Subdomain vs subdirectory

Long definition

The debate has lived in SEO forums for fifteen years. Google's public position, repeated by John Mueller and others, is that the two are "essentially equivalent" — Googlebot crawls both the same way, and rankings shouldn't differ based on URL structure alone. Practitioner observation, including controlled migrations published as case studies, consistently shows subdirectories outperforming after a content move from subdomain.

The mechanical difference: a subdomain is treated by Google as a related but distinct site. It can pass authority to the parent (and vice versa) but the strength of that flow is variable. A subdirectory is unambiguously part of the same site — internal links, internal anchor text, and crawl signals work without the cross-site discount.

When a subdomain makes sense:

• Functionally separate property. A SaaS platform's customer-facing app at app.example.com versus marketing at example.com. Different stacks, different teams, different intents. The subdomain is honest about the separation.
• Different language/region without a CCTLD. de.example.com paired with hreflang. (Subdirectory /de/ is also valid here; the choice is operational.)
• Different brand or partnership. A co-branded product line that shouldn't share the parent's reputation directly.
• Technical constraint. Some legacy infrastructure can't host new content under a path. Reverse proxies fix most of these, but not all.

When a subdirectory wins:

• Content marketing. Blog, knowledge base, glossary, resources — anything whose purpose is to feed authority and traffic to the main site. /blog consolidates link equity, internal linking, and topical signals into one entity.
• Hosted platforms. Shopify, Wix, and similar were historically subdomain-only and lost ranking lift versus self-hosted subdirectories. Many now offer reverse-proxy setups precisely because of this.
• Migration risk minimization. Moving from a strong subdomain to a subdirectory is a known-recoverable operation; moving in the other direction often drops rankings without a clear path back.

The decision is rarely about ranking signals in isolation — it's about engineering cost, team boundaries, and content strategy. SEO weighs in as: when in doubt, subdirectory.

Common misconceptions

• "Subdomains and subdirectories rank identically." Google says so; observed migrations rarely confirm it. The most reliable case studies show 20-50% organic traffic increases after subdomain-to-subdirectory moves, holding content constant. Treat the official position as a starting point, not the end of the analysis.
• "You can't combine both." You can. Many sites use subdirectories for content (/blog, /help) and subdomains for functional surfaces (app., status.). The decision is per-property, not site-wide.
• "A subdomain inherits the parent's domain authority." Authority transfers, but not 1:1. Treat a new subdomain closer to a fresh site that gets a head start, not a same-site URL.
• "Switching is risky and not worth it." It's a standard URL migration: 301 every old URL to its new path, update sitemap, monitor Search Console. Most sites recover or improve within 30-90 days. The risk is overstated; the case for inaction is usually inertia.