Are subdomains really evaluated separately from the main domain?

What reasoning does Google really apply to subdomains?

The official statement remains deliberately vague on the precise criteria. Google indicates that it evaluates the relevance and relationship of the content to decide whether a subdomain is treated as a distinct entity or as an extension of the main domain.

In practice, this means there is no binary rule. A subdomain blog.example.com can inherit authority from example.com if the content is thematically aligned and well-linked with the root domain. Conversely, a completely disconnected subdomain — like partner.example.com hosting third-party content — will likely be assessed in isolation.

How does this evaluation impact crawling and indexing?

Each subdomain technically has its own crawl budget. Google can thus allocate different resources based on the quality and frequency of the content hosted.

If the subdomain is considered an integral part of the main domain, trust and authority signals can flow more easily. But beware: a low-quality subdomain can also negatively impact the overall perception of the root domain, particularly if the content is perceived as thin or spammy.

Why does this ambiguity pose a problem for SEOs?

Because it leaves a considerable margin for interpretation. Some sites use subdomains to segment geographical areas (fr.example.com, uk.example.com), while others use them to isolate technical content (support.example.com). The observed results vary greatly depending on sectors and internal structuring.

Without clear criteria, it becomes difficult to anticipate whether a subdomain will benefit from the link equity of the main domain or if it will have to build its own authority from scratch. This uncertainty complicates architectural decisions, especially during migrations or redesigns.

• Google evaluates subdomains on a case-by-case basis, without a predictable universal rule
• Theme relevance and internal linking influence the treatment
• A subdomain can benefit from or suffer due to the authority of the root domain depending on the context
• The crawl budget is allocated independently for each subdomain
• Quality signals can propagate both ways (positive or negative)

Is this statement consistent with real-world observations?

Yes and no. Tests carried out by the SEO community indeed show very heterogeneous behaviors. Some subdomains rank immediately with visible inherited authority, while others stagnate for months despite quality content.

The real problem is that Google never quantifies what constitutes a "sufficient content relationship". Is it a dense internal link structure? Semantic coherence measured by NLP? A history of shared inbound links? We navigate in the dark. [To be verified]: no public metric allows measuring the degree of integration of a subdomain to the root domain.

What nuances should we add to this claim?

Google speaks of independent evaluation, but forgets to mention that external signals (backlinks, domain authority) are never completely isolated. A link pointing to the root domain can indirectly enhance the perceived quality of all its subdomains.

Let’s be honest: the notion of "content relevance" remains fuzzy. Relevant to what? The main domain? The user query? The entities recognized by the Knowledge Graph? Google mixes several dimensions without clearly separating them. This ambiguity is likely intentional to preserve algorithmic flexibility.

In which cases does this rule not really apply?

Generic platform subdomains (like user.wordpress.com or shop.shopify.com) are treated differently. Google knows that these structures have no content relationship with the root domain — they are separate sites hosted on a shared infrastructure.

Similarly, subdomains used for non-indexable technical content (staging.example.com, admin.example.com) are generally ignored or excluded via robots.txt. In these cases, independent evaluation becomes purely theoretical.

Should you prefer subdomain or subdirectory for a new project?

The answer depends on your branding and content strategy. If the project is thematically aligned with the main domain and needs to benefit immediately from its authority, a subdirectory (example.com/blog) remains the safest choice.

On the other hand, if you are launching a completely different vertical — say a marketplace on a media site — a subdomain allows you to isolate metrics, crawl budget, and potential risks. But beware: you will then need to build the authority of this subdomain as if it were a new domain.

How can you maximize synergies between the main domain and subdomains?

Strategic internal linking is crucial. If Google evaluates the relevance and relationship of the content, explicit connections need to be created: contextual links, shared navigation, cross-references in the content.

Another often-overlooked lever: semantic coherence. If the main domain deals with finance and the subdomain suddenly addresses gardening, do not expect an authority transfer. Conversely, a subdomain dedicated to financial news or investment guides is likely to be perceived as a natural extension.

What critical mistakes should be avoided with subdomains?

Never duplicate identical content between the main domain and subdomain. Google could view this as cannibalization and devalue the whole platform. Each space must have a distinct editorial purpose.

Also, avoid neglecting technical configurations: a poorly configured subdomain (missing SSL certificate, broken redirects, overly restrictive robots.txt) will be treated as a low-quality site, regardless of the authority of the root domain. And this is often where problems arise.

• Check the thematic coherence between the main domain and the subdomain
• Establish an explicit and contextual internal linking
• Correctly configure SSL, redirects, and robots.txt files on each subdomain
• Avoid any content duplication between the root domain and subdomains
• Monitor crawl and indexing metrics separately for each subdomain in Search Console
• Plan a specific backlink strategy if the subdomain is treated as a distinct entity