A developer built this because he was tired of waiting.

Before a developer can bid on a New York lot, somebody has to answer one question: what can legally be built here? The usual route is to hire an architect or a zoning attorney for a feasibility study. That costs roughly $1,500 to $10,000 and takes one to three weeks.

That price is fine once you are in contract. It is useless at the moment you actually need it, which is while you are deciding whether to bid at all. So most lots never get studied. People bid on instinct, or they pass on sites that were worth buying, or they buy sites that were not.

Envelope was built to collapse that study into minutes, so it can be run on every lot you look at rather than the one you already chose. It models every as-of-right scenario a lot permits, draws the floor plans and the massing, and cites the Zoning Resolution section behind each number.

A zoning engine that is confidently wrong is worse than no engine. Three things keep this one honest.

• ■Every bulk rule is mirrored from the verbatim text of the Zoning Resolution, stored alongside the number it produces, so a figure can be traced to the sentence that created it.
• ■Reports are scored against real feasibility studies drawn by practicing architects. That score is recorded, and a release cannot ship if it drops.
• ■The full test suite and an accuracy gate run before every deployment. A failure stops the release rather than warning about it.