Building a B2B SaaS application involves critical architectural choices, primarily between multi-tenant and single-tenant models, each affecting cost, scalability, security, and customization. Multi-tenancy allows multiple customers to share a single application instance and database, promoting cost efficiency and scalability while requiring robust tenant isolation measures to ensure data security. Conversely, single-tenancy provides each customer with their own dedicated resources, offering enhanced security and customization at the expense of higher infrastructure costs and maintenance complexity. The decision hinges on factors such as the target market, compliance requirements, and operational capacity. Although many startups favor multi-tenancy for its scalability and cost benefits, hybrid approaches can provide tenant-specific isolation within a shared environment. Modern tools like Clerk, Supabase, Prisma, and Neon help streamline tenant management and isolation, enabling businesses to focus on core functionalities while maintaining security and efficiency. Ultimately, the choice should align with the company’s goals and customer needs, leveraging the appropriate tools to balance performance, security, and cost-effectiveness.