Tenancy, billing and permissions are architectural decisions, not features to add later. Retrofitting them is where SaaS builds go badly wrong, and it is almost always more expensive than settling them at the start.
The three decisions
How tenants are isolated, how usage becomes revenue, and who can see what. Almost everything else in a SaaS product can be changed later. These three cannot be changed cheaply.
Shared schema, schema-per-tenant or database-per-tenant is not a preference. It follows from your isolation requirements, the size distribution of your customers, and what your largest prospect's security review is going to ask for.
We make that call explicitly and write down what it costs, because migrating between them afterwards is a project of its own — usually one that has to happen under load.
Plans, seats, usage, trials, mid-cycle upgrades, proration, failed payments, dunning. Billing looks like a checkout page and is really a state machine that has to stay in agreement with your ledger.
We build it as a state machine and we make it reconcilable, so the invoice, the entitlement and the access a customer has all derive from the same source rather than from three that drift.
The tools your own team needs — impersonation, plan changes, refunds, usage inspection, feature flags — decide whether support scales or consumes engineering.
They go into the first release rather than the fourth, because a product without them generates a support queue that only engineers can serve, and that cost compounds with every customer.
Tenancy, billing and permissions are architectural decisions, not features to add later — retrofitting them is where SaaS builds go badly wrong. We settle them before the first screen is designed.
Three failures we have seen often enough to design against by default.
Entitlements end up derived from three places that disagree, and the first refund is what exposes it.
You cannot price on usage you never recorded. Metering goes in before anybody needs the numbers.
It works until the second customer, and the fix is a data migration with production traffic on top of it.
Tell us the shape of the product and who buys it. The tenancy and billing answers follow from that, and they are worth getting right first.