How Should I Structure a Reusable Next.js Application Without Overengineering It?

Don’t design around a hypothetical mobile client. Keep your business rules in shared service modules, and make both server actions and route handlers thin adapters around those services. If another client becomes a real requirement, add an API at that point. That keeps the initial application simple without preventing a clean expansion later.

Next.js doesn’t require you to choose one universal architecture. Server actions are convenient for mutations that belong closely to the web application, while route handlers or a separate API are better when external clients need a stable contract. Either way, put authorization and business logic below those entry points so the choice of transport doesn’t duplicate core behavior.

For a product that genuinely targets web and mobile, a shared workspace can work well: separate client and server applications, shared database and validation packages, a reusable API client, authentication utilities, and common UI components. That approach is useful when multiple clients are already part of the plan, but it’s probably more structure than a web-only project needs on day one.

A layered structure like Web → API → Service → Repository → Database isn’t automatically over-engineering. Small, well-defined abstractions can make testing and future changes easier, especially around data access. The important part is avoiding layers that only pass arguments through without adding a useful boundary.