Is it really necessary to create a separate sitemap for each subdomain?

Why does Google require a sitemap for each subdomain?

The reason is simple: Google considers each subdomain as a separate site during crawling. blog.example.com is not identified in the same way as example.com or shop.example.com. Each entity therefore has its own crawl budget, its own indexing history, and its own trust metrics.

In practical terms, if you upload a global sitemap to example.com that references URLs from blog.example.com, Google will completely ignore it. The sitemap must be hosted on the same subdomain as the URLs it lists. This is a non-negotiable technical principle.

What role does the robots.txt file play in this configuration?

The robots.txt file allows you to declare the location of the sitemap using the Sitemap: directive. Each subdomain should therefore have its own robots.txt containing the appropriate line. For example, blog.example.com/robots.txt will indicate Sitemap: https://blog.example.com/sitemap.xml.

This approach offers two advantages. First, it facilitates automatic discovery by Googlebot without going through Search Console. Second, it centralizes configuration: a human or a bot consults robots.txt and immediately knows where to find the sitemap. This is a good technical hygiene practice.

Do all sites need a multi-subdomain architecture?

No, and this is where the issue lies. Many subdomain structures are historical or poorly designed. They date back to times when tech teams separated blogs, e-commerce, and corporate sites for infrastructure reasons, without considering the SEO implications.

A subdirectory architecture (example.com/blog/, example.com/shop/) simplifies everything: a single crawl budget, centralized authority, a unique sitemap. If you don't have a compelling technical reason to separate your content onto subdomains, avoid it. Mueller's recommendation is valid, but it shouldn't serve as an excuse to multiply subdomains thoughtlessly.

• One subdomain = a distinct site in Google's eyes, with its own crawl budget and metrics.
• Each subdomain requires its own XML sitemap, hosted on that same subdomain.
• The robots.txt file for each subdomain must reference its sitemap through the Sitemap: directive.
• A subdirectory architecture is generally preferable unless there are strong technical or organizational constraints.
• Avoid fragmenting your authority by creating subdomains without a clear strategic necessity.

Does this recommendation apply to all contexts?

Yes, but it mainly reveals a poor initial choice. Most sites that use subdomains do so out of habit or technical constraint, not as an SEO strategy. If you are a marketplace with hundreds of independent sellers (each on seller.example.com), or a multi-tenant SaaS platform, subdomains make sense. In these cases, you consciously accept the fragmentation of the crawl budget.

But if you've simply put the blog on blog.example.com because "it was easier for the dev team," you've shot yourself in the foot. You disrupt your internal linking, disperse authority, and complicate sitemap management. You then have to carry the maintenance burden: one sitemap per subdomain, one Search Console property per subdomain, and separate monitoring of indexing errors.

What common mistakes are observed in practice?

The first classic mistake: submitting a cross-domain sitemap via Search Console. You are on the example.com property and add a sitemap listing URLs from blog.example.com. Google accepts it without issue in the interface, but crawls nothing. No errors reported, just radio silence.

The second mistake: forgetting the robots.txt on certain subdomains. You set everything up correctly on the main domain, but the subdomains return a 404 on /robots.txt. As a result, Google has to guess where the sitemap is, or you have to manually submit it in Search Console. Waste of time, risk of forgetting during a new launch.

The third observed case: poorly managed wildcard subdomains. Some CMSs or platforms dynamically generate user subdomains (user123.example.com). If you do not master the automatic generation of sitemaps and robots.txt for each, you end up with thousands of unindexed or chaotically crawled subdomains. [To verify]: Google does not clearly document how it prioritizes crawling in these massive wildcard configurations.

In what situations should you really keep subdomains?

If you have independent geographic or linguistic content (fr.example.com, de.example.com), using subdomains remains defensible, even though subdirectories with hreflang are often preferable. If you manage a heterogeneous technical infrastructure (blog on WordPress, shop on Shopify, corporate site on a custom CMS), and migrating everything to a single stack is unrealistic in the short term, then accept the subdomains.

But be honest with yourself: is it a real constraint or just a choice of convenience? Because you will pay for this initial convenience in SEO complexity for years. Each subdomain is a silo, and you will have to compensate with aggressive internal linking to artificially recreate the authority you would have naturally had with subdirectories.

What should you do if you are already using subdomains?

First, audit the existing setup. List all your active subdomains, check that each has an updated XML sitemap and a robots.txt that references it. Use a crawler like Screaming Frog in list mode to ensure that the URLs of each sitemap are actually crawlable and not blocked by robots.txt or noindex.

Then, consider migration. If your subdomains serve no strong business logic, seriously consider bringing everything back under subdirectories. Yes, it's a project. Yes, it requires clean 301 redirects, rigorous management, and monitoring in Search Console. But over 12-18 months, you will regain centralized authority and drastically simplify your technical stack.

How do you properly configure a multi-subdomain architecture?

Each subdomain must have its own dedicated robots.txt file at the root, containing at least the Sitemap: directive pointing to its own XML sitemap. No generic sitemap, no copying and pasting the robots.txt from the main domain. Each entity is autonomous.

Create a distinct Search Console property for each subdomain. Google allows you to create a "Domain" type property that aggregates everything, but it does not replace individual properties for detailed tracking. You must be able to isolate indexing errors, crawling performance, and Core Web Vitals by subdomain.

Set up automated monitoring that regularly checks that each sitemap is accessible, up-to-date, and does not contain URLs with 4xx or 5xx errors. An overlooked subdomain for six months means that content gradually disappears from the index without you noticing.

What mistakes should you absolutely avoid?

Never deploy a sitemap that lists cross-domain URLs. Google will silently ignore it, and you'll think you are compliant while nothing is crawled. Also, ensure you do not have unintentional cross-domain canonicals: a canonical on blog.example.com pointing to example.com breaks the logic of distinct subdomains.

Avoid creating multiple subdomains for cosmetic reasons. Each new subdomain is a permanent SEO maintenance cost. If you have no compelling reason, stay in subdirectory. And if you have doubts, test first with a subdirectory: migrating from /blog to blog.example.com is doable; the reverse is more complicated.

• Ensure each subdomain has its own XML sitemap hosted on that subdomain.
• Create a distinct robots.txt file for each subdomain, referencing the sitemap via Sitemap:.
• Configure a separate Search Console property for each subdomain.
• Regularly audit sitemaps for error URLs, duplicates, or cross-domain issues.
• Seriously evaluate the opportunity for migration to subdirectories if no strong technical constraints justify the subdomains.
• Implement automated monitoring of sitemap accessibility and freshness.