Subdomains vs Subdirectories: Is there really an SEO advantage to favoring one over the other?

Is this statement consistent with field observations?

Yes and no. On medium-sized sites, there is indeed little difference in ranking between subdomains and equivalent structured subdirectories. A/B migration tests show minor variations, often attributable to other factors — poorly managed redirects, temporary duplicate content, disrupted crawl budget.

But on high-authority domains, some SEOs report that subdirectories seem to benefit more quickly from existing authority during the rollout of new sections. Coincidence? Confirmation bias? [To be verified] with large-scale data. Google does not publish any figures on this point, and third-party studies often lack methodological rigor.

What nuances should be added to this statement from Google?

Mueller speaks of "essentially equivalent" treatment — that word matters. Essentially, it’s not strictly. In practice, a subdomain may be crawled at a different rate if Googlebot perceives it as a more autonomous entity — even temporarily.

Crawl budget may be allocated differently, especially on large sites. A subdomain with its own XML sitemap and update frequency may see Googlebot adjust its behavior. This is not a penalty, but it’s also not perfectly transparent. If your main site is crawled every hour and your subdomain waits 48 hours, you have a problem — not SEO, but responsiveness.

In what cases does this rule not fully apply?

When the content of the subdomain is radically different from that of the main domain. A corporate blog at blog.example.com with a distinct tone, audience, and themes may be perceived as more autonomous than a simple /blog/ integrated. Google may adjust its assessment of relevance and authority accordingly — not a penalty, just a different evaluation.

Multilingual or multi-regional sites also raise questions. Google officially advises using subdomains or distinct domains for very different languages. If fr.example.com targets France and jp.example.com targets Japan, the treatment will necessarily be distinct — geographical targeting, language, user behavior. Saying they are treated "equivalently" becomes a technical abstraction that masks practical realities.

What should you do if you are launching a new project?

Forget SEO as your main decision criterion. Ask yourself the right questions: who manages this content? What technical team? What CMS? If your blog runs on WordPress and your main site on a custom framework, a subdomain simplifies life — two stacks, two deployments, two SSL certificates if required.

If you want unified analytics tracking and your tool doesn’t handle cross-domain well, a subdirectory avoids complications. Google Analytics 4 manages multi-domain better than Universal Analytics, but it’s not always smooth. SEO won’t decide — your analytics stack will.

What mistakes should be avoided during an existing migration?

Do not migrate simply because "subdirectories are better for SEO." This is a myth that Mueller explicitly dismantles. If your subdomain works well, don’t change anything without a solid strategic reason — technical overhaul, team merger, infrastructure simplification.

If you do migrate anyway, manage 301 redirects surgically. Each URL of the old subdomain must point to its exact equivalent in the subdirectory. Test the XML sitemap, verify that Search Console recognizes the new structure, monitor the crawl for 4 to 6 weeks. A poorly managed migration can result in a 20 to 30% loss of organic traffic for several months — for no SEO gain.

How to check if your current structure is penalizing your SEO?

Go to Search Console and compare the performance of your subdomains and your main domain. If a subdomain shows an abnormally low indexing rate or very spaced crawl, it’s not the fault of its subdomain nature — it's a problem of content, internal linking, or XML sitemap.

Ensure your internal links do not treat the subdomain as an external site. A subdomain should interlink from the main domain with the same logic as a subdirectory — no rel="external", no unjustified nofollow, no artificial separation. If Google sees a coherent interlinking, it will treat everything as a whole, regardless of the technical structure.

• Choose based on your technical constraints: stack, CMS, teams, deployment — not based on a hypothetical SEO bonus
• If you use subdomains, ensure they are well-linked from the main domain and referenced in the global XML sitemap
• Don’t migrate without a strategic reason: the risk of error far outweighs the theoretical SEO gain (none)
• Monitor crawl and indexing in Search Console for any unjustified differentiated treatment
• Test your analytics tracking before making a choice: some tools handle cross-domain poorly, making subdirectories more practical
• For multilingual sites, document your choice with hreflang and geographical targeting in Search Console, regardless of the chosen structure