Building a SaaS application should not start with software or architecture, but rather with a monolith that can be evolved over time. A better approach is to begin with a monolithic structure and then focus on the essential building blocks of SaaS applications, including multi-tenancy, tenant isolation, and audit logs. Multi-tenancy is critical in SaaS as it allows multiple customers (or tenants) to operate within a single installation of software. Tenant isolation provides security by allowing code to be written in a way that avoids accidental disclosure of information from other tenants. Scoping operations down to the current tenant is also crucial to prevent security problems and protect tenant data. Audit logs are vital for uncovering what dangerous operations were executed on a customer's tenant, especially when dealing with issues like deleted data. By focusing on these building blocks, developers can build a solid foundation for their SaaS application.