

When implementing a cookie banner, there are essentially two roads to take: you can either hardcode it directly into your website source code or deploy it through Google Tag Manager. Both options come with their own set of advantages and trade-offs. The GTM route is often chosen for its flexibility. It allows teams to make changes without involving developers, manage scripts centrally, and adapt quickly to new design or regulatory requirements. Most consent management platforms even provide ready-to-use GTM templates, making the setup relatively easy.

However, this convenience comes at a cost. When the banner is loaded via GTM, its appearance depends on GTM’s own loading sequence. This creates a small but critical dependency: if GTM takes even a fraction of a second longer to load, there’s a risk that certain tracking tags fire before consent is properly registered. In theory, the setup might still be compliant, but in practice, this timing gap can lead to data being collected before consent is given. Beyond the compliance risk, the marketing impact is substantial. With Google Consent Mode v2, delayed consent signals can cause Google’s behavioural and conversion modelling to fall back into less accurate “no consent” pathways, reducing model quality. Google Ads may receive too few high-quality pings to reliably reconstruct conversions, weakening bidding signals and decreasing optimisation performance. In addition, if the consent signals are not collected correctly we risk that these hits become ineligible for audience building altogether, resulting in shrinking remarketing lists and lost eligibility for key segments. This combination undermines both compliance and the integrity of your measurement data.

Hardcoding the banner, on the other hand, means embedding it directly into the website’s source code, usually before any other scripts run. It’s a more rigid setup as every update or change requires development support, but it provides full control over when and how the banner is executed. By loading the banner before GTM or any tracking script, you can be sure that no tags fire before the user’s consent is established. This not only strengthens compliance but also guarantees that your analytics and conversion data reflect the real consent status from the very first page load.

This control can make all the difference. While GTM is the more flexible option, hardcoding remains the more reliable one, especially from a measurement accuracy point of view. Therefore, we prefer to hardcode the banner because it removes the risk of timing delays altogether. It ensures that the consent logic is in place before any other technology starts doing its work, protecting data quality and user privacy in one move.

Local storage vs 1st party cookie : Same but different

Both cookies and localStorage serve a similar purpose: they allow a website to remember something about a user across pages or visits. In the context of consent management, they’re used to store whether a user has accepted, rejected, or customized their preferences. While they achieve the same goal, they work in fundamentally different ways and thus the choice between them has real implications for how your tracking setup functions.

Cookies are small pieces of data that the browser automatically sends to the server with every request. They have built-in expiration dates, can be restricted to specific domains or paths, and are traditionally used for things like authentication, personalization, and of course also consent storage. Because they travel with each page request, cookies are accessible very early during page load, which is helpful when tags or server-side systems need to know the consent state immediately.

LocalStorage, on the other hand, stores data entirely in the user’s browser and never sends it to the server. It can store larger amounts of data and doesn’t expire unless explicitly cleared, making it a convenient and flexible option for front-end applications. However, it only becomes available once the page’s JavaScript is running, which can introduce timing differences: the consent signal might not be available as quickly as it would be with a cookie.

In practice, both options can work perfectly well for storing consent choices, but they just behave differently. One important behavior to consider is what happens when a user clears their browser data.
If consent is stored in a cookie, removing cookies will also remove the user’s preferences. On their next visit, the CMP will ask for consent again. This is expected behavior, but it also means you risk re-prompting users more frequently, which can influence consent rates and therefore impact your measurement. If consent is stored in localStorage, deleting cookies does not remove the consent preferences. Unless the user explicitly clears all site data, localStorage remains intact and the consent choice persists. This reduces the number of times a user sees the banner and can lead to more stable consent rates over time , which in turn stabilizes your tracking.

There is no universally “better” option. What matters is understanding how each method interacts with your tracking setup, especially regarding timing, persistence, and user experience. The right choice ensures your consent signal is consistent, accurate, and available exactly when your measurement tools need it. To help you choose the most appropriate storage method, the following guidelines can serve as a practical decision framework:

Use a 1st-party cookie if:

Use localStorage if:

Conclusion

Consent management is far more than a legal checkbox. The technical decisions you make, from how your CMP is selected, to where the banner is implemented, to how consent is stored will directly shape the accuracy, stability, and reliability of your measurement framework. Subtle choices that seem purely operational or UX-driven can have significant downstream effects on analytics quality, conversion tracking, and attribution.

By looking beyond compliance and considering timing, storage strategy, and integration methods, organizations can ensure that their consent setup protects both user privacy and the integrity of their data. A well-designed consent framework doesn’t just meet regulatory standards; it creates the foundation for trustworthy insights and more resilient marketing performance.

Now that we have a clearer understanding of how these choices influence measurement, our next article will dive into the most efficient implementation setups, as well as practical tips and tricks to ensure clean, accurate, and privacy-safe data collection from day one.

publication auteur Lotte Vranckx
AUTHOR
Lotte Vranckx

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