Detached ADUs: a separate home in your back yard
A detached backyard ADU in Simi Valley: the 800 sq ft streamlined path, 16-foot height, 4-foot setbacks, the 6-foot separation HCD struck, and real cost data.
A detached ADU is a new, freestanding house on your lot with its own kitchen, bathroom and entry. It costs the most of the four ADU types because you are building a foundation, frame, roof and utility runs from scratch. It also gives the most privacy, which is why it is the usual choice for a parent moving in or a long-term rental.
In Simi Valley the questions that decide a detached ADU are physical: how much flat yard you have, where the sewer lateral runs, whether you are in a mapped fire hazard zone, and how close the unit can sit to the property line. State law settles more of that than most homeowners expect.
The size and height state law guarantees you
The fastest route is the streamlined category in Government Code section 66323(a)(2): one detached unit up to 800 square feet with 4-foot side and rear setbacks. For a larger unit, section 66321 says a city cannot cap detached ADUs below 850 square feet, or 1,000 square feet for more than one bedroom, and must always allow an 800 square foot unit with 4-foot side and rear setbacks.
| Standard | What the City must allow |
|---|---|
| Size, streamlined path | Up to 800 sq ft |
| Size cap, standard path | Not below 850 sq ft (1,000 sq ft for 2+ bedrooms) |
| Side and rear setbacks | 4 ft |
| Height | 16 ft; 18 ft within a half mile of major transit, plus 2 ft for a matching roof pitch |
| Parking | No more than 1 space; none in several exempt cases |
The 6-foot separation rule HCD struck down
Simi Valley's Ordinance 1349 defined a detached ADU as one separated from the house and every other structure by at least 6 feet, with no covered breezeway connecting them. In its October 29, 2025 findings, HCD told the City that the streamlined detached category requires no separation beyond what fire and safety codes demand, and that the City's rule could not block an 800 square foot unit with 4-foot setbacks.
That matters on the narrow back yards common in older valley-floor tracts, where a 6-foot gap could push a unit into the middle of the yard. The City's replacement Ordinance 1373 (reported adopted July 2026) answers this; confirm the current separation standard at plan review rather than relying on the old number.
Hillside and fire-zone lots
Many lots in Wood Ranch, Big Sky and the hills above Santa Susana are sloped, and some sit in mapped fire hazard severity zones. Slope can mean retaining walls, a soils report and a stepped or raised foundation. A fire-zone address brings ignition-resistant construction requirements for roofing, vents, siding and windows under Chapter 7A of the California Building Code.
Neither condition stops you from building, but both add design time and cost, and both should be priced before you sign. Older flat lots on the valley floor rarely carry either issue.
- Check whether your parcel is in a fire hazard severity zone before design starts.
- Ask whether a soils or geotechnical report will be required for your pad.
- Look for protected oak trees near the build area; the City regulates work around them.
Questions homeowners ask
How big can a detached ADU be in Simi Valley?
Under state law the City must allow at least 800 square feet with 4-foot side and rear setbacks, and any size cap it sets cannot be lower than 850 square feet (1,000 for two or more bedrooms). Simi Valley's exact maximum is in Ordinance 1373 (reported adopted July 2026); confirm it with Building & Safety before design.
Can a detached ADU be two stories?
The state floor is 16 feet for a detached unit, or 18 feet near major transit. HCD's handbook says that if a two-story ADU fits within the allowed height and meets the building code, a city cannot deny it because of underlying zoning. On most Simi Valley lots 16 feet is tight for two full stories, so it depends on the design.
Do I lose a parking space?
The City can require at most one parking space for the ADU, and it can be tandem on your driveway. No space is required if you are within a half mile walk of public transit, among other exemptions listed in Government Code 66322.
What does a detached ADU cost in Simi Valley?
A competitor, Safeway Remodel, publishes roughly $240,000 to $330,000 for 600 square feet and $355,000 to $485,000 for 1,000 square feet (factual update September 16, 2026). Aguila prices from your lot; hillside and fire-zone conditions push a job toward the top of any range.
How long does a detached ADU take to build?
Safeway Remodel's September 2026 page estimates about 6 to 11 months of active construction for a detached unit, after the permit is issued. Design and City review come before that.
Sources
Facts on this page last verified: .
- California Government Code section 66321 (size, height and setback floors): https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=GOV§ionNum=66321. (2026-10-05)
- HCD findings on Simi Valley ADU Ordinance No. 1349, October 29, 2025: https://www.hcd.ca.gov/sites/default/files/docs/policy-and-research/ordinance-review-letters/simi-valley-adu-findings-102925.pdf (2026-10-05)
- HCD ADU Handbook, updated for law in effect January 1, 2026: https://www.hcd.ca.gov/sites/default/files/docs/policy-and-research/adu-handbook-update.pdf (2026-10-05)
- Ordinance 1373 (reported adopted July 2026): the adopted text was not published online when this page was written. Confirm current local ADU standards with City of Simi Valley Building & Safety: https://www.simivalley.org/departments/environmental-services/building-safety-division/online-permit-submittal-instructions (2026-10-05)
- Competitor's published range: Safeway Remodel, ADU cost in Simi Valley (factual update September 16, 2026): https://www.safewayremodel.com/blog/adu-cost-simi-valley-2026 (2026-10-05)
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