Feature-Based Folder Structure: Why It Matters for Scaling SaaS Codebases

A new guide in the Full Stack SaaS Masterclass series argues that grouping code by technical role — using separate controllers, services, and models folders — breaks down once an application grows beyond a handful of features. The author advocates for vertical slicing, where all files related to a single feature, such as invoices or organizations, are kept together in one folder. This approach is demonstrated using a NestJS API structure, where each module encapsulates its own controllers, services, DTOs, and tests, aligning with the framework's own design conventions. A parallel structure is shown for Next.js, where the App Router's filesystem-based routing naturally encourages feature-adjacent organization using route groups. The article emphasizes that establishing this structure early prevents the costly, disruptive rewrites that typically arise when codebases grow without clear boundaries.