What's the difference between a PRD and a build specification?

A product requirements document, or PRD, usually explains what a product should do and why: the problem, the users, the goals and the main features. It's often written to get a team aligned. A build specification is written to be built from: every requirement can be tested and traced back to a person or decision, disagreements are settled before the build, and the people responsible approve it.

The two overlap a lot. The difference is mostly in how finished and how checkable they are.

What does a PRD usually cover?

The problem, the target users, goals and success measures, the main features or user stories, and what's out of scope. Many PRDs are living documents that change as the team learns, and they often leave detail to be worked out during the build.

What does a build specification add?

• Every requirement testable, with acceptance criteria someone can check.
• Every item traceable, with a trace ID and a link to the answer or decision behind it.
• The business rules and exceptions, spelled out rather than left for the developer to ask about.
• Data and connected systems, so integrations can be estimated.
• Decisions recorded, including who made each one and why.
• Ownership after the build: who will run, support and eventually retire it.
• Sign-off, so everyone knows which version is approved.

Which one do I need?

If you're exploring an idea and the team will keep learning together, a PRD may be the right tool. If you're about to hand the work to a developer, an outside team or an AI coding tool, and you want quotes and results that match what you meant, you need something closer to a build specification. Many teams start with a PRD and turn it into a build specification before the build.

Can a PRD become a build specification?

Yes. Take each feature, break it into single testable requirements, ask the people who do the work about the rules and exceptions, settle where they disagree, and add the data, the integrations and the acceptance criteria. Then have the sponsor sign off.

How can I tell if my PRD is ready to build from?

Ask whether a developer could start without coming back with questions. If you'd like a quick read, the free spec check lists where a builder would likely get stuck. Mochlion produces the full build specification from your experts' answers.