Consent Mode v2 is effectively mandatory for preserving measurement and ad personalisation signals under UK and EU privacy rules. When a user grants or denies consent, that signal — ad_storage, analytics_storage, ad_user_data, ad_personalization — has to propagate consistently across every subdomain they visit next, not just the page where they saw the banner.
Setting defaults before anything else loads
window.dataLayer = window.dataLayer || [];
function gtag(){ dataLayer.push(arguments); }
gtag('consent', 'default', {
'ad_storage': 'denied',
'analytics_storage': 'denied',
'ad_user_data': 'denied',
'ad_personalization': 'denied',
'wait_for_update': 500
});The part that actually breaks: cookie-domain scope
If a user accepts consent on example.com, that choice needs to apply on app.example.com or shop.example.com too — otherwise they see a duplicate consent banner and, worse, analytics silently fails between the disagreement. Set the consent management platform's cookie domain explicitly to the root host with a leading dot (.example.com), which propagates the consent cookie across every subdomain on that root rather than scoping it to a single host.
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Written by
Paul Lovell
International SEO Consultant & Founder of Always Evolving SEO. 15+ years running technical audits, log-file analysis, and root-cause fixes for multimillion-dollar businesses — speaker at SMX London, Search & Content Summit, and SEMrush events.