1. What is API development?
API stands for Application Programming Interface. In practice, an API is a set of rules and protocols that allows one piece of software to talk to another. When you sign into a website using your Google account, Google's API is handling that authentication. When a payment page charges your card through Stripe, Stripe's API is doing the actual work. When a travel booking site shows live flight prices from multiple airlines at the same time, it is making simultaneous API calls to each airline's system.
WebAnts is a London web development agency building custom APIs, REST endpoints, GraphQL schemas, and third party integrations for startups, SMEs, and enterprise teams. We have built APIs for clients across the UK, US, and Europe, connecting payment systems, CRMs, booking platforms, AI services, and bespoke internal tools. This guide is for business owners and product managers who want to understand what API development actually involves before they start a project — and what it should realistically cost.
Already know you need an API built?
This is an educational guide explaining what API development involves. If you have a project in mind and want a quote, visit our API Development service page for scope examples, platform details, and how to get a fixed price.
API development is the work of designing, building, documenting, and maintaining these communication layers. Custom API development is when you build your own API rather than simply connecting to one that already exists. This becomes necessary when you have proprietary data, custom business logic, or specific integration requirements that off-the-shelf connectors cannot handle.
The distinction between these scenarios matters practically. "API development" can mean several different things depending on context: building a new API that other systems or apps call, integrating a third party API into your existing product, building a middleware layer that connects two or more external services, or some combination of all three. The cost, timeline, and technical approach are different for each. A developer who does not establish which scenario applies before quoting is not someone you want building your infrastructure.
2. When does a business actually need a custom API?
The honest answer is: less often than some agencies will tell you. There are situations where a third party integration tool — Zapier, Make (formerly Integromat), a native CRM connector — will do exactly what you need without any custom development. Before committing to a custom API build, it is worth asking honestly whether a ready-made solution already exists that meets your requirements.
That said, a custom API is the right decision in the following situations:
Situation 1
You have proprietary data or internal systems that third party tools cannot access
Your ERP system, your inventory database, your internal booking platform — if it does not have an existing API or an integration plugin, a custom API is often the only way to connect it to other systems. This is extremely common in businesses that have grown on legacy software.
Situation 2
Your business logic is too complex for no-code middleware
If you are manually maintaining a Zapier workflow because the logic requires conditional branching and data transformations that Zapier handles awkwardly, you have probably outgrown the no-code approach. A custom API layer gives you full control over the logic, and it tends to be more reliable and cheaper to run at scale than per-operation middleware pricing.
Situation 3
You need to expose your platform to third party developers or partners
If you want other businesses or developers to build integrations on top of your product — marketplace sellers, affiliate partners, enterprise clients who need custom data feeds — you need a documented, versioned, secure public or partner API. This is fundamentally a different build to an internal integration.
Situation 4
You are building a mobile app or a single page web application
Nearly all modern mobile apps and React-based frontends communicate with a backend via API. If you are building a SaaS product, a mobile app, or a React dashboard, you are almost certainly building an API as part of that project. The question is whether it is designed properly from the start.
3. REST, GraphQL, and webhooks explained simply
Three types of API architecture come up in almost every brief. Here is what each one means and when it is the right choice for your project.
REST — Representational State Transfer
REST is the most common API architecture and the default choice for the majority of web and mobile integrations. A REST API organises data around resources — users, orders, products, bookings — and uses standard HTTP methods: GET to retrieve data, POST to create it, PUT or PATCH to update it, DELETE to remove it. REST is well understood, widely supported, and easy to test and document.
For most business integrations and the majority of web application backends, a well-designed REST API is the right answer. It is predictable, maintainable, and every developer on the market knows how to work with it.
Learn more about REST API design principles at restfulapi.net →
GraphQL
GraphQL is an alternative API query language developed by Meta. Unlike REST, where the API determines what data gets returned, GraphQL lets the client specify exactly which fields it needs in a single query. This eliminates over-fetching (receiving more data than you need) and under-fetching (needing multiple requests to assemble a complete view of the data).
GraphQL is particularly well suited to complex frontend applications with many different data views — a React dashboard that displays different combinations of data depending on user role, for example, or a mobile app that needs to minimise data transfer on slower connections. It is more complex to implement and test than REST, and for simple CRUD operations, the overhead is rarely worth it.
Webhooks
Webhooks are a different concept entirely. Rather than a client asking an API for data — a pull model — webhooks push data to a URL you specify when an event occurs. When a payment completes on Stripe, Stripe sends a webhook to your server with the payment details. When a form is submitted in Typeform, Typeform sends a webhook to your CRM. Webhooks are event-driven and relatively simple to implement, but they require a publicly accessible endpoint and careful error handling for cases where the receiving server is unavailable or slow.
Webhooks are not a replacement for a full API — they are a specific tool for event-driven data flows. Most mature integrations combine a REST or GraphQL API for querying data with webhooks for real-time event notifications.
The vast majority of London business API projects use REST with webhooks for specific event-driven requirements. GraphQL becomes relevant when the frontend complexity genuinely justifies it — which is less often than the technology press suggests.
4. Building your own API vs integrating third party services
This is probably the most consequential decision in any API project, and it gets confused more than almost anything else in our discovery calls with London businesses. The confusion is understandable — the terminology overlaps — but the implications are significant.
Third party API integration means consuming an API that already exists — calling Stripe's payment API, pulling contact data from HubSpot, reading inventory levels from Shopify, pulling flight prices from an airline's system. This is typically faster, cheaper, and lower risk than building from scratch. You are relying on the third party to maintain the API, handle security, and manage uptime. The trade-off is that you are also dependent on their pricing, their rate limits, and their decision about whether to deprecate a feature.
Building your own API means creating a new interface that other systems will call — your own frontend, a mobile app, partner tools, or third party developers. This is more complex, requires upfront design work, and carries ongoing maintenance responsibility. But it gives you full control over your data model, your business logic, the interface you expose, and the pricing model you can charge for API access.
The decision tree
The key mistake we see is agencies recommending a fully custom API build when an integration layer and a few third party API connections would achieve the same outcome in a fraction of the time and cost. The reverse also happens: businesses trying to stretch Zapier workflows into complex logic flows that cry out for a properly designed integration layer. Neither is good for the client.
5. API security: what you cannot afford to overlook
APIs are the most targeted attack surface in modern web infrastructure. The OWASP API Security Top 10 documents the most common vulnerabilities, and the list makes uncomfortable reading for any business running a production API without a formal security review. These are not theoretical risks — they are the vulnerabilities that get exploited in real incidents against real businesses.
Broken object level authorisation
An authenticated user who can access their own data but can also access another user's data simply by changing an ID parameter in the URL. This is the most common API vulnerability and one of the easiest to exploit. It is also one of the most damaging for GDPR compliance.
Excessive data exposure
An API that returns a complete user object — including email, phone, address, and payment method — when the client only asked for the display name and profile picture. Everything extra that gets returned is a potential data leak and a GDPR liability.
Mass assignment
An API that blindly accepts all parameters from a POST request, allowing an attacker to set fields — admin status, account balance, subscription tier — that they should have no ability to modify. A subtle vulnerability that requires deliberate allowlisting of acceptable fields on every endpoint.
No rate limiting
An API with no request rate limits can be targeted by brute force credential attacks or used as a vector for data scraping. Rate limiting is not optional for any production API — it is a baseline security control.
Weak authentication
Expired tokens that are not properly invalidated, JWTs signed with weak or shared keys, OAuth flows with insecure redirect handling. Authentication is where junior developers most often introduce vulnerabilities, and it is where the most severe breaches originate.
Proper API security requires threat modelling during the design phase — not a security checklist bolted on after the build is complete. At WebAnts, authentication, authorisation, rate limiting, input validation, and response sanitisation are built into every API we ship, not added as an afterthought during QA.
6. What API development costs in London (2026 guide)
This is the question most businesses are searching for, so here are realistic figures for 2026. These are London market rates for competent senior developers with a track record in production API work — not offshore rates, not junior rates, and not the kind of estimates you get from agencies that have not properly scoped your project.
Simple third party integration
Connecting your website or web app to a single well-documented third party service — a payment gateway, CRM, email platform, or booking system. Low complexity, well-trodden ground.
£1,500 to £5,000
Medium complexity custom REST API
A custom API with several endpoints, JWT or OAuth authentication, database integration, basic role-based access, automated tests, and API documentation. Typical for startup MVPs and internal tooling.
£5,000 to £15,000
Complex custom API or microservices
GraphQL schema or microservices architecture, multiple integration points, granular role-based access control, comprehensive test coverage, full OpenAPI documentation, and a versioning strategy. Typical for established SaaS platforms and enterprise integrations.
£15,000 to £40,000+
Ongoing API maintenance retainer
Monitoring, updates, performance optimisation, security patching, and incremental feature development. Pricing depends on API complexity and update frequency.
£500 to £2,500/mo
A word on cheap development. API development is an area where cutting costs almost always creates technical debt you pay for later — sometimes much later. An API with poor documentation, inadequate testing, and no versioning strategy is expensive to maintain and risky to extend. The initial saving on the build is typically outweighed by the rework costs within 12 to 18 months.
WebAnts provides fixed project quotes for all API development work after a scoping session. We do not give estimates on calls before we understand the full technical scope — because estimates without scope are not estimates, they are guesses. See our API development service page for more detail on how we scope and price API projects.
7. How to brief an API developer properly
A well-constructed brief does two things. It reduces the scope for miscommunication during the build. And it makes the resulting quote far more accurate and trustworthy than one produced after a 30-minute call with no written brief. Here is what to include.
What systems need to connect, and what data needs to flow between them
Be specific. Not "our CRM needs to talk to our billing system" but "HubSpot Deals need to trigger subscription creation in Stripe when a deal moves to Closed Won, and Stripe subscription events need to update the deal status back in HubSpot." The more specific the data flow, the more accurate the quote.
Who will consume the API
Is it your own frontend team building a React app? Third party developers building integrations? A mobile app? Each audience has different implications for documentation requirements, authentication approach, and versioning strategy.
Expected transaction volume
How many API calls per hour or per day? What are the peak demand patterns? This affects architecture choices, rate limiting thresholds, and hosting infrastructure requirements.
Security and compliance requirements
Are you handling personal data subject to GDPR? Payment card data subject to PCI-DSS? Healthcare data? Each adds specific requirements to authentication, encryption, logging, and data retention — and each adds cost and time to the build.
Existing systems and constraints
What technology stack are you already using? Are there existing APIs that need to be integrated? Are there legacy systems with access restrictions or data quality issues that might constrain the design?
Timeframe and budget range
Both constraints matter. "We need this before our product launch on 1 September" and "we have £8,000 to spend" are information a developer needs to give you honest advice. Developers who never ask about budget are usually hiding something.
8. Choosing an API development partner in London
The London development market is deep. There are hundreds of agencies and freelancers who will tell you they can build your API. Here is how to distinguish the ones who actually can.
Look at API-specific portfolio work
Generic web development experience is not the same as API architecture experience. Ask to see examples of APIs they have built in production. Ask about the technology choices they made and why: REST rather than GraphQL for this project, OAuth rather than JWT for that one. Good developers have considered opinions and can explain their reasoning.
Ask about documentation practice
An API without documentation is only useful to the person who built it. Ask how they document their APIs. The answer should reference OpenAPI (Swagger) specifications, developer-facing reference documentation, and a changelog for versioned APIs. If the answer is "we write inline comments," that is a concern.
Ask about versioning strategy
APIs consumed by third parties cannot simply be changed without breaking their integrations. How do they handle API versioning? How do they communicate breaking changes? If the developer does not have a clear, considered answer to this, they have probably not built many production APIs that outlived their first version.
Ask about testing approach
Unit tests, integration tests, and load tests are all part of professional API development. Ask what test coverage they target and how they handle regression testing when the API evolves. An API with no test suite is a liability from the moment it launches.
Ask about post-launch support
APIs that fail at 2am on a Sunday are not a hypothetical. What is their incident response process? What does ongoing support cost? Is there a monitoring setup included? These questions separate developers who have run production systems from those who have only ever handed over a build and walked away.
WebAnts builds APIs as part of our custom web application service, typically using Node.js or Laravel on the backend with PostgreSQL or Supabase for data. Every API we ship includes OpenAPI documentation, a test suite, rate limiting, and a defined post-launch support arrangement. If you are scoping an API project in London, we are happy to give you an honest assessment of whether a custom build is actually the right approach for your situation — and a flat written quote if it is.
Useful resources
The following resources are worth bookmarking if you are researching API development seriously:
- OWASP API Security Top 10 — the definitive reference for API security vulnerabilities and how to prevent them.
- OpenAPI Specification (Swagger) — the industry standard for describing REST API interfaces.
- GraphQL official documentation — if you are evaluating whether GraphQL is the right choice for your project.
- Also see our related posts: What is AIEO and why it matters in 2026 and WordPress vs Webflow vs Framer.