Time box the first version and write the spec down

In YC's Startup School lecture on planning an MVP, Michael Seibel advises you to time box your spec: pick a date, such as launching in a few weeks, and include only what can be built by then. An MVP should be buildable in weeks, not months.

He also stresses writing the spec down. An unwritten plan changes without anyone noticing, and a three-week plan quietly becomes a three-month one. A written spec makes drift visible.

Basecamp's Shape Up describes the same idea as an appetite: a fixed time budget set before the design, which "start[s] with a number and end[s] with a design". For a side project, an appetite might be six weeks of evenings and weekends.