How Does the Mobile First Index Transition Work Based on Your Site Architecture?

Google has clarified the transition mechanism to the Mobile First Index (MFI): this process operates at the domain name level and not site by site or page by page.

Concretely, this means that when your main domain (example: www.example.com) transitions to MFI, all associated subdomains (blog.example.com, shop.example.com, etc.) switch simultaneously.

This approach has a major implication for site migrations: a site that migrates to a new domain name is considered completely new and therefore transitions directly to MFI, without a transition period.

• The MFI transition happens at the root domain level, not at the individual subdomain level
• All subdomains of the same domain switch together and simultaneously
• A domain migration = treatment as a new site = immediate transition to MFI
• This rule applies uniformly, regardless of the size or complexity of your architecture

This statement is consistent with Google's technical approach which manages indexing by domain, particularly for reasons of crawl efficiency and trust signal processing.

However, a gray area remains concerning shared platforms like WordPress.com, Blogger or Wix, where thousands of sites coexist on subdomains of the same main domain. It is unlikely that Google strictly applies this rule in these cases, as it would create massive indexing quality problems.

In practice, we observe that Google can apply a differentiated qualitative assessment for large groups owning numerous subdomains with very different content. The search engine maintains some flexibility to avoid side effects.

• Audit all your subdomains: identify all active subdomains linked to your main domain and verify their mobile compatibility
• Harmonize the mobile experience: ensure that each subdomain offers a quality mobile experience (responsive design, loading speed, touch UX)
• Test with Search Console: use Google's mobile-friendly test for each subdomain before the transition
• Prioritize mobile for migrations: if you're planning a domain migration, design your new site from the outset with a mobile-first approach
• Monitor post-transition performance: set up precise monitoring of rankings, traffic and crawl for each subdomain after the transition to MFI
• Unify your technical strategy: define common standards for all your subdomains (loading time, Core Web Vitals, structure)