ADU Constructionin Simi Valley, CA
ADU construction in Simi Valley starts with state law, which protects four kinds of unit, including an 800 square foot detached ADU with 4 foot setbacks. The City's ADU Guide, dated July 2023, adds its own sizes and design rules, and Planning says it is updating them, so confirm current numbers before anyone draws plans.
Simi Valley is a city of single-family lots. The Census Bureau's 2024 estimates count 33,431 detached houses among 45,201 housing units, 74.0 percent of homes are owner-occupied, and the median year built is 1979. Most of those lots were laid out for 1960s and 1970s tract houses, which usually leave room behind the garage and a garage that is ready to convert.
This page runs in the order that governs a real project. State law comes first because it sets the floor, the City's ADU Guide second because it adds Simi Valley's own numbers, then garages, clearances from other agencies, hillside and fire questions, and the rules that stay with the unit after it is built. The City's website blocked our tools, so we read stamped Wayback Machine copies of its pages and PDFs, and we read the municipal code through third-party mirrors. Price ranges and phases are on our ADU construction page.
Simi Valley rules that apply to ADU construction
Where adu construction permits come from in Simi Valley: City of Simi Valley Department of Environmental Services, Building & Safety Division.
- Simi Valley's ADU Guide (dated July 2023) says an attached ADU can be 800 square feet or 50 percent of the house (up to 1,000 square feet, whichever is larger), a detached ADU is 800 square feet when combined with a JADU or proposed on a hillside slope, otherwise 1,000 square feet on lots up to 8,000 square feet and 1,200 square feet on larger lots, the minimum is 220 square feet, and a detached ADU is limited to 18 feet and one story. Planning said in October 2026 that the ordinance and standards were being updated, so confirm current numbers.
- The ADU Guide's setback and design rules include 4 feet minimum from side and rear lot lines (with extra fire-rating requirements under 5 feet), 6 feet separation from other structures, 20 feet front setback for ADUs over 800 square feet, a separate exterior entrance, one material and one color from the main home on detached ADUs, a detached ADU garage capped at 400 square feet, and a patio cover limited to 10 percent of the ADU.
- The ADU Guide: a conversion of a legally permitted accessory structure or part of the house has no size limit and can add up to 150 square feet only for ingress and egress; parking is not required for studios, conversions, or ADUs within a half mile of a public transit bus stop; owner-occupancy is not required for an ADU but is for a JADU (with a recorded deed restriction); rentals must be for 30 days or more; an RV cannot be an ADU.
- City ADU Checklist: plans plus site plan, Title 24 energy calculations, and structural calculations if the project is not conventional light-frame; plan check takes 20 to 25 business days and so do corrections; clearances include a Public Works sewer will-serve letter, a Ventura County Fire 610 form (and sometimes a 625 form) for detached ADUs, school district payment for ADUs over 750 square feet, traffic impact fees over 750 square feet, and a Ventura County Watershed Protection District waiver or receipt for all new ADUs and JADUs, with that district's fees collected for ADUs of 750 square feet or more.
- The ADU Guide adds that fire flow must be tested for detached ADUs by completing a 625 application with Public Works Waterworks District No. 8 or Golden State Water Company, depending on the property, and sending it to the Fire Department with the 610 application.
- SVMC 9-44.160 (third-party Zoneomics copy) requires ministerial Zoning Clearance review of ADU building permit applications in residential zones, lets the City limit ADUs where water or sewer service, traffic flow or public safety is inadequate, and says ADUs do not count toward lot density; the code also sets a 60-day action deadline for ADU applications.
- SB 9 two-unit developments and lot splits in single-family zones are handled under City Ordinance No. 1352, effective December 18, 2023, and the City asks owners proposing an SB 9 unit to email Planning before submitting anything.
What does California law let you build as an ADU in Simi Valley?
The state sets a floor that no city can lower. Under Government Code Section 66323, a city must approve a building permit for any mix of four protected types: an ADU and a junior ADU converted inside a house or an existing accessory structure, with up to 150 square feet added only for entry and exit; one new detached ADU of up to 800 square feet with 4 foot setbacks; conversions of non-livable multifamily space; and up to eight detached ADUs on a lot with an existing multifamily building. A city cannot add other objective standards to those types, and a rental must run longer than 30 days.
Fees follow Section 66311.5, which allows no impact fee on an ADU of 750 square feet or less, while utility connection charges are separate. HCD's published handbook also describes decision clocks of 15 business days for completeness and 60 days to act on a complete application. Read the HCD ADU handbook for the current text, because the 2025 and 2026 amendments moved several rules.
The state clocks come from HCD's handbook. Simi Valley's own deadline wording was cut off in the copy we could read, so we do not state it as the City's rule.
How big can an ADU be under Simi Valley's own ADU Guide?
The City's ADU Guide is dated July 2023. It allows an attached ADU of 800 square feet or 50 percent of the house, up to 1,000 square feet, whichever is larger. A detached ADU is 800 square feet when combined with a junior ADU or built on a hillside slope, and otherwise 1,000 square feet on lots up to 8,000 square feet and 1,200 square feet on larger lots. The minimum is 220 square feet, and a detached unit is limited to 18 feet and one story.
Planning's own page said in October 2026 that the ADU ordinance and development standards were being updated, so these numbers may already be stale. Where the Guide and state law disagree on one of the protected types, state law controls, and we ask Planning in writing which path a given design takes. A conversion of a legally permitted accessory structure or part of the house has no size limit under the Guide.
What setbacks, entrances and exterior design rules apply to a Simi Valley ADU?
The Guide asks for at least 4 feet from the side and rear lot lines, with extra fire-rating requirements for walls under 5 feet, and 6 feet between the ADU and other structures. An ADU over 800 square feet needs a 20 foot front setback. It also requires a separate exterior entrance and, for a detached ADU, one material and one color matching the main home. A detached ADU garage is capped at 400 square feet and a patio cover at 10 percent of the ADU.
A wall that sits 4 feet from the fence line is a wall with few windows, which shapes the floor plan before anyone picks finishes. We did not read Simi Valley's residential zone setbacks in Chapter 9-24 or its sample plot plan exhibit, and the lot coverage rules were not opened either, so no lot-coverage number appears here. The City says home improvements normally need a Zoning Clearance from Planning first, and the code text we read treats an ADU permit application in a residential zone as a ministerial Zoning Clearance review.
Can you convert a Simi Valley garage into an ADU, and what happens to the parking?
Yes, and conversion is the lightest path. The Guide says parking is not required for studios, for conversions, or for ADUs within a half mile of a public transit bus stop. The City's ADU Checklist adds that structural calculations are not required for a garage conversion unless square footage is added. It requires a Ventura County Fire clearance for every detached ADU, including a detached garage converted in place, and for an attached garage only if square footage is added to the home or garage.
The work itself is mostly the slab, the old door opening and the new walls. The City's inspection sequence requires an approved slab inspection before any concrete is placed, with the vapor barrier, sand and reinforcement in place, so a new slab is a visit you schedule. Door infill, egress, a bathroom and a separate panel go in the plan set. An unpermitted conversion built before 2020 may qualify for the state's legalization route in Section 66311.7, and we ask a client for any old permit record first.
Which sewer, fire, school and watershed clearances does an ADU need?
Plan check takes 20 to 25 business days, and so does each round of corrections, according to the City's checklist, which adds that submittals take one to two weeks to reach plan check. Clearances come from several offices: a Public Works sewer will-serve letter, a Ventura County Fire 610 form and sometimes a 625 form for detached ADUs, school district payment for ADUs over 750 square feet, traffic impact fees over 750 square feet, and a Ventura County Watershed Protection District waiver or receipt for every new ADU.
Fire flow is its own step. The Guide says a detached ADU needs a flow test through a 625 application filed with Public Works Waterworks District No. 8 or Golden State Water Company, depending on the property, and sent to the Fire Department with the 610. The City also tells owners to check the FEMA flood map and to call Public Works at (805) 583-6786 if the lot is in a flood zone. We did not read any fee amounts, so none are printed.
Does a hillside lot, an oak tree or a fire zone change an ADU in Simi Valley?
It can change all three of the early decisions. The Hillside Performance Standards in SVMC 9-32 cover any parcel outside the valley floor boundary, keep development off natural slopes of 20 percent or more except as allowed, and call for a slope map signed by a civil engineer or land surveyor. Mature native oaks, roughly 5 inches across at 4.5 feet, are protected even in a single-family yard, and removing one takes a Tree Removal Permit with seven days of notice to the Council. Both come from the Zoneomics copy of the code.
Fire follows the Ventura County Fire Protection District, whose board adopted the State Fire Marshal's 2025 zone maps by Ordinance 33 on May 20, 2025. We did not find whether or how the City adopted them, or the zone for any address. The City adopted the 2025 California Wildland-Urban Interface Code, and ABC7 named Simi Valley among the Ventura County cities with the most very high hazard acres. The Simi fault bounds the north side of the valley, so framing and foundation drawings take seismic design seriously.
Build an ADU in Simi Valley: owner occupancy, rentals, SB 9 and HOAs
Operating rules stay with the unit. The Guide says owner occupancy is not required for an ADU but is required for a junior ADU, with a recorded deed restriction, rentals must be for 30 days or more, and an RV cannot count as an ADU. Recent state changes narrowed the junior ADU occupancy rule, so ask Planning which version applies. Separately, SB 9 two-unit developments and lot splits run under City Ordinance No. 1352, effective December 18, 2023, and the City asks owners to email Planning before submitting anything. HCD says state law voids CC&Rs that effectively prohibit an ADU, and we found no count of homeowners associations in Simi Valley.
MDG Builders, CSLB #1040337, serves as the design-build general contractor: we draw and permit the unit, then coordinate the slab, framing, plumbing and electrical crews, who are licensed subcontractors whose license numbers you can request. Our ADU versus JADU guide explains the junior unit.
ADU photos from Simi Valley and nearby homes
ADU scenes tied to Simi Valley and the Ventura County and San Fernando Valley communities around it, from garage conversions and backyard cottages to utility trenches and slab work. Tap a photo to enlarge it.
What adu construction costs around Simi Valley
Our planning range for a Simi Valley garage conversion to a studio or one bedroom is $100,000 to $200,000, and for a new detached unit of 400 to 800 square feet it is $200,000 to $400,000; the method is on our ADU construction page. Three outside studies support those figures. UC Berkeley researchers surveyed owners who finished or applied for an ADU in 2018 and 2019 and reported a Los Angeles County median of $100,000, the Terner Center put Los Angeles near $148,000 in 2020, while Remodeling magazine's 2025 Cost vs. Value report lists $178,536 for a Los Angeles ADU and says 39.9 percent comes back when the house sells. None is a Simi Valley bid, and Ventura County costs may differ. A longer sewer run, a utility upgrade, a fire flow test or a slope report can each add a line, and permit and plan check charges appear separately in a written proposal. Our ADU cost guide shows how the ranges were built. Nearby pages cover Agoura Hills, Calabasas, Santa Clarita and Los Angeles. In the same city we also handle bathroom remodeling, room additions and windows, and the Simi Valley overview lists every trade.
| What it covers | Typical range |
|---|---|
| Garage conversion to a studio or one bedroom | $100,000 to $200,000 |
| New detached ADU, 400 to 800 sq ft | $200,000 to $400,000 |
| UC Berkeley survey, Los Angeles County median, 2018 and 2019 projects | $100,000 overall. $90,000 garage. $180,000 detached. |
| Terner Center and Berkeley CCI, 2020 | $148,000 Los Angeles estimate. $167,000 statewide average. |
| Remodeling Cost vs. Value 2025, Los Angeles | $178,536 job cost |
| Electrical and plumbing, Angi Los Angeles, January 2026 | Electrical $2,600 to $7,800. plumbing $3,900 to $10,400. |
| City permit and plan check fees | Not priced here |
Other work we manage in Simi Valley
Simi Valley ADU questions, answered
How do I build an ADU in Simi Valley?
Start with the City's ADU Guide and call Planning to confirm that its July 2023 numbers are current. Then draw a site plan and plans, add Title 24 energy calculations, gather the sewer, fire, school and watershed clearances, and submit to Building & Safety through the online portal. Plan check takes about 20 to 25 business days.
How big can an ADU be in Simi Valley?
Under the City's July 2023 guide, attached ADUs reach 800 square feet or 50 percent of the house, up to 1,000, and detached ADUs reach 1,000 or 1,200 square feet depending on lot size, or 800 with a JADU or on a hillside. State law protects an 800 square foot detached unit. Planning is updating these standards.
How much does it cost to build an ADU in California?
For a Simi Valley garage turned into a studio or one bedroom we budget $100,000 to $200,000, and for a new detached unit of 400 to 800 square feet, $200,000 to $400,000. Those planning ranges rest on Los Angeles and statewide studies instead of local bids, so only a site visit and a written proposal fix your price.
Do I need to provide parking for my ADU?
Not in many cases. The City's guide says parking is not required for studios, conversions, or ADUs within a half mile of a public transit bus stop, and state law adds protections. Ask Planning about your lot, since the guide predates the update Planning says is underway.
What is the difference between an ADU and a JADU?
An ADU is a full second dwelling that can be detached, attached or converted, while a junior ADU is a unit of up to 500 square feet inside the main house with an efficiency kitchen. Simi Valley's guide requires a recorded deed restriction and owner occupancy for a junior ADU, though state changes may narrow the occupancy rule.
Do I have to live on the property to build or rent an ADU?
No for a full ADU. The City's guide says owner occupancy is not required for an ADU, though rentals must run 30 days or more. A junior ADU needs a deed restriction and owner occupancy in the guide, and recent state changes may have narrowed that, so confirm with Planning.
Can I convert my garage into an ADU in Simi Valley?
Yes. A conversion has no size limit under the City's guide, may add up to 150 square feet for entry and exit, and needs no parking. A Ventura County Fire clearance is required for a detached garage conversion. Structural calculations are not required unless square footage is added.
How long does it take to get an ADU permit in Simi Valley?
The City's checklist puts plan check at 20 to 25 business days, with the same for each round of corrections, plus one to two weeks to send submittals to plan check. State law sets 15 business days for completeness and 60 days to act on a complete application. Clearances from other agencies add time.
Can my HOA stop me from building an ADU?
Not by banning it outright. HCD says state law voids CC&Rs that effectively prohibit an ADU, and a 2025 amendment bars reasonable HOA restrictions from including fees. Other design rules in your documents may still apply, and we found no count of Simi Valley associations, so read yours and ask the board.
Tell us about your Simi Valley ADU
Send the street or tract, whether you are weighing the garage or the back yard, whether the lot slopes or has an oak, and which water company serves it, and the first visit will begin with the state-law path and the clearance list.