Multi-tenancy is one of those architectural decisions that's cheap to get right at the start and expensive to fix later, because it touches almost every layer of a SaaS product — from the database schema to how you bill customers.
Tenant isolation is the first decision to make explicitly: will tenants share a database with row-level separation, or get isolated schemas or databases? Shared infrastructure is cheaper to operate at scale, but a mistake in isolation logic is a data-leak risk between customers — the single most damaging failure mode a SaaS product can have.
Billing model and tenant isolation are more connected than most teams expect. Usage-based billing, seat-based tiers, and enterprise custom contracts each put different demands on how you track and attribute resource consumption per tenant — decide the billing model early, or the metering logic becomes a retrofit.
Infrastructure decisions that are genuinely expensive to change later include your approach to tenant-specific customization, your data residency strategy for regulated customers, and how you handle a single large tenant that outgrows the assumptions your architecture was built on. Plan for that outlier before it arrives, not after.
