Subdomains and SEO: what changes when content lives on its own subdomain, and when a subfolder is simpler
What Google's own documentation says about subdomains for robots.txt, Search Console, sitemaps, site names, international sites and site moves, read on 22 October 2026, and how to decide between blog.example.com and example.com/blog.
Published by AI Knows Us · Updated 22 October 2026
A subdomain such as blog.example.com can be found and ranked by Google like any other part of your site, but it is a separate host, and several things in Google's own documentation are set per host. It needs its own robots.txt file, its own Search Console property unless you use a Domain property, its own place in your sitemap plan, and its own site name markup. A subfolder such as example.com/blog keeps everything on one host, so it is the simpler choice when the content is part of the same business and runs on the same platform. Choose a subdomain when there is a real technical or organisational reason, such as a different platform, a different team or a separate country site, and then do the extra setup on purpose.
This page goes through each place where Google's documentation treats a subdomain differently, then gives a short way to decide, and the checklist for moving content between the two.
The words first
In https://shop.example.com/shoes, the host is shop.example.com. The part before the main domain, shop, is the subdomain. In https://www.example.com/shop/shoes, the host stays www.example.com and /shop/ is a subfolder, also called a subdirectory. The question "is a subdomain bad for SEO" usually means "if I put my blog, help centre or store on a subdomain, does the main site lose anything". The useful answer is to look at what Google actually documents per host.
Where Google's documentation treats a subdomain as its own site
robots.txt is per host
Google's robots.txt specification, updated 31 August 2026, says the rules in a robots.txt file "apply only to the host, protocol, and port number where the robots.txt file is hosted" and that a robots.txt file on a subdomain "is only valid for that subdomain". So the file at www.example.com/robots.txt does nothing for blog.example.com. Each subdomain needs its own file, and a staging or test subdomain left open is crawlable unless you block or protect it.
Search Console needs the right property type
Search Console's help page on adding a property describes two kinds. A URL-prefix property "Includes only URLs with the specified prefix, including the protocol". A Domain property includes "all subdomains (m, www, and so on) and multiple protocols", and it needs DNS verification. If you only verified https://www.example.com, the reports will not show blog.example.com. Add a Domain property, then use a page filter in the Performance report to look at one subdomain at a time.
Sitemaps can cover several hosts, with verification
Google's sitemap guide, updated 8 July 2026, says that unless you submit it through Search Console, "a sitemap affects only descendants of the parent directory". It also describes cross-submission: one sitemap can list URLs for several sites, as long as you have verified ownership of all of them in Search Console, or you reference it from each site's robots.txt. The simple rule is to give each subdomain its own sitemap and list it in that subdomain's robots.txt.
Site names are set per domain or subdomain, never per subfolder
The site names documentation says Google Search "only supports one site name per site, where a site is defined by the domain or subdomain", and that "Google Search does not support site names at the subdirectory level." A subdomain can carry its own site name through WebSite structured data on its own home page. If it has none, the documentation says "the domain-level site name may be used for the subdomain as a fallback." A subfolder always shows the main site's name.
Moving between hosts is a site move
Google's site move guide, updated 20 August 2026, says to submit a Change of Address in Search Console "If you're changing domain names or subdomains". Moving blog.example.com to example.com/blog therefore gets the full site move treatment: a URL map, permanent redirects for every old URL, updated internal links and canonicals, and new sitemaps.
International sites: the one place Google compares the options
Google's guide to managing multi-regional sites sets out the choices side by side.
| Structure | Example | Pros Google lists | Cons Google lists |
|---|---|---|---|
| Country domain | example.de | Clear geotargeting, server location irrelevant, easy separation of sites | Expensive, more infrastructure, strict ccTLD requirements, targets one country only |
| Subdomain | de.example.com | Easy to set up, allows different server locations, easy separation of sites | Users might not recognise geotargeting from the URL alone |
| Subfolder | example.com/de/ | Easy to set up, low maintenance (same host) | Users might not recognise geotargeting from the URL alone, single server location, separation of sites harder |
That table is the clearest statement of the trade off: a subdomain separates things, a subfolder keeps maintenance low because it is the same host. Our guide to SEO for international sites covers hreflang and the rest of a multi-country setup.
How to decide for a blog, a help centre or a store
- Same platform, same team, same audience: use a subfolder. One host, one robots.txt, one property, one site name, and internal links that need no special care.
- A different platform that cannot run under your main host: a hosted help desk, a store on a separate system, a documentation tool. A subdomain is the practical answer, unless your platform supports serving it under a path through a reverse proxy, which your developer can tell you.
- A separate brand, product or country site: a subdomain, or a separate domain, keeps it apart, and the site name can differ.
- A staging or test copy: a subdomain behind a password, so nobody and no crawler reaches it by accident.
Whichever you choose, link the two properly. A blog on a subdomain should link to the product pages on the main site in its articles, and the main site's navigation should link to the blog, so people and crawlers can move between them.
Setup checklist for a subdomain
- Its own robots.txt at the root of the subdomain, with a Sitemap line.
- A Domain property in Search Console, verified by DNS, plus a page filter or URL-prefix property for the subdomain.
- Its own XML sitemap.
- WebSite structured data on its home page if you want a site name of its own.
- Self-referencing canonicals, and no copies of the same pages living on both hosts.
- Analytics that follows people across both hosts, so a visit that starts on the blog and ends in the store is counted as one journey.
Subdomain SEO questions
Does a subdomain get the main domain's authority?
Google does not publish a rule that passes or withholds anything by subdomain. What it documents is that robots.txt, site names and Search Console properties work per host. Treat a subdomain as a site you must link, verify and maintain on its own, and link it well from the main site.
Should my blog be on a subdomain or a subfolder?
If your blog can run on the same platform as the main site, a subfolder is simpler to run, and Google's multi-regional guide lists "low maintenance (same host)" as the subfolder's advantage. Use a subdomain when the platform or the team forces it.
Is www a subdomain?
Technically yes. Google's site names documentation notes that subdomain names starting with www or m "are generally considered as being equivalent" to the main domain for site names. Pick one version, redirect the other to it, and use it in every canonical and sitemap.
How do we move a subdomain into a subfolder?
Treat it as a site move. Map every old URL to its new one, add permanent redirects, update internal links, canonicals and sitemaps, and submit a Change of Address in Search Console as the site move guide says. Keep the redirects in place for the long term.
Do we need a separate robots.txt for every subdomain?
Yes. Google's specification says a robots.txt file on a subdomain is only valid for that subdomain, and the main site's file does not cover it.
When an assistant cites your help centre instead of your home page
The same split shows up when somebody asks ChatGPT, Perplexity or Gemini about your product. The answer may cite a page on help.example.com or docs.example.com rather than the main site, and that page may be older than you think. AI Knows Us is built for this newer check: it asks the questions your buyers ask on the assistants and lists the pages each answer leaned on, so you can see which of your subdomains is speaking for you.