Building a SaaS product involves more than designing screens. Early architectural decisions — multi-tenancy model, authentication, and billing — are expensive to change later, so it's worth getting them right from the start.
A typical build order: define the core workflow and validate it with a narrow MVP, design the data model with tenancy in mind, integrate subscription billing, then layer in admin tooling and integrations as the product matures.
Infrastructure choices — how you handle scaling, background jobs, and monitoring — matter more once you have paying customers than they do during the prototype stage, so it helps to plan for them even if you don't build them all immediately.
Most successful SaaS products launch with a narrower feature set than founders initially imagine, then expand based on real usage data.