by The Computing Australia Group | Are Subdomains Killing
Your SEO
When to use a subdomain, when to use a subdirectory, and how to protect your rankings
Subdomains are one of the web’s longest-running SEO debates. Some teams swear by them for clean architecture and brand clarity; others blame them for diluted link equity and crawl inefficiencies. The truth is more nuanced: subdomains can help or hurt-it depends on your use case, your execution, and how you connect the dots (technical, content, and analytics).
This guide modernises the conversation with practical, testable advice. You’ll learn what subdomains actually are, how search engines treat them today, when a subdomain makes strategic sense, when a subdirectory is better, and the precise technical steps to keep your visibility intact either way.
Quick definitions (with a mini URL anatomy)
Take the URL: https://blog.oneexample.com/articles/launch-checklist
- Protocol: https:// - how your browser talks to the server (use HTTPS, full stop).
- Subdomain: blog. - an optional label before the root domain.
- Second-level domain (SLD): oneexample - the name you purchased.
- Top-level domain (TLD): .com - the extension (e.g., .com, .org, .com.au).
- Path: /articles/launch-checklist - the location of a resource.
Subdomain vs subdirectory
- Subdomain: blog.oneexample.com (treated as a separate “site” in many contexts).
- Subdirectory (subfolder): oneexample.com/blog/ (lives under the main site).
How search engines treat subdomains today
Search engines like Google are pretty good at associating brands across subdomains, but they still evaluate content largely on a per-host basis. That means:
- Signals (links, behavioural data, some quality metrics) are not automatically pooled between www.oneexample.com and blog.oneexample.com.
- You’ll often need separate verification in Google Search Console (GSC) for each subdomain (domain properties help with oversight, but reporting is still segmented).
- Crawl management (sitemaps, robots.txt rules, canonicalisation) applies per host.
- Cookies and some session data are not shared across subdomains unless you configure them to do so (e.g., .oneexample.com cookie domain).
Bottom line: Subdomains aren’t “bad.” They’re simply separate hosts. If you split your content across hosts, be ready to build and maintain SEO signals for each, and implement strong internal linking and canonical hygiene to show the relationships clearly.
What subdomains are good for (legitimate, defensible use cases)
- Example: app.oneexample.com (SPA or SaaS) vs marketing site on www.
- Benefits: independent deployments, security boundaries, uptime isolation.
2. Internationalisation where language/market truly needs siloing
- Example: fr.oneexample.com, de.oneexample.com when teams, legal requirements, and hosting locations differ.
- Must-haves: robust hreflang across all alternates; consistent architecture.
3. Support portals, docs, or developer hubs
- Example: support.oneexample.com, docs.oneexample.com.
- These often rely on separate platforms (e.g., Zendesk, ReadMe).
4. Status pages and security-sensitive zones
- Example: status.oneexample.com, auth.oneexample.com.
- Clear separation protects reliability and simplifies incident comms.
5. Temporary microsites for large campaigns with heavy traffic
- Example: event2025.oneexample.com.
- Can be tuned and scaled separately, then archived cleanly.
If your scenario matches one of the above and you resource the subdomain like a first-class site, it can be a strong choice.
When subdirectories usually win (and why)
For most content marketing, SEO landings, and resource hubs, subdirectories are simpler and more effective:
- Consolidated authority: Links earned by /blog/ reinforce the root domain.
- Simpler analytics & attribution: One primary property/stream, cleaner funnels.
- Easier internal linking & discovery: Breadcrumbs, related posts, and taxonomy share signals more naturally within one host.
- Crawl efficiency: One robots.txt, one sitemap set, one server performance profile.
Rule of thumb: If the content’s primary job is to rank and assist conversions for the main site, keep it on the main host in subdirectories unless there’s a strong operational reason not to.
Pros & cons—applied to SEO outcomes
Subdomains: Pros
- Strong isolation for security, performance, and engineering velocity.
- Independent branding and UI/UX (helpful for docs, support, app).
- Can host closer to regional audiences (latency wins).
- Easy vendor integrations that prefer/require a separate host.
Subdomains: Cons
- Signal fragmentation: Links and behavioural signals split across hosts.
- Double the ops: Separate sitemaps, robots, canonicalisation, and GSC management.
- Internal link dampening: Cross-host links can be weaker than same-host contextual links (practically speaking).
- Tracking gaps: More work to align cookies, consent, cross-domain measurement.
Subdirectories: Pros
- Faster to build authority (signals accrue to one host).
- Simpler governance: one code path, one analytics setup, one cache strategy.
- Lower maintenance overhead (especially for small teams).
- Clearer topical consolidation for E-E-A-T.
Subdirectories: Cons
- Less isolation for incidents and performance.
- Platform constraints might make /blog/ hard if your app and site are tightly coupled.
- Some third-party platforms are easier on a subdomain.
“Are subdomains killing your SEO?” — the honest answer
They can, if you:
- Move your blog/docs/store to a subdomain and don’t rebuild internal links, sitemaps, and authority from the root.
- Treat the subdomain as an afterthought (thin content, weak topical clustering).
- Introduce duplicate content (same articles on www and blog) without correct canonicalisation.
- Ship a subdomain that’s slow, poorly cached, or on a weaker infrastructure.
They won’t, if you:
- Have a justified use case and resource the subdomain with real content and link acquisition.
- Implement tight internal linking from the main site (header/footer + contextual).
- Maintain consistent branding, navigation, and taxonomy so users (and crawlers) understand it’s a unified experience.
- Nail the technical SEO plumbing (see next sections).
Technical checklist for subdomains (to avoid ranking leakage)
- Configure CNAME/A records correctly; use wildcard SAN or individual TLS certs.
- Enforce HTTPS + HSTS (include preload if you know what you’re doing).
2. Performance
- Same performance bar as your root site: HTTP/2 or HTTP/3, Brotli compression, TLS 1.3, fast TTFB, CDN edge caching, image optimisation (WebP/AVIF).
- Monitor Core Web Vitals (LCP/INP/CLS) per subdomain, not just at root.
3. Indexation & discovery
- Dedicated XML sitemaps for each subdomain; submit in its own GSC property.
- Reference sitemaps in each subdomain’s robots.txt (e.g., Sitemap: https://blog.oneexample.com/sitemap.xml).
- Avoid noindex misconfigurations when cloning environments.
4. Canonicalisation
- Use self-referencing canonicals on unique pages.
- If content is intentionally duplicated across hosts, set canonical to the preferred URL.
- Align hreflang across subdomains for language/market variants (point to canonical versions and include return tags).
5. Internal linking
- Link from the root navigation and footer to key subdomain sections.
- Add contextual links from related articles/pages on root → subdomain (and back).
- Maintain consistent anchor text that reflects search intent.
6. Analytics & consent
- Implement cross-domain measurement (e.g., GA4 with cross-domain linking; ensure cookie domain is .oneexample.com where appropriate).
- Sync consent management so tracking behaviour is consistent across hosts.
7. Brand & UX
- Keep headers, footers, and design tokens aligned to signal unity.
- Use breadcrumb and schema patterns that mirror the main site’s IA.
8. Security & governance
- Apply the same WAF/DDoS rules, patching cadence, and vulnerability scanning.
- Use least-privilege access and audit for every host.
Migration playbooks
1. Moving content from subdomain → subdirectory (often a win for SEO)
- Inventory & map URLs (blog, docs, categories, tags).
- Stand up /blog/ on the root with identical or improved structure.
- 301 redirect every old URL to its new /blog/... equivalent (page-level, not just host-level).
- Update canonicals, hreflang, and internal links across both hosts.
- Refresh sitemaps; submit Change of Address is not applicable (same domain).
- Monitor crawl stats, coverage, and Core Web Vitals; fix any spikes in 404/5xx.
2. Moving content from subdomain → subdirectory (often a win for SEO)
- Mirror IA and slug patterns to minimise disruption.
- Create page-level 301s from /blog/... → blog. equivalents.
- Update hard-coded links in nav, footers, and content.
- Re-submit sitemaps for both hosts; ensure robots.txt doesn’t block the new host.
- Put real effort into internal links from www to blog (category hubs, “related reading” components, homepage modules).
- Track rankings/traffic by section and expect a temporary volatility window.
Content strategy: how to keep topical authority across hosts
- Clusters and hubs: Build topic clusters (pillar pages + supportive articles) consistently. If clusters live on a subdomain, ensure cluster hubs receive links from the root.
- Avoid split topics: Don’t scatter a single topical cluster across hosts unless there’s a strong UX reason.
- E-E-A-T consistency: Author pages, editorial guidelines, and entity markup should look and feel the same on both hosts.
- Schema markup: Use Article, FAQPage, BreadcrumbList, and Organization consistently across hosts to reinforce brand cohesion.
Measurement: know if your structure is working
- Google Search Console
- Add a Domain property for helicopter view and URL-prefix properties for each subdomain to get granular reports (Coverage, Enhancements, Page Experience).
- GA4
- Enable cross-domain measurement (linking), align streams and filters, test attribution.
- RUM + Synthetic
- Real User Monitoring for field Web Vitals per host; synthetic checks for baseline uptime/TTFB.
- Link intelligence
- Track how many unique referring domains point to each subdomain; build where weak.
- Goal conversion paths
- Ensure multi-host journeys (e.g., blog → product → signup) aren’t broken by cookie domain or redirect quirks.
Common pitfalls to avoid
-
“We’ll put the blog on blogspot/medium/hosted platform.”
You’re building authority for another domain. If you must, use a custom subdomain and map DNS. -
Robots.txt mismatches
One permissive, one overly restrictive-accidental deindexing of entire sections is common during launches. -
Mixed canonical signals
Cross-host duplicates with conflicting canonicals confuse crawlers. Choose one canonical per cluster. -
Thin, orphaned subdomains
Five articles on blog. with no internal links from www will struggle to index and rank. -
Inconsistent consent/analytics
Breaks attribution and re-marketing; can also cause compliance issues.
Decision framework (5 questions)
5. Can we execute the plumbing flawlessly (links, canonicals, sitemaps, analytics)?
30-day subdomain optimisation plan
Week 1 - Audit
- Inventory all hosts, sitemaps, robots, canonicals, hreflang.
- Benchmark CWV (LCP/INP/CLS), TTFB, and uptime per host.
- Crawl each host; identify orphaned content and duplicate clusters.
Week 2 - Architecture & linking
- Decide on consolidation vs continued split.
- If keeping subdomains: implement strong cross-host nav/footer and contextual links.
- Standardise schema and author profiles.
Week 3 -Technical hardening
- Enable HTTP/3, Brotli, image optimisation, edge caching.
- Fix canonicals, hreflang return tags, sitemap coverage gaps.
- Align analytics and consent; test multi-host conversions.
Week 4 - Content & monitoring
- Strengthen topic clusters on weaker host(s); publish/refresh cornerstone pieces.
- Build or reclaim links to the subdomain if authority is thin.
- Set alerts for 5xx errors, CWV regressions, and indexing anomalies.
What about mobile subdomains (m.)?
Legacy patterns like m.oneexample.com are largely obsolete with responsive design. Unless you’re maintaining a truly separate mobile experience (rare and complex), converge to a single responsive host and use device-appropriate breakpoints.
Jargon buster (expanded)
- Canonical URL: The version of a page you tell search engines to treat as primary to avoid duplication issues.
- Hreflang: Tagging system indicating language/region variants and their relationships.
- Crawl budget: Practical limit of pages a search engine will fetch over a timeframe; impacted by site health and importance.
- Core Web Vitals (CWV): Field metrics-LCP (loading), INP (interactivity), CLS (visual stability).
- RUM (Real User Monitoring): Collects live performance data from actual users.
- WAF: Web Application Firewall-filters malicious requests before they hit your app.
The pragmatic verdict
Subdomains aren’t inherently good or bad for SEO. They’re a strategic tool. If you need isolation (app, docs, region) and you’ll invest in that host’s authority and UX, go for it. If your goal is to maximise organic visibility for marketing content, keep it under the main domain in subdirectories and enjoy simpler ops, stronger signal consolidation, and cleaner analytics.
Jargon Buster
URL – Uniform Resource Locator or the website address is the address of a given unique resource or page on the web.
Protocol – The protocol determines how your web browser should communicate with the web server when fetching the page.
FAQ
Do subdomains automatically hurt rankings?
No. They’re just separate hosts. Many brands succeed with subdomains-when properly resourced.
Should my blog live on a subdomain or subdirectory?
Will moving my blog to /blog/ improve SEO?
Often, yes-because signals consolidate. But results depend on your redirects, internal links, and content quality.
Can I use subdomains for languages (fr., de.)?
Do I need separate sitemaps & GSC for each subdomain?
Yes-submit sitemaps and verify each host to access full reporting.
