ADU Constructionin Brea, CA
Yes, Brea allows an ADU on a single-family lot, and the ordinance text we read permits one conversion ADU, one new ADU and one JADU. Detached units top out at 1,200 square feet. State law gives the City 15 business days to check completeness and 60 days to act. Fire zone and oil-field rules can change a lot.
Brea is a foothill city built over an old oil field. Of its 17,373 housing units, 9,415 are single-family detached homes, 54.2 percent, and another 1,986 are single-family attached. The median home was built in 1978, the median owner-occupied value is $922,300, and 40.3 percent of occupied homes are rented, so a backyard unit here is often a rental conversation as much as a family one. Lots range from downtown tract parcels to the large horse properties of Olinda, a neighborhood revived from 1964 with about 120 homes, and to hillside land where the Olinda Ranch Specific Plan converted 284 acres of oil land to a community of up to 662 homes.
The sections below cover what the ordinance allows, how big a unit can be, garage conversions, the wildfire plan for hillside lots, the oil-field and soil rules, older and hillside lots, and the permit sequence. Brea's Municipal Code text comes from the GoCodebook mirror because the City's official host blocks automated readers, and we could not confirm whether the state housing department has reviewed Brea's ADU ordinance, so every number here is one to confirm with the Building and Safety Division. Our general ADU construction page holds the cost tiers and the phase list.
Brea Building and Safety rules that apply to ADU work
Where adu construction permits come from in Brea: City of Brea Community and Economic Development Department, Building and Safety Division.
- The City's ADU and SB 9 page says Brea permits ADUs and JADUs in all zones allowing single-family, multifamily and mixed-use housing, and permits two-unit developments and urban lot splits in single-family zones, under Zoning Chapters 20.52 and 20.56. It advises a preliminary sketch to the Planning Division at planner@cityofbrea.gov before filing, and Building and Safety takes all permit intake and routes the package to Planning, Public Works and Fire.
- The City's ADU package must include the building permit application, project plans, the ADU Checklist and Questionnaire (and the SB 9 checklist if applicable), a waste management form, a Water Quality Management Plan checklist, an encroachment permit and address change form, a Best Management Practices form and a Fire Hazard Severity Zones form. Applicants email one PDF set to building@cityofbrea.gov, wait for a completeness email, then pay by phone at 714-671-4406 or by check, and the plan check is not approved until every department approves.
- Under Brea's ADU ordinance (Zoning Chapter 20.52, Ordinances 1242, 1247, 1254 and 1276, GoCodebook copy), a lot with a single-family home may have one conversion ADU, one new attached or detached ADU and one JADU. Detached ADUs are limited to 1,200 square feet, attached ADUs to 50 percent of the main home or 1,200 square feet whichever is less, JADUs to 500 square feet, side and rear setbacks are 4 feet, detached units must sit no closer to the street than the front-most wall of the main home and 10 feet from it, and height is 16 feet with state-law extra height allowed.
- Brea's ADU chapter requires an independent exterior entrance and an efficiency kitchen, says ADUs cannot be sold separately except as state law allows (Gov. Code 66341) and must be rented for at least 30 consecutive days, requires owner occupancy only for a JADU with shared sanitation (with a covenant recorded within 30 days of permit issuance), and says fire sprinklers are not required in an ADU if they are not required for the main home. An ADU that does not meet the standards can go through a Certificate of Compatibility.
- The City's pre-approved ADU plans program lets designers and builders get plans approved in advance for use by Brea homeowners, though some site-specific customization is still needed. Plans must include Fire Hazard Severity Zones design where applicable, and the City accepts electronic submittals only.
- The Brea Fire Department's Fuel Modification Guideline says all new single-family homes, multifamily structures and ADUs built in or adjacent to a wildfire-risk area need a Fuel Modification Plan under Chapter 49 of the California Fire Code, with a typical layout of Zone A 30 feet from the structure (irrigated), Zone B out to 100 feet (thinned so coverage is no more than 50 percent) and Zone C out to 200 feet (thinned at least 30 percent).
Can I build an ADU in Brea, and how many units fit on one lot?
Yes, in any zone that allows single-family, multifamily or mixed-use housing. The City's ADU and SB 9 page says Brea permits ADUs and JADUs there, and permits two-unit developments and urban lot splits in single-family zones, under Zoning Chapters 20.52 and 20.56.
On a lot with a single-family home, the ordinance text we read allows one conversion ADU, one new attached or detached ADU and one JADU. State law sits underneath that. Government Code 66317 calls for ministerial approval with no hearing, and since January 1, 2026 the City has 15 business days to say whether an application is complete and 60 days after that to approve or deny it, with deemed approval if it stays silent.
The chapter also requires an independent exterior entrance and an efficiency kitchen, and it requires rentals of at least 30 consecutive days. An ADU cannot be sold apart from the house except as state law allows under Government Code 66341. Owner occupancy applies only to a JADU with shared sanitation, backed by a covenant recorded within 30 days of permit issuance.
How big can an ADU be in Brea? Square feet, height and setbacks
The ordinance caps a detached ADU at 1,200 square feet and an attached ADU at 50 percent of the main home or 1,200 square feet, whichever is less. A JADU tops out at 500 square feet. Side and rear setbacks are 4 feet, and height is 16 feet, with the extra height state law allows.
One local rule deserves a question: a detached unit may sit no closer to the street than the front-most wall of the main home, and must stand 10 feet from that home. State law limits what a city can demand. It bars a maximum below 850 square feet, or 1,000 with more than one bedroom, and it protects one detached unit of up to 800 square feet with 4 foot setbacks. If the City's answer and the ordinance disagree, the state-protected types give the safer path.
A unit that misses a standard can apply for a Certificate of Compatibility. We could not confirm whether the state housing department has reviewed this ordinance, so treat every figure as one to confirm, and ask the Planning Division by email at planner@cityofbrea.gov before drawing anything.
Can I convert my Brea garage to an ADU, and what happens to the parking?
Yes. State law lets an ADU and a JADU be created inside an existing house or accessory structure, with up to 150 square feet added only for entry and exit, and it sets no setback for a converted structure. Government Code 66314 and 66322 also say no replacement parking can be required when a garage or carport is converted.
Brea's chapter says sprinklers are not required in the ADU if the main home does not need them. We found no Brea bulletin listing garage conversion details such as wall rating, slab thickness or a separate panel, and Los Angeles bulletins do not govern here. Ask the Division for its checklist before the layout is drawn.
The City's foundation policy gives a useful yardstick for an old floor: a 5 inch slab with #4 bars at 18 inches each way over 4 inches of base and a 10 mil vapor retarder, tied to a footing 12 inches wide and 24 deep. Compare the existing garage slab against it. A garage slab was poured for cars, so it is the first thing to test.
Brea's fire zone and the fuel modification plan for a new ADU
The City's annual Wildfire Safety Notice says Brea is within a Very High Fire Hazard Severity Zone. Its Wildland-Urban Interface amendment sets 200 feet of defensible space from a structure in High and Very High zones. We could not confirm whether Brea has adopted the State Fire Marshal's 2025 map, and the City's own map is dated November 2010, so ask the Brea Fire Department which tier your parcel sits in. Brea has its own fire department and does not use the Orange County Fire Authority.
The Fuel Modification Guideline says new homes, multifamily buildings and ADUs in or next to a wildfire-risk area need a Fuel Modification Plan under Chapter 49 of the California Fire Code. A typical layout has Zone A at 30 feet from the structure, irrigated, Zone B out to 100 feet thinned so coverage stays at or below 50 percent, and Zone C out to 200 feet thinned by at least 30 percent. Final plans are approved before a grading permit issues.
The ADU package includes a Fire Hazard Severity Zones form, and pre-approved plans must show that design where it applies. An accessory building 3 to 50 feet from a main building must be noncombustible or ignition-resistant. New buildings in High and Very High zones must be sprinklered, while the ADU chapter exempts a unit when the house has none, so ask which rule the Division applies.
Methane, old wells and soil: what the oil field changes under an ADU
Brea's Fire Code adds Section 326: projects with combustible soil gas need methane testing, passive or active mitigation, and detection with alarms under the Fire Department's soil gas guideline. It applies to identified methane areas, not every lot, and installing or changing a mitigation system takes a construction permit. Ask whether your address is in one before design starts.
The Olinda Ranch Specific Plan requires special venting for homes within 10 feet of an abandoned oil well. Its 2004 revision counted 129 wells on the old oil land, with about 48 expected to keep producing. We found no source naming which streets sit near a plugged well, so the answer has to come from the City for your parcel.
Soil matters as well. The City's 2025 foundation policy puts most of Brea in Seismic Design Category D, notes expansive soil in places and sets tighter size limits in liquefaction areas. A one-story detached accessory structure up to 1,200 square feet outside a liquefaction area is excepted from the geotechnical report, and the limit drops to 500 square feet inside one. The Flood Hazard Overlay adds Title 18 floodplain rules on parcels in FEMA mapped areas.
ADUs behind a Union Oil house, on a hillside lot or under protected trees
The Historic Resource Register lists 58 sites, mostly 1911 to 1938 houses on South Madrona, South Redwood and South Walnut Avenues. Zoning Section 20.60.060 sends exterior alterations of a designated resource to plan review, and state law removes parking requirements in a historic district. A detached unit behind a register house is a planning conversation first, so ask early whether the lot is designated.
On Hillside Residential land, building or grading needs a hillside development permit, with exceptions that include accessory structures under 500 square feet that need no grading permit. The zone also limits accessory structures to 12 feet and keeps them out of the front yard, while the ADU chapter allows 16 feet. Retaining walls cannot exceed 6 feet cumulatively. Which rule governs an ADU on an HR lot is a question for the Planning Division.
Large lots, such as the horse properties in Olinda, may also meet the Tree Preservation chapter, which covers parcels of 20,000 square feet or more and protects native oaks, sycamores, willows and other species. It treats paving, trenching or grading inside a protected tree's root zone as destroying the tree, and a utility trench to a rear unit can do exactly that.
How do you build and permit an ADU in Brea, step by step?
Start with a preliminary sketch to the Planning Division. Building and Safety then takes all permit intake and routes the package to Planning, Public Works and Fire. The package holds the permit application, plans, the ADU Checklist and Questionnaire, a waste management form, a Water Quality Management Plan checklist, an encroachment permit and address change form, a Best Management Practices form and the Fire Hazard Severity Zones form.
Email one PDF set to building@cityofbrea.gov, wait for the completeness email, and pay by phone at 714-671-4406 or by check. The plan check is not approved until every department signs off. The City also runs a pre-approved ADU plans program, electronic submittals only, and some site customization is still needed. Brea's 2025 codes apply to plans submitted on or after January 1, 2026.
MDG Builders, CSLB #1040337, designs the unit, assembles that package and manages the build as a general contractor, with licensed subcontractors doing the framing, plumbing, electrical and finish work. Plan on 4 to 10 weeks of design, the state clocks, then 3 to 5 months for a garage or 5 to 9 months for a detached unit.
ADU photos from Brea and nearby lots
ADU scenes tied to Brea and the Orange County communities around it, from garage conversions and framing to finished backyard units. Tap a photo to enlarge it.
What adu construction costs around Brea
The sourced ranges on our ADU construction page are roughly $100,000 to $200,000 to turn a garage into a studio or one bedroom, and roughly $200,000 to $400,000 for a new detached unit of 400 to 800 square feet. Three independent reads support those ranges: a UC Berkeley survey of 2018 and 2019 ADU households that found a $100,000 median in Los Angeles County, the Terner Center's 2020 estimate of $148,000 for Los Angeles, and Remodeling magazine's 2025 Cost vs. Value report, which put the Los Angeles job cost at $178,536 with about 39.9 percent recouped at resale. None is a Brea or Orange County quote. State law bars impact fees on an ADU of 750 square feet or less, but utility connection charges are separate, and we did not summarize the City's fee schedule, so permit, plan check and utility charges appear as their own lines in a written proposal. The ADU cost guide explains what moves the number, and ADU vs JADU in California explains the two unit types. For nearby Orange County work, compare ADU construction in Fullerton, La Habra and Yorba Linda. A bath inside the unit is on our Brea bathroom page, an attached addition on the Brea room addition page, windows for the unit on the Brea window page, and the Brea area page lists every trade we cover in the city.
| 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 |
Brea ADU questions, answered
Can I build an ADU in Brea?
Yes. Brea permits ADUs and JADUs in zones that allow single-family, multifamily or mixed-use housing, and its ordinance allows one conversion ADU, one new ADU and one JADU on a single-family lot. Confirm the numbers with the Planning Division, since we could not verify a state housing department review of the ordinance.
How do you build and permit an ADU in Brea?
Send a sketch to Planning, then email one PDF set with the application, plans and the City's forms to building@cityofbrea.gov. Staff reply with a completeness email, you pay, and every department must approve the plan check. Then construction starts, and inspections run to final. Hillside and fire zone lots add a fuel modification plan.
How big can an ADU be in California?
State law lets a detached ADU reach 1,200 square feet and an attached one 50 percent of the existing house, and a city cannot set a maximum below 850 square feet, or 1,000 with two bedrooms. Brea's ordinance text uses 1,200 for detached and 500 for a JADU. Confirm both with the City.
How much does it cost to build an ADU in California?
Our sourced ranges are roughly $100,000 to $200,000 for a garage conversion to a studio or one bedroom and $200,000 to $400,000 for a new detached unit of 400 to 800 square feet. They rest on Los Angeles area studies because no Brea survey turned up, so a written proposal sets the price.
Do I need to provide parking for my ADU?
Usually not for a conversion. State law bars replacement parking when a garage or carport becomes an ADU, and it allows no more than one space per unit or bedroom otherwise. It also requires none in a historic district or within a half mile walk of transit. Ask Brea which exemption applies to your lot.
What is the difference between an ADU and a JADU?
A JADU is smaller, capped at 500 square feet, and sits inside the single-family house, including an attached garage. An ADU can be detached or attached and larger. Brea requires owner occupancy for a JADU only when sanitation is shared, and both need an exterior entrance and an efficiency kitchen.
Can I convert my garage to an ADU?
Yes. State law allows a garage conversion with no setback and up to 150 square feet added for access, and it forbids replacement parking. Brea adds a separate entrance and an efficiency kitchen. The slab, the electrical panel and the plan check checklist should be settled with the Division before you draw.
Can I sell my ADU separately from the main house?
Generally no. Brea's chapter bars separate sale except as state law allows, and the state exceptions are narrow: a qualifying low-income buyer through a nonprofit under Government Code 66341, or a condominium ordinance the city has adopted under 66342. Most owners keep the unit as a rental or family space.
How long does it take to get an ADU permit in Brea?
State law gives the City 15 business days to decide completeness and then 60 days to approve or deny a complete application, with deemed approval if it does not act. The City's undated handout lists plan check at 10 to 14 working days, but ADU plan check waits on every department.
Do I have to live on the property to rent out an ADU?
Not for an ADU. State law bars owner occupancy rules for ADUs, and Brea requires only that rentals run at least 30 consecutive days. A JADU with shared sanitation is the exception, with a covenant recorded within 30 days of permit issuance. Short-term rental rules, if any, are separate.
Tell us about your Brea ADU
Tell us which part of Brea the lot is in, whether you have a garage, a flat backyard or a slope, and whether the main house is rented, and the first conversation starts there.