Calculate density bonus from base and proposed units, or find maximum total units from lot area, density, and bonus percentage for a project.
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Density Bonus Formula
The calculator uses one of four formulas depending on the inputs you have:
Bonus % = ((Proposed Units - Base Units) / Base Units) * 100
Base Units = Base Density * Site Area
Bonus % = ((Built Density - Base Density) / Base Density) * 100
Max Units = Base Units * (1 + Bonus % / 100)
- Base Units – units allowed by the underlying zoning before any incentive.
- Proposed Units – total units in the project, including bonus units.
- Base Density – allowed density (units/acre, units/hectare, or units/m²).
- Built Density – proposed or as-built density on the site.
- Site Area – the buildable lot area used to compute base units.
- Bonus % – the percentage increase over base allowance.
The calculator converts mixed units to a common basis (units/acre) before computing. Results are pre-rounding; your local code may require rounding fractional units up or down, and may exclude certain land (rights-of-way, easements, slopes) from the area used to compute base units.
Reference Tables
Bonus tiers vary by jurisdiction. The values below are typical ranges, not legal limits.
| Affordable / Set-aside Provision | Typical Bonus |
|---|---|
| 5% very low income | 20% |
| 10% low income | 20% |
| 10% moderate income (for-sale) | 5–10% |
| 15% very low income | 35–50% |
| 100% affordable project | 80% or unlimited |
| Senior housing (100%) | 20% |
| Base Units | +20% | +35% | +50% |
|---|---|---|---|
| 20 | 24 | 27 | 30 |
| 50 | 60 | 68 | 75 |
| 100 | 120 | 135 | 150 |
| 250 | 300 | 338 | 375 |
Example
A 4-acre site is zoned for 20 units/acre. Base units = 20 × 4 = 80. The developer proposes 100 units in exchange for 10% low-income set-aside.
Bonus % = ((100 − 80) / 80) × 100 = 25%. That is 20 extra units above the base allowance, or a built density of 25 units/acre.
FAQ
Does the bonus apply to fractional units? The math produces decimals. Most codes round fractional bonus units up to the next whole unit, but check your local ordinance.
Is the set-aside counted from base units or total units? Most state and local programs (including California's) calculate the affordable set-aside as a percentage of the base units, not the bonus total.
Can the bonus stack with other incentives? Density bonuses are usually paired with concessions, waivers, or reduced parking. Those are separate from the unit count and are not included in this calculation.
What area do I use for site area? Use the area your jurisdiction defines as the developable or net lot area. Public rights-of-way and undevelopable land are often excluded.
