WordPress Full Site Editing (FSE) is no longer experimental. It shipped in WordPress 5.9, matured through 6.0 to 6.5, and is now the default approach for new theme development. Yet many agencies are still building with classic themes and page builders, unsure whether FSE is ready for client work or still a work in progress.
The short answer: FSE is production-ready for most agency use cases. But adopting it requires understanding what has changed, what it replaces, and where it still has limitations. This post covers the practical reality of FSE for agencies building WordPress sites in 2026.
What Full Site Editing Actually Changes
In classic WordPress, themes are built with PHP template files. Header, footer, sidebar, and page layouts are coded in PHP, styled with CSS, and modified through the Customizer or custom theme options. Content authors work within the Gutenberg block editor for post and page content, but the surrounding site structure — navigation, headers, footers, archive layouts — is controlled by developers through code.
Full Site Editing changes this model. The entire site — not just post content — is built with blocks. Headers, footers, navigation menus, template structures, and archive layouts are all composed in the block editor. Theme templates are defined as HTML files containing block markup rather than PHP files containing function calls.
This means a block theme is essentially a collection of HTML templates, a theme.json configuration file, and optional CSS. There is no functions.php with dozens of hooks and filters. There are no PHP template tags. The theme.json file controls typography, colors, spacing, layout widths, and block-level design settings — all in one centralized configuration.
Why This Matters for Agencies
The shift to FSE has three practical implications for how agencies build and deliver WordPress sites.
Faster Development Cycles
Building a classic theme from scratch typically takes weeks. Building a block theme takes less time because much of the layout work happens in the editor rather than in code. Developers define the design system in theme.json — fonts, color palette, spacing scale, default block styles — and then compose templates visually using blocks. For agencies handling high volumes of WordPress projects, this time savings compounds quickly. A site that took three weeks to develop as a classic theme can often be delivered in two weeks as a block theme, assuming the team is proficient with the FSE workflow.
Client Self-Service Capability
The biggest operational benefit of FSE is that clients can modify site-wide elements without developer involvement. In a classic theme, changing the header layout, updating the footer, or modifying the blog archive design requires a developer to edit PHP template files. In a block theme, these changes happen in the Site Editor — the same visual interface clients use to edit page content.
This reduces the volume of small change requests that agencies handle post-launch. Clients who previously needed support tickets for minor layout adjustments can now make those changes themselves. For agencies on maintenance retainers, this frees up development time for higher-value work.
Reduced Plugin Dependency
Many agencies currently rely on page builders like Elementor, Beaver Builder, or WPBakery for layout flexibility. FSE provides much of that flexibility natively. The block editor supports columns, grids, groups, cover blocks, query loops, and template parts — the building blocks of most marketing websites. For standard marketing sites, brochure sites, and content-driven projects, FSE eliminates the need for a third-party page builder. This reduces plugin licensing costs, removes a potential performance bottleneck, and simplifies the technology stack.
Understanding theme.json
The theme.json file is the foundation of every block theme. It replaces what used to require hundreds of lines of PHP in functions.php and scattered CSS across multiple stylesheets. A single JSON file now controls the entire design system.
Typography settings define font families, sizes, and fluid typography scales. Color settings establish the palette available to content editors. Spacing settings control padding and margin presets. Layout settings define content width and wide alignment dimensions. Block-level settings allow you to configure defaults for individual blocks — disabling certain features, setting default styles, or restricting color options to maintain brand consistency.
For agencies, theme.json is a governance tool. It gives developers precise control over what content editors can and cannot change. You can lock down the color palette so editors cannot introduce off-brand colors. You can restrict font choices to the approved brand fonts. You can set spacing presets that maintain consistent vertical rhythm. This level of design system enforcement was difficult to achieve with classic themes without extensive custom development.
Block Patterns and Reusable Components
Block patterns are pre-designed block compositions that editors can insert with a single click. A testimonial section, a pricing table, a team grid, a call-to-action banner — each of these can be saved as a pattern and made available in the editor’s pattern library.
For agencies, patterns serve as a design system delivery mechanism. Rather than handing clients a style guide and hoping they follow it, you deliver a library of patterns that enforce correct layout, typography, and spacing. The client picks the pattern, drops it into the page, and replaces the placeholder content. The design stays consistent because the structure is baked into the pattern.
Synced patterns (formerly called reusable blocks) take this further. A synced pattern updates everywhere it appears when edited in one location. This is ideal for elements like newsletter signup forms, promotional banners, or legal disclaimers that need to be consistent across the entire site.
Where FSE Still Has Limitations
Full Site Editing is not a universal replacement for every WordPress development approach. Several scenarios still benefit from alternative methods.
Complex Custom Functionality
FSE excels at layout and design. It does not replace the need for custom PHP development when projects require complex business logic, custom database queries, API integrations, or advanced form processing. A membership site with tiered access levels, a real estate portal with map-based property search, or a SaaS application dashboard still requires traditional WordPress development alongside the block editor. FSE handles the presentation layer, but custom functionality lives in plugins and custom code.
Legacy Theme Migration
Converting an existing classic theme to a block theme is not a simple find-and-replace operation. PHP template logic needs to be reconceived as block markup. Custom theme options need to be mapped to theme.json settings or block attributes. Shortcodes and custom widgets need block equivalents. For clients with heavily customized classic themes, a gradual migration — using a hybrid theme approach that supports both classic templates and block templates — is often more practical than a complete rebuild.
Advanced Design Requirements
The block editor’s layout capabilities have improved significantly, but they do not match the pixel-level control of CSS Grid or advanced CSS techniques that a skilled front-end developer can implement. Complex animations, intricate grid layouts with overlapping elements, and highly custom responsive behaviors still require custom CSS or JavaScript. For most marketing sites, the block editor’s layout tools are sufficient. For design-forward agencies where every pixel matters, a hybrid approach — block theme structure with custom CSS enhancements — delivers the best results.
How to Start With FSE in Your Agency
Agencies that have not yet adopted FSE do not need to overhaul their entire workflow overnight. A phased approach reduces risk and builds team confidence.
Start with a small project. Choose a brochure site or a simple content site — a project where the layout requirements are straightforward and the client is open to a modern editing experience. Build it as a block theme from scratch. Use theme.json for design tokens. Create a library of block patterns for common sections. Let the team learn the workflow in a low-stakes environment.
Invest in theme.json proficiency. Understanding how to configure typography scales, color palettes, spacing presets, and block-level defaults in theme.json is the single most valuable skill for FSE development. It replaces what used to require scattered CSS and PHP hooks with a single, well-structured configuration file.
Build a pattern library. As your team completes FSE projects, save reusable block patterns — hero sections, feature grids, testimonial carousels, pricing tables, FAQ accordions. Over time, this library becomes a competitive advantage. New projects launch faster because you are composing pages from proven patterns rather than building from scratch.
Document the editorial workflow for clients. FSE gives clients more control, but that control needs guardrails. Create a brief onboarding guide showing clients how to use the Site Editor, where to find patterns, and what elements they should and should not modify. This investment in documentation reduces support requests and improves client satisfaction.
The Competitive Advantage for Agencies
Agencies that adopt FSE early gain several advantages. Development is faster because layout work happens in the editor rather than in code. Client handoffs are cleaner because the editing experience is more intuitive. Maintenance costs decrease because clients can handle more changes independently. And the sites themselves perform better because block themes generate cleaner, leaner markup than page-builder-dependent alternatives.
The WordPress ecosystem is moving decisively toward full site editing. Classic themes are not deprecated, but new features, performance improvements, and community energy are concentrated in the block editor and block theme ecosystem. Agencies that delay adoption will eventually face a larger migration effort and a steeper learning curve.
Where a White-Label Partner Fits
Transitioning from classic theme development to FSE requires both technical skill and production experience. The learning curve is real — theme.json configuration, block template structure, pattern creation, and the Site Editor workflow all require hands-on practice. A white-label development partner that has already delivered FSE projects can accelerate your agency’s transition. They handle the technical execution while your team learns the workflow, reducing the risk of delivery delays on client projects during the learning period.