What Full Site Editing actually is
Full Site Editing arrived with WordPress 5.9 in early 2022 and extended the block editor beyond post content to cover the entire page structure. Instead of a developer writing header.php and footer.php in PHP, a block theme defines those as HTML files built from blocks, located in a /templates and /parts folder, editable live in the Site Editor under Appearance → Editor.
Underpinning FSE is theme.json, a structured JSON file that declares available colors, font sizes, spacing scales, and layout constraints. Rather than writing custom CSS for every option, a theme author defines these values once, and WordPress generates the corresponding CSS custom properties and enforces the same options across the block editor UI.
Traditional theme workflow
In a classic theme, structural changes require editing PHP files directly, or relying on a page builder plugin like Elementor to compensate for what the theme cannot do visually. The Customizer, accessed through Appearance → Customize, exposes only what the theme developer specifically coded as an option — colors, a logo upload, maybe a hero layout toggle — and nothing more.
This makes classic themes predictable and stable: once built, they rarely surprise you, and the ecosystem of classic-theme plugins and page builders is enormous and battle-tested after nearly two decades of use.
Site Editor workflow
With a block theme, editing the header, footer, or single-post template happens directly in the browser through the Site Editor, with changes previewed live and saved as template revisions. You can create template parts, reuse them across templates, and adjust global styles — like heading font or overall spacing — from one settings panel that applies sitewide.
This is a genuinely different editing model: less code, more direct manipulation, and version-controllable templates stored partly in the theme and partly in the database once you start customizing them, which is a nuance worth understanding since it complicates moving a site between environments.
Trade-offs to know before switching
Full Site Editing is still evolving with each WordPress release, so some advanced layouts that are trivial in a mature page builder can require more block-pattern fiddling in FSE. Plugin compatibility is also uneven: many older plugins assume classic theme hooks like wp_head() and wp_footer() and were not built with block templates in mind, though this gap has narrowed significantly since 2022.
On the other hand, FSE sites tend to load less third-party JavaScript because styling comes from theme.json rather than a bundled builder runtime, and editing does not require touching PHP at all, which lowers the barrier for non-developers maintaining the site long-term.
- Classic theme: mature ecosystem, PHP-based, Customizer-limited options
- Block theme + FSE: visual template editing, theme.json driven styling, newer and evolving
- Both: can be extended with the same plugin ecosystem for functionality
How to tell which one you are using
Go to Appearance in your dashboard: if you see "Editor" with a full Site Editor interface, you are on a block theme with FSE support. If you see "Customize" opening a sidebar panel instead, you are on a classic theme.
- Open wp-admin and go to Appearance
- If the menu shows "Editor", your theme supports Full Site Editing
- If the menu shows "Customize", you are on a classic theme
- Check the theme folder for a theme.json file to confirm block-theme support
Which one should you choose
Choose a block theme with Full Site Editing if you want to control layout yourself without hiring a developer for every template change, and you are comfortable with a platform that is still actively evolving. Choose a classic theme, often paired with a page builder, if your site relies on plugins that expect classic PHP hooks, or if you value the maturity and predictability of a workflow that has not changed structurally in years.
Frequently asked questions
- Does Full Site Editing replace the Customizer?
- For block themes, yes: theme.json and the Site Editor replace most Customizer functions. Classic themes still use the Customizer since they were never built around block templates.
- Can I use a page builder plugin with a block theme?
- Yes, most page builders still work on block themes for individual page content, though some deeper integrations with header and footer editing overlap awkwardly with the Site Editor's own template system.
- Is theme.json required for every theme?
- No, classic themes do not need theme.json at all. It is specific to block themes and to classic themes that opt into partial block-editor support for content areas.
- Will my classic theme plugins work after switching to Full Site Editing?
- Most will, since plugin functionality is generally independent of theme type, but plugins that hook deeply into classic theme templates or the Customizer may need updates or replacements built for block themes.
- Is Full Site Editing only for new WordPress sites?
- No, you can switch an existing WordPress installation to a block theme at any time; existing posts and pages remain intact, though custom classic-theme layouts will need rebuilding as blocks.