Technical SEO is the part of search engine optimisation that deals with how a website is built and served, so that search engines can find its pages, read them and add them to their index. It covers the server, the code, the URLs and the structure of the site rather than the words on any one page.
How technical SEO works
Before a page can rank, three things have to happen. Google’s crawler has to discover the URL and fetch it, which is crawling. It then has to process the HTML, run any JavaScript and work out what the page contains, which is rendering. Finally it decides whether the page is worth storing and showing in results, which is indexing. Technical SEO removes whatever gets in the way at each stage.
In practice the work falls into a few groups:
- Access. Robots.txt rules, server status codes, redirects and login walls that decide whether crawlers can reach a page at all.
- Indexing signals. Noindex tags, the canonical tag, XML sitemaps and duplicate URLs that tell Google which version of a page to keep.
- Rendering. Whether important content and links exist in the HTML or only appear after scripts run.
- Performance. Load speed and stability, measured by Google as Core Web Vitals.
- Structure. URL patterns, internal linking, pagination and how deep important pages sit below the homepage.
- Markup. Structured data, hreflang where relevant, and mobile-friendly templates.
Most of this is invisible to a visitor. A page can look perfect in a browser and still be blocked, duplicated forty times through filter parameters, or pointing its canonical at a different page entirely.
Why it matters
Content and links only pay off if the pages they support are in the index. A London law firm can publish an excellent guide to lasting powers of attorney, but if a plugin has quietly set it to noindex, it will never appear. Technical faults rarely announce themselves; traffic just never arrives, or drops after a redesign, and nobody connects it to a single setting.
The stakes are highest when something changes: a new CMS, a move from a .com to a .co.uk domain, a Shopify theme swap, or a developer pushing a staging site live with its “discourage search engines” setting still ticked. When traffic falls sharply soon after a change like this, a technical cause is the first thing worth ruling out.
Common mistakes
- Chasing a tool score. Audit tools flag hundreds of issues. Many are trivial; a few block indexing. Fix by impact, not by count.
- Treating it as a one-off. Plugins update, developers change templates and new sections get added. Faults return unless someone checks.
- Blocking resources in robots.txt. Blocking CSS or JavaScript files stops Google rendering the page as visitors see it.
- Leaving duplicate versions live. HTTP and HTTPS, www and non-www, with and without trailing slashes, all resolving with a 200 status.
- Ignoring Search Console. The Page indexing report shows exactly which URLs Google has excluded and why, and it is free.
How to act on it
Start with Google Search Console. Look at the Page indexing report for important pages listed as “Crawled – currently not indexed”, “Excluded by ‘noindex’ tag” or “Duplicate without user-selected canonical”. Then crawl the site with a tool such as Screaming Frog to find broken links, redirect chains and missing canonicals. Check one key template in the URL Inspection tool to see the rendered HTML Google gets.
Fix what stops indexing first, then duplication, then speed and structure. If the list is long or the causes sit in the server or theme, a technical SEO service will find the faults and write fixes your developer can act on.