Site capacity: how many units a lot holds

The units a lot holds are set by whichever limit binds first: density, floor area, height, the yards, parking, open space or the floor plate itself. A test fit draws the building to find it, because the limits interact and the zoning table alone does not show which one governs.

The limits

• Density: units per acre, or one unit per a set area of lot.
• Floor area ratio: the building's floor area divided by the lot area.
• Height, in feet and in stories, and the step-backs above a base height.
• Yards and setbacks, which set the footprint, and the building code's limits on windows near lot lines.
• Parking, where it is still required, and the ramp and drive aisles it needs.
• Open space, by the code's measure.

The floor plate

Inside those limits, the plate has to work: a corridor of code width, two stairs at the code's separation, an elevator, a trash room, and units that each have a door, a window wall and a regular shape. Corners hold the larger units. The efficiency of the plate, the share of it that is rentable, decides how much of the permitted floor area becomes units.

Bonus programs change the answer

A density bonus or an incentive program can relax density, floor area, height or the yards, so the binding constraint moves. Each pathway gets its own test fit, and the comparison between them is made on the drawn units and the returns.

Questions

How do you calculate how many units fit on a lot?

By drawing a test fit within every limit (density, floor area, height, yards, parking and open space) and a working floor plate; the limit that binds first sets the count.

Sources