Allowed area is not the same as achievable units.
A credible unit count comes from more than dividing an area allowance by an average apartment size. It needs a consistent area basis, a workable building layout, and the applicable density rules.
Same gross area.
Different planning yield.
80% efficiency30,000 × 0.80 = 24,000 sq ft30 units at an 800 sq ft average
75% efficiency30,000 × 0.75 = 22,500 sq ft28 whole units at the same average
Hypothetical planning arithmetic, before a floor-plan test or application of density controls.
Keep the different ceilings visible
FAR, height, lot coverage, and any dwelling-unit density control answer different questions. Do not treat a units-per-acre assumption as a substitute for a floor-area analysis, or vice versa. Determine the locally applicable rules for the proposed housing type and any bonus or affordability program before incorporating them into a scenario.
Convert gross area into a planning allowance
Assume a hypothetical 30,000-square-foot residential building and an illustrative 80% net-to-gross efficiency. That leaves 24,000 square feet for the defined net program. Dividing by an assumed average 800-square-foot unit produces an initial 30-unit planning estimate. The 80% and 800-square-foot assumptions are not design standards, and the arithmetic is not a floor-plan test.
Make the mix add up
An example mix of ten 600-square-foot units, ten 800-square-foot units, and ten 1,000-square-foot units totals 24,000 square feet across 30 units. The average is 800 square feet, but those different unit shapes have different implications for frontage, depth, circulation, and market positioning. Record the area convention used for every unit type.
Test the building, not just the spreadsheet
Lay out the circulation, stairs, elevators where applicable, building services, structure, common spaces, and units together. Review daylight, accessible design, egress, and other applicable requirements. A unit schedule that fits numerically can still fail to fit the floor plate. Do not silently treat the remainder after units as sufficient common and service space.
Change efficiency and follow the consequences
At 75% efficiency, the same 30,000-square-foot building has 22,500 square feet for the defined net program. At the same 800-square-foot average, the arithmetic estimate falls to 28 whole units, with 100 square feet remaining. That is two fewer units than the first estimate before any plan test. Revisit revenue, parking, amenities, and cost assumptions rather than changing only the unit-count label.
Compare scenarios on the same basis
Use a common area convention and identify what changes in each option. Show the zoning ceiling, gross area, assumed efficiency, unit mix, preliminary yield, and parking basis side by side. Distinguish a by-right concept from one dependent on an approval or incentive. A clear comparison should show which assumption drives the result and what evidence would change it.
Primary references
Use these sources as a starting point, not a project-specific determination. Check the full context, applicable rules, and current project assumptions before using a result.