The fit question
A California ADU site plan answers one question before any other: does a unit of 800 sq ft fit on the lot while keeping 4 ft from the side and rear lot lines? The state has already settled the zoning side of that question. Under Gov. Code 66321, local standards on lot coverage, floor area ratio, open space and the front setback cannot block a unit of at least 800 sq ft with 4 ft side and rear setbacks, and under 66314(d)(7) a city may require no more than 4 ft at the side and rear lines [1, 2]. Both sections belong to the recodified ADU law, Gov. Code 66310 to 66342, as it read on October 10, 2026 [1, 2]. Approval of an ADU on a lot with a single-family or multifamily dwelling is ministerial [1], so the plan is checked against the numbers rather than weighed at a discretionary hearing.
What is left is geometry: the house, the yards, the access and the city's own lines, drawn around that envelope. State rules for every state are tabled in the ADU laws by state guide; this page draws the California case.
The envelope, drawn
The base envelope this page draws comes from Gov. Code 66321(b)(3) and the HCD handbook: an 800 sq ft unit, 16 ft high, 4 ft from the side and rear lines, with the front setback set aside [1, 2]. The diagrams are static and not to scale; footprint dimensions are left out because they depend on the lot.
Plan view, street at the top:
| From front to rear | Left side | Center of the lot | Right side |
|---|---|---|---|
| Street | Street and front lot line | ||
| Front yard | Front setback: cannot block an 800 sq ft unit (66321) [1, 2] | ||
| House | Side yard | Primary dwelling, its footprint and square footage on the plan [4] | Side yard |
| Between | Separation between the ADU and the primary dwelling, dimensioned [4] | ||
| ADU | Side setback of 4 ft at most (66314(d)(7)) [2] | ADU footprint: the 800 sq ft unit local standards cannot block [1] | Side setback of 4 ft at most [2] |
| Rear yard | Rear setback of 4 ft at most [2] | ||
| Rear lot line | Rear lot line |
Section and area figures:
| Figure | State number | Rule |
|---|---|---|
| Height | 16, 18 or 20 ft, depending on conditions [2] | Government Code, as the HCD handbook sets it out [2] |
| Height, some ADU types in San Francisco | Up to 25 ft [5] | Planning Director Bulletin No. 3 [5] |
| Clearance to the side and rear lot lines | 4 ft at most [2] | 66314(d)(7) [2] |
| Clearance for a conversion within existing space | No setback [2] | 66314(d)(7) [2] |
| Floor area local standards cannot block | 800 sq ft [1] | 66321 [1] |
| Floor area a compliant ordinance must allow | 850 sq ft, or 1,000 sq ft for more than one bedroom [2] | HCD handbook [2] |
| Maximum for a new detached ADU | Up to 1,200 sq ft where the ordinance is compliant [2] | HCD handbook [2] |
Reading the diagram
Each band of the plan view is a line the site plan dimensions.
- The side and rear lines are capped; the front line is overridden. The two side setbacks and the rear setback are the distances the state caps at 4 ft [2]. The front setback is different in kind: for the 800 sq ft unit it is not a limit at all, because 66321 keeps it, with lot coverage, floor area ratio and open space, from blocking that unit [1, 2]. A site data table can still report lot coverage and floor area, but under 66321 those figures do not defeat an 800 sq ft ADU [1].
- The separation is drawn, not assumed. Los Angeles puts the separation between the ADU and the primary dwelling, and the primary dwelling's footprint with its square footage, on the plot plan [4]. The space between the two buildings is what is left after the 4 ft lines are placed, so it is a measured result on the plan.
- Height belongs to the section. The 16 ft base and the 18 or 20 ft heights the Government Code allows under other conditions [2] do not change the footprint, but they fix the section a reviewer reads with it.
- Size has a floor and a ceiling. Above the 800 sq ft protected unit, a compliant ordinance must allow 850 sq ft, or 1,000 sq ft with more than one bedroom, and a new detached ADU can run to 1,200 sq ft where the ordinance is compliant [2].
Where a city adds distance
The state caps the setbacks a city may require at the side and rear; city programs still add lines of their own to the plan.
- Pasadena. The ADU Standard Plans Program shows the side and rear setbacks at 4 ft, and a distance of 5 ft where a wall has doors or windows [3]. Placing new construction within 5 ft of a property line brings a property line survey, and mechanical equipment stays 5 ft from the property line, screened [3]. Where a wall facing a side or rear line has doors or windows, that 5 ft distance, not the 4 ft line, is what the footprint is drawn to.
- San Francisco. Planning Director Bulletin No. 3, first issued August 2025 and updated June 2026, lists heights up to 25 ft for some ADU types [5].
- Los Angeles. The plot plan is drawn to scale, and the separation between the ADU and the house is one of its items [4]; the full list is compared in ADU site plan requirements.
- San Diego County. The county's zoning plot plan instructions, PDS-1009 (Rev. 06/10/2016), ask for "An engineer's scale" [6].
City context for the region sits on the Los Angeles, San Diego and Sacramento metro pages, and the statewide rules for development on the California site plan page.
Conversions, catalogs and lot splits
A conversion within existing space needs no setback at all [2], so a garage or other structure already on the lot keeps its position when it becomes the ADU.
Two other California paths use the same lot, both cited as read on October 10, 2026. A detached ADU built from a city's preapproved plan is decided within 30 days under Gov. Code 65852.27 [7], and the plan still has to be placed on the lot, as set out in site plans for pre-approved ADUs. A lot split under SB 9, Gov. Code 66411.7, divides the lot itself [8].
Who signs which sheet
Seals attach to sheets, not to the fit. Los Angeles County's One- and Two-Family Dwelling Plan Review List (v2.0, 1-1-2023) wants final plans "stamped and signed by the California registered architect or engineer when appropriate" [9], and Dublin's Policy BDP 17 has a licensed professional prepare the structural plans when the building is one story with a basement or more than one story [10]. The question is set out in who can draw ADU plans.
The studio draws the site plan: the lot lines, the house, the ADU footprint inside the 4 ft lines, the separation and access, and the site data the city asks for. It is a preliminary plan without the engineering layer; structural, drainage, grading and utility work, and any sheet a jurisdiction wants sealed, are by your architect or engineer of record, who builds on it. Developers placing ADUs across many lots will find the lotting plan on the single-family and townhome site plan hub, and the other homeowner plans are under permit site plans.
To have the fit drawn for your California lot, start a project and choose "Residential: ADU (accessory dwelling unit)" as the project type; attach one file, a survey or a recent plot plan, if you have one.
Frequently asked questions
Will an 800 sq ft ADU fit on my California lot?
The state answers the zoning half: lot coverage, floor area ratio, open space and the front setback cannot stop a unit of at least 800 sq ft that keeps 4 ft from the side and rear lines (Gov. Code 66321) [1, 2]. The other half is geometry, whether that footprint fits between those lines and the house, and the site plan is where it is drawn.
How tall can a California ADU be?
Does a garage converted to an ADU need setbacks in California?
Not for a conversion within existing space: the statute's side and rear setback cap of 4 ft gives way to no setback at all there (Gov. Code 66314(d)(7)) [2].
How big can a California ADU be?
A compliant local ordinance must allow at least 850 sq ft, or 1,000 sq ft for an ADU with more than one bedroom, and the maximum for a new detached ADU runs up to 1,200 sq ft where the ordinance is compliant [2].
Does Pasadena allow an ADU 4 ft from the property line?
Its ADU Standard Plans Program shows 4 ft side and rear setbacks, and 5 ft where a wall has doors or windows; new construction within 5 ft of a property line needs a property line survey [3].
Do I need stamped plans for a California ADU?
For some sheets. Los Angeles County wants final plans "stamped and signed by the California registered architect or engineer when appropriate" [9], and Dublin's Policy BDP 17 has a licensed professional prepare the structural plans when the building is one story with a basement or more than one story [10].
Sources
- 1)ADU State Laws Summary and Checklist (dated 7.1.25), Association of Bay Area Governments (abag.ca.gov). Accessed 2026-10-10.
- 2)Accessory Dwelling Unit Handbook (update): Gov. Code 66310 to 66342, California Department of Housing and Community Development. Accessed 2026-10-10.
- 3)ADU Standard Plans Program, City of Pasadena Planning and Community Development Department. Accessed 2026-10-10.
- 4)ADU Standard Plan Building Permit Submittal Checklist (dated 7.7.2023), Los Angeles Department of Building and Safety. Accessed 2026-10-10.
- 5)Planning Director Bulletin No. 3: State Accessory Dwelling Unit Program (first issued August 2025, updated June 2026), San Francisco Planning Department. Accessed 2026-10-10.
- 6)Zoning plot plan instructions, PDS-1009 (Rev. 06/10/2016), County of San Diego Planning and Development Services. Accessed 2026-10-10.
- 7)AB 1332, Accessory dwelling units: preapproved plans (Gov. Code 65852.27), California Legislature. Accessed 2026-10-10.
- 8)Senate Bill 9, County of Santa Cruz Community Development and Infrastructure. Accessed 2026-10-10.
- 9)One- and Two-Family Dwelling Plan Review List (v2.0, 1-1-2023), Los Angeles County Public Works, Building and Safety. Accessed 2026-10-10.
- 10)Policy No. BDP 17 (original effective date August 16, 2001), City of Dublin, California, Building Division. Accessed 2026-10-10.
How these sources were chosen and when the page is reviewed: how we research.