WordPress ships with over 90 core blocks. Combined with third-party block libraries, most sites can be built without writing a single line of block code. But there is a category of client projects where the default blocks are not enough — where the content structure, design requirements, or editorial workflow demands something purpose-built.
Custom block development fills that gap. It gives agencies the ability to create editor experiences that match exactly what the client needs, with no extra features and no missing ones. A testimonial block that enforces the brand’s layout. A pricing table that automatically calculates annual versus monthly rates. A location finder that pulls data from an API and renders an interactive map.
This post covers the practical considerations for agencies evaluating custom block development — when to build, when to avoid it, and what the development process looks like.
When Custom Blocks Make Sense
Building custom blocks is a meaningful investment. It requires JavaScript and React knowledge, familiarity with the WordPress block API, and ongoing maintenance as WordPress core evolves. The investment is justified in specific scenarios.
Repeating Content Structures
When clients need to create the same type of content repeatedly — team member profiles, product feature cards, case study summaries, event listings — a custom block ensures consistency. Each instance follows the same structure, enforces required fields, and renders with the same layout. Content editors focus on entering information rather than arranging blocks into the correct pattern each time.
Data-Driven Content
When a block needs to display data from an external source — an API, a custom database table, or a third-party service — core blocks cannot help. A custom block can fetch pricing data from a SaaS API, display real-time inventory from a warehouse management system, or render a chart from a data analytics platform. The block handles the data fetching and rendering while the editor provides configuration options.
Controlled Editorial Experiences
Some clients need guardrails. A marketing team that should not change the layout of a hero section but should be able to update the headline, image, and call-to-action text. A content team that needs to add FAQ entries without worrying about formatting. Custom blocks let you define exactly which fields are editable, what options are available, and how the output renders — removing the possibility of editorial mistakes that break the design.
When to Use Alternatives Instead
Not every content need requires a custom block. Simpler solutions often deliver the same result with less development and maintenance overhead.
Block patterns solve the layout consistency problem without custom code. A pattern is a pre-arranged group of core blocks — a heading, paragraph, image, and button arranged in a specific layout. Editors insert the pattern and replace the placeholder content. If the content need can be met by combining existing blocks in a specific arrangement, a pattern is the better choice.
Block variations let you create modified versions of existing core blocks with preset attributes. A “Team Member” variation of the Media and Text block could pre-configure the layout, add a specific CSS class, and set default placeholder text — without building a new block from scratch.
Block styles add visual variants to existing blocks without modifying their structure. A “Bordered” style for the Group block, a “Highlight” style for the Paragraph block, or a “Card” style for the Column block can be registered with a few lines of PHP and CSS.
The Development Process
Custom block development uses JavaScript (specifically React) for the editor experience and either JavaScript or PHP for the front-end rendering. The WordPress block API provides a structured framework for registering blocks, defining attributes, and building the editor interface.
Setting Up the Development Environment
WordPress provides the @wordpress/create-block package — a scaffolding tool that generates a complete block plugin with the correct file structure, build configuration, and development server. Running a single command creates a ready-to-develop block plugin with hot reloading, linting, and build scripts preconfigured. This eliminates the manual webpack and Babel configuration that was previously required.
Block Registration and Attributes
Every block starts with a block.json file. This file defines the block’s name, title, description, category, icon, supported features, and attributes. Attributes define the data the block stores — text fields, URLs, numbers, booleans, arrays, and objects. Each attribute specifies its type and the source from which the block editor reads and writes it.
The block.json file also declares the editor and front-end scripts, stylesheets, and render template. WordPress uses this manifest to register the block, enqueue assets only when the block is used, and handle server-side rendering when specified.
The Edit Component
The edit function defines how the block appears and behaves in the editor. It is a React component that receives the block’s attributes and a setAttributes function for updating them. WordPress provides a library of built-in components — RichText for editable text, MediaUpload for image selection, InspectorControls for sidebar settings, ColorPalette for color pickers, and dozens more.
The edit component is where the editorial experience is defined. A well-designed edit component makes content entry intuitive. A poorly designed one frustrates editors and leads to support requests. Invest time in the edit experience — it is what the client interacts with daily.
The Save Function and Server-Side Rendering
The save function defines the HTML output stored in the database. For static blocks — blocks whose output does not change based on external data — the save function returns JSX that WordPress converts to HTML and stores in the post content.
For dynamic blocks — blocks whose output depends on external data, database queries, or complex logic — server-side rendering is the better approach. Instead of a save function, a PHP render callback generates the HTML at page load time. This is essential for blocks that display recent posts, API data, or any content that changes independently of the post being edited.
Testing and Maintenance Considerations
Custom blocks require ongoing maintenance that agencies must account for in their retainer pricing and workflow.
WordPress core updates can change the block API, deprecate functions, or modify the editor’s behavior. Each major WordPress release should trigger a compatibility review of custom blocks. The block deprecation system helps manage markup changes — allowing old saved content to render correctly while new instances use updated markup — but it requires proactive management.
Test custom blocks across the editorial workflow. Verify that they work correctly when inserted, edited, saved, and re-opened. Test block transformations, copy-paste behavior, and interaction with other blocks. Test the front-end rendering across browsers and devices. Automated testing with the @wordpress/e2e-test-utils package can streamline this process for blocks used across multiple client sites.
Where a White-Label Partner Fits
Custom block development sits at the intersection of WordPress expertise and modern JavaScript development. Agencies whose teams are primarily PHP developers may find the React-based block API challenging to adopt. A white-label development partner with block development experience can build, test, and document custom blocks while your agency defines the editorial requirements and manages the client relationship. This lets you deliver custom editing experiences without requiring every developer on your team to become proficient in React and the WordPress block API.