Build vs. buy: a framework, not a rule

“Should we build this or buy it” comes up constantly when architecting a SaaS platform — authentication, payments, email delivery, search, analytics. There’s no universal answer, but there is a consistent way to think it through.

The question that actually matters

Not “can we build it” — a capable engineering team can build almost anything given enough time. The more useful question is: does building this specific piece ourselves make the product meaningfully better for our users, or is it invisible infrastructure that a specialist already does well?

Authentication, payments and transactional email are rarely differentiators. Users don’t choose a product because it rolled its own auth system — they choose it despite that, if it’s done adequately. These are strong candidates to buy, because a specialist provider has already solved edge cases (fraud detection, deliverability reputation, compliance) that would take real time to rediscover in-house, and that time isn’t spent on anything users actually value.

The parts of a product that are genuinely core to what makes it different are the opposite case. If a specific workflow, algorithm or data model is the actual reason customers choose the product over alternatives, that’s usually worth owning directly — outsourcing it means outsourcing the thing that’s supposed to be defensible.

The part that’s easy to underweight: ongoing operational cost

The build decision is often evaluated only on build time, which understates the real cost. Buying a service also means paying for someone else’s uptime, someone else’s security team, and someone else’s ongoing feature development. Building it in-house means all of that becomes a permanent line item on your own team’s plate — including the parts that only show up eighteen months later, like a compliance requirement the original build never accounted for.

That’s not an argument for always buying. It’s an argument for evaluating both options on their full lifetime cost, not just the initial build estimate.

A reasonable default

Buy the undifferentiated, well-solved infrastructure. Build the parts that are actually the product. Revisit the decision when the calculus changes — a “buy” that made sense at low volume can become genuinely worth building in-house once usage and cost both scale, and that’s a legitimate reason to reassess later rather than a mistake made earlier.