Density Bonus Calculator
A density bonus lets a housing developer build more homes on a site than base zoning allows, in exchange for including affordable homes or another public benefit the local program rewards. The arithmetic is one multiplication, but the two inputs hide the real work: the base unit count comes from your zoning, and the bonus percentage comes from the affordable housing rules you qualify under.
Enter the base number of units your zoning allows without any bonus, then the bonus percentage your project earns, and this calculator returns the bonus units and the total units allowed.
Estimates only. The calculator counts units. It does not check height, floor area, setbacks, parking, or whether your project qualifies for the percentage you entered. Your planning department confirms all of that.
Quick Answer
Key takeaways
- The bonus is a percentage of the base, never of the total. 35% on a base of 100 is 35 extra units, and the affordable homes that earn the bonus are counted inside the project, not added on top of the total.
- The percentage is not yours to choose. It comes from a program table tied to how many affordable homes you include and at what income level, so find your table row before you run any numbers.
- The tool reports decimals, such as 56.25 units, because rounding is a legal step, not arithmetic. California rounds fractional bonus units up; many other jurisdictions round down or to the nearest whole unit, so check your local code before quoting a total.
- A unit count is not a design. Height, floor area, setbacks, and parking can still cap what fits on the site even when the unit count is allowed.
- Quick Answer
- How a density bonus is earned, step by step
- The density bonus formula
- Worked example: 100 base units at a 35% bonus
- Where the percentage comes from: the California table as a model
- What this unit count does not settle
- What competing calculators miss
- Frequently Asked Questions
- Add This Calculator to Your Website
- References
How a density bonus is earned, step by step
A bonus is earned in a fixed order: base first, qualification second, arithmetic last. Skipping to the arithmetic with a guessed percentage is how feasibility studies go wrong.
Find the base units. This is the maximum number of homes allowed by the zoning and general plan for the site, with no bonus. Where zoning is stated as units per acre, multiply by the site area. If you are still measuring the parcel, the acreage calculator settles the area figure first.
Find your percentage in the program table. The share of affordable homes in the project, and the income level they serve, maps to one bonus percentage. Deeper affordability earns a larger bonus per home committed.
Run the multiplication. Bonus units = base units x bonus percentage / 100. Total units = base units + bonus units. That is all this calculator does, and it is the part worth automating.
Apply your jurisdiction rounding rule. Convert the decimal result to whole homes the way your code requires, then test whether the total physically fits the site under height and floor area limits.
The density bonus formula
The formula scales the base allowance by the bonus percentage. Nothing else enters the calculation.
Two reading traps catch people here. First, the percentage applies to the base only. A 50% bonus on 100 base units is 150 total, not 200: the bonus homes do not themselves earn a further bonus. Second, the base is the maximum allowed on the date you apply, under whichever controls, zoning or general plan, and some programs let you elect the site floor area ratio as the base instead. The definition in California Government Code Section 65915 is the model most state programs follow: a density increase over the otherwise maximum allowable residential density, scaled by the amount of affordable housing provided.
Worked example: 100 base units at a 35% bonus
The calculator is pre-filled with this case: a mid-size apartment site where zoning allows 100 homes, and the project mix qualifies for a 35% bonus.
| Scenario | Bonus units | Total units allowed | Note |
|---|---|---|---|
| 100 base at 20% | 20.00 | 120.00 | Entry level bonus in most programs |
| 100 base at 35% | 35.00 | 135.00 | The worked example above |
| 80 base at 35% | 28.00 | 108.00 | Same percentage, smaller site |
| 45 base at 25% | 11.25 | 56.25 | Fractional result: rounding rule decides the buildable count |
| 120 base at 50% | 60.00 | 180.00 | The standard California cap |
The 45 unit row is the one to study. The arithmetic honestly produces 56.25 units, and no one can build a quarter of a home. California resolves this by rounding fractional units up, so the same inputs yield 57 units there. A jurisdiction that rounds down yields 56. One unit on a small project can be the difference between a viable and an unviable scheme, which is why the rounding rule belongs in every feasibility note next to the percentage itself. Once the count looks viable, the build cost per square foot calculator helps price what those extra units cost to construct, since bonus units still have to be financed and built.
Where the percentage comes from: the California table as a model
Percentages come from statute or ordinance tables, and California’s is the most used reference because so many programs copy its shape. The deeper the affordability, the faster the bonus grows.
| Affordable homes in the project | Share of base units | Density bonus |
|---|---|---|
| Very low income | 5% | 20% |
| Very low income | 10% | 32.5% |
| Very low income | 11% | 35% |
| Very low income | 15% | 50% (standard cap) |
| Low income | 10% | 20% |
| Low income | 20% | 35% |
| Low income | 24% | 50% (standard cap) |
| Moderate income, for-sale only | 10% | 5% |
| Moderate income, for-sale only | 20% | 15% |
| Moderate income, for-sale only | 44% | 50% (standard cap) |
Read the worked example against this table: the 35% bonus used above is earned either by 11% very low income homes or by 20% low income homes, two different affordability commitments that land on the same percentage. The Association of Bay Area Governments model guidelines walk through the same tables with parcel level examples, including how the affordable count itself rounds up. Two cautions before you borrow these figures: the standard 50% cap is not the ceiling for every project, since 100% affordable projects and stacking under AB 1287 can go higher in California, and every state and city table differs in detail. City pages such as Santa Rosa’s density bonus summary show how a local program presents the same state framework to applicants.
Reasons to take the bonus
- More saleable or rentable homes on land you already control, which spreads land cost across more units.
- The extra market rate units often carry the affordable homes financially, making a mixed income project pencil out.
- Bonuses usually arrive with concessions, such as reduced parking or setbacks, that lower build cost further.
Costs and obligations to price in
- Affordable homes are typically deed restricted for decades, with rents or prices capped and compliance reporting required.
- A larger building can trigger added review, parking, or infrastructure requirements that eat into the gain.
- Entitlement timelines vary widely; the bonus is a right in some states and a negotiation in others.
What this unit count does not settle
A total of 135 units is a permission about count, not a promise that 135 homes fit or will be approved as drawn.
Floor area ratio, height limits, setbacks, open space, and parking all shape the building envelope, and a bonus project often needs a concession or waiver on one of them before the full count is buildable. California applicants can also elect to express the bonus through floor area on some sites, which this units-only calculator does not model. Treat the result as the first feasibility screen: if the project does not work at the bonus count on paper, it will not work in review either, and if it does work on paper, the envelope checks come next.
Disclaimer: this calculator provides planning estimates only and is not legal advice or an entitlement determination. Density bonus eligibility, percentages, rounding, and concessions are set by your state law and local ordinance and confirmed by your planning department. Verify every figure against the current code before relying on it in a pro forma or application.
What competing calculators miss
The density bonus tools ranking for this term mostly solve the problem backwards, or bury the one step that matters.
Miss 1: they solve for the percentage, not the units. The best known competing tools take a built density and a typical density and return the bonus percentage between them. That grades a finished design. A developer at feasibility stage has the opposite problem: a known percentage from the program table and a need for the unit count it produces, which is the direction this calculator runs.
Miss 2: they make you build the base yourself, then fumble the rounding. One competitor starts from units per acre times acres, which is fine, then presents a 56.25 unit result with a garbled note calling part of it additional units. The fractional case is exactly where readers need care: this page shows the decimal honestly, names the rounding step as a legal decision, and states which way California and other jurisdictions resolve it.
Miss 3: no program table behind the percentage box. Competing pages accept any percentage without showing where percentages come from, so a reader can type 80% and get a confident total that no program would grant. The California table on this page, with its 50% standard cap and the affordability shares that earn each tier, gives the input a reality check before the arithmetic starts.
Frequently Asked Questions
A density bonus is a zoning incentive that lets a developer build more homes on a site than the base zoning allows, in exchange for a public benefit, most often homes reserved at affordable rents or prices. The bonus is a percentage increase over the base unit count: a 35% bonus on a base of 100 units allows 135 units in total.
Your state law and local ordinance set it, through tables that tie the percentage to the share of affordable homes in the project and the income level they serve. In California, Government Code Section 65915 sets bonuses from 20% up to a standard cap of 50%. You do not pick the percentage; you read it off the row your project qualifies for.
The base is the maximum number of homes the zoning and general plan allow on your site with no bonus, measured on the date you apply. If your zoning states a rate such as 20 units per acre, multiply that rate by the site area in acres. Where zoning and the general plan disagree, many programs, including California’s, use the greater of the two, so check both documents.
Because the multiplication honestly produces a fraction: 45 base units at 25% is 11.25 bonus units, for a total of 56.25. Homes cannot be built in fractions, so your jurisdiction rounding rule converts the figure. California rounds fractional units up, which would make this 57 units. Other places round down or to the nearest whole unit, so confirm the rule in your local code.
Yes, in defined cases. The standard California tables cap at 50%, but projects that are 100% affordable can qualify for larger bonuses, and AB 1287 lets some projects stack an additional moderate income bonus on top of a maximum low or very low income bonus. Local programs elsewhere set their own caps. Enter only the combined percentage your program actually grants, since this calculator applies one percentage to the base.
Not by itself in this calculator, which counts units only. In practice, bonus projects often also receive concessions or waivers, such as reduced parking, smaller setbacks, or added height, and some California projects can elect a floor area based bonus. Those are separate determinations by the planning department and are not reflected in the unit total shown here.
No. The result is an arithmetic estimate based on the base units and percentage you enter. Eligibility, the correct percentage, rounding, and any cap are confirmed by your city or county planning department under the current state law and local ordinance. Use the figure for feasibility screening, and verify it before it goes into an application or a financial model.
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References
- California Government Code Section 65915, “Density Bonuses and Other Incentives” – the statute defining the bonus, the affordability tables, and the 50% standard cap used in the model table. FindLaw: California Government Code Section 65915. Accessed October 2026.
- California Government Code, Chapter 4.3, Sections 65915 to 65918, “Density Bonuses and Other Incentives” – the full chapter text, including the low income and very low income bonus tables. Justia: California Density Bonuses and Other Incentives, Sections 65915-65918. Accessed October 2026.
- Association of Bay Area Governments (ABAG), “Density Bonus Model Program Guidelines,” April 2025 – parcel level worked examples of base density, bonus rounding up, and affordable unit counts. ABAG: Density Bonus Model Program Guidelines. Accessed October 2026.
- City of Santa Rosa, “Density Bonus” – how one city presents state density bonus eligibility, incentives, concessions, and waivers to applicants. City of Santa Rosa: Density Bonus. Accessed October 2026.
- Terner Center for Housing Innovation, UC Berkeley, “San Diego’s Success in Spurring Missing Middle Housing: The Accessory Dwelling Unit Bonus Program” – research on a local bonus program delivering affordable homes in practice. Terner Center: San Diego ADU Bonus Program. Accessed October 2026.
- California Department of Housing and Community Development (HCD), City of Duarte Housing Element, Section 5.3, “Density Bonus Law” – a housing element summary of the state law, its 35% standard tier, and higher bonuses for fully affordable projects. California HCD: Duarte Housing Element, Density Bonus Law. Accessed October 2026.