ADU Constructionin Montebello, CA
You can build an ADU in Montebello, but which rules apply is unsettled. The state housing department wrote the City on October 8, 2025 that its ADU ordinance on file dates from 2019 and may be out of compliance, and the code text still shows 2004 limits. State law controls where they conflict, so ask Planning.
Most Montebello houses came with a garage, and a lot of those garages are now past 60 years old: the median home was built in 1963, 84.1 percent predate 1980 and 22.3 percent date from the 1950s alone. The city also has an unusual ADU past. In 1988 it ran a deferment program that let owners of R-A and R-1 lots keep an illegal garage or detached-structure conversion if they applied before October 31, 1988, and a recorded certificate must be cleared by reconverting the space and getting a certificate of occupancy before the title transfers (Municipal Code 17.10.250).
This page covers which ADU rules apply, how the old code text and state law differ, the City's garage conversion checklist, fire and egress, utilities, detached units and apartment lots, and the hills. For cost tiers, the timeline and our steps, see the main ADU construction page.
What the City of Montebello says about ADUs
Where adu construction permits come from in Montebello: City of Montebello Planning and Community Development, Building and Safety Division.
- California's housing department (HCD) wrote the City on October 8, 2025 that the most recent ADU ordinance on file for Montebello is from 2019, that it may be outdated and out of compliance with state ADU law, and that a noncompliant ordinance is null and void so that state standards apply until a compliant one is adopted. HCD asked for a response by November 7, 2025; whether the City answered or adopted a new ordinance was not verified.
- HCD's letter lists state ADU rules the City's older ordinance may conflict with: applications are deemed approved if not acted on within 60 days; one ADU and one JADU per lot are ministerially approved; owner occupancy cannot be required; maximum ADU size cannot be set below 850 square feet (1,000 if more than one bedroom); no replacement parking for garage or carport conversions and none for a JADU; no lot size minimums; ADUs up to 750 square feet are exempt from impact fees and larger ones pay proportionally; and no deed restrictions or HOA review.
- HCD's letter adds that an unpermitted ADU built before January 1, 2020 cannot be denied a permit for building code violations unless correction is needed to avoid a substandard finding, that enforcement against a qualifying substandard ADU must be delayed five years on request, that up to eight detached ADUs are allowed on a lot with an existing multifamily building (not more than the existing units), and that uncovered parking removed for an ADU need not be replaced.
- Montebello's own code (MMC 17.10.280, text from Ordinances 2109 of 1994 and 2274 of 2004) allows one accessory dwelling unit by right on an R-1 or R-A lot that has one house, either attached within the living area or detached on the same lot, and it says the unit must be clearly subordinate in size. It is not allowed on substandard lots, and a project that does not meet the standards can be appealed only to the city manager without a public hearing.
- Under MMC 17.10.280 a detached ADU must be at least 10 feet from the house (20 feet if its entrance faces the rear of the house), living area runs from 350 square feet up to 35 percent of the house's living area for attached units or 1,200 square feet for detached, and the unit follows the zone's height, setback, coverage, lot area and design standards. These local size caps conflict with the state minimums HCD lists, so a written ADU estimate should say state law may control.
- MMC 17.10.280 also requires one enclosed off-street parking space per ADU, in addition to the house's two enclosed spaces, the same driveway as the house unless there is alley access, utilities metered with the house, underground utility connections, a separate entrance that is not visible from the street, hardwired smoke detectors, fire sprinklers in both units unless infeasible, address numbers on both buildings, and the same or similar architectural features, materials and color as the house. State law (per HCD) limits several of these.
- The local ADU code bars a guest house once an ADU exists, bars renting individual bedrooms to boarders, requires an owner with at least 50 percent ownership to live in either unit, and bars selling the ADU separately from the house (MMC 17.10.280.C.20 to 22). It does not validate an existing illegal ADU, but an owner may apply to convert one to a legal ADU under the same standards as new construction (MMC 17.10.280.E).
- The City has no ADU page; its ADU Handout sits on the Forms and Handouts page. The handout tells owners to contact Planning for zoning requirements, says approvals and permits with plans are required before any work, and says plan check and permit fees follow the valuation of the work, with mechanical, electrical and plumbing permits charged separately per fixture.
Which ADU rules apply in Montebello: the 2004 code text, a 2019 ordinance or state law?
State law wins where it conflicts, and that matters here because the state has already questioned the City's ordinance. On October 8, 2025, California's Department of Housing and Community Development (HCD) wrote that the most recent ADU ordinance on file for Montebello is from 2019, may be outdated and out of compliance, and that a noncompliant ordinance is null and void, so state standards apply until a compliant one is adopted. HCD asked for a response by November 7, 2025.
Whether the City answered, or has adopted a new ordinance since, we could not verify. The code text on Municode, current through Ordinance 2491 of February 2026, still shows Municipal Code 17.10.280 with language from 1994 and 2004, and one adopted ordinance is listed as not yet codified, with its topic unstated. The City has no ADU page, only an ADU Handout on its Forms and Handouts page, so start with the Building and Safety Division and Planning. Ask Planning at 323-887-1478 which ordinance it applies to your lot, and get the answer in writing before design starts.
ADU size, parking and owner occupancy: what the old code says and what HCD says instead
Montebello's code text allows one ADU by right on an R-1 or R-A lot that has one house, attached to it or detached, and sets its own limits. HCD's letter lists where the state says limits like these cannot stand.
- Size: the code text sets a 350 square foot minimum, 35 percent of the house's living area as the cap on an attached unit and 1,200 square feet for a detached one. HCD says a city cannot set a maximum below 850 square feet, or 1,000 if the unit has more than one bedroom.
- Parking: the code text asks for one enclosed space per ADU on top of the house's two. HCD says no replacement parking for a garage or carport conversion, and none for a JADU.
- Owners: the code text requires an owner with at least 50 percent ownership to live in one of the units. HCD says owner occupancy cannot be required.
- Lots and timing: HCD says no lot size minimums, and an application the City has not acted on within 60 days is deemed approved.
- Fees: an ADU of 750 square feet or less is exempt from impact fees, and a larger one pays in proportion to the house.
A written estimate for a Montebello ADU should say plainly that state law may control.
Build an ADU from a Montebello garage: the footings, slab and walls the City's handout lists
Turning a permitted garage into an ADU is a change of use from garage to dwelling, and the City's handout says that triggers current code for fire-resistant construction, light, ventilation, heating, room area, ceiling height, bath spaces, glazing, emergency escape, guards, alarms, site address and flood-resistant construction, even when only partitions, ceilings and fixtures are added.
The structural checklist is specific. New or underpinned footings are at least 12 inches wide and 12 inches deep, or match a deeper existing one, with 2,500 psi concrete and four deformed bars. Anchor bolts are at least 10 inches long with 7 inches of embedment, spaced no more than 6 feet apart. The old door opening is infilled with 2x4 studs at 16 inches on center, and walls with new openings get 10 feet of wood structural panel bracing. A detached garage with no foundation needs a new one on the plans, and a slab with no footing, or one under 12 inches, must be underpinned.
The old slab may stay if it gets a vapor retarder under section R506.2.3 or an approved membrane, and the handout lists products with ICC evaluation reports for a slab that has none. Floor finishes wait on that decision.
Fire walls, windows and exits in a garage ADU: the 3 foot and 5 foot lines
Distance to the property line drives the wall. On a non-sprinklered conversion, an exterior wall less than 5 feet from the line must be 1-hour rated, and on a sprinklered one the line is 3 feet. No openings are allowed within 3 feet, and between 3 and 5 feet a non-sprinklered conversion is capped at 25 percent of the wall in openings. The handout leaves sprinklers to the Fire Department, the municipal fire code requires them in new R1 and R3 buildings, and state law says an ADU cannot be made to have them when the main house does not (Government Code 66314). Ask which of those the Fire Department applies.
Inside, the handout sets the entry door at 3 feet by 6 feet 8 inches, interior doors at 32 inches, ceilings at 7 feet, habitable rooms at 70 square feet and 7 feet in each dimension, natural light at 8 percent of floor area and ventilation at 4 percent, heating to 68 F and a separate bathroom with a 50 cfm fan. A bedroom escape opening must be at least 24 inches high and 20 inches wide, with 5 square feet of opening and a sill no higher than 44 inches. Our page on window replacement in Montebello covers the glass side.
Sewer, panel, heating and plumbing for a Montebello ADU: what the handout allows
An ADU sewer can tie to the house drain or run its own line, and a separate connection goes through the City's Utilities Division. A separate meter or service panel is not required, and a larger service is needed only if load calculations show the existing one is too small. Electrical plan check is not needed for a panel rated 399 amps or less, and a subpanel in a detached ADU needs its own grounding electrode and a main breaker of at least 60 amps if more than six breakers are installed.
Two rules surprise owners. A single heating unit may not serve both the house and an attached ADU, so each dwelling gets its own heater and thermostat. And water piping cannot run in or under a slab inside a building under Municipal Code 15.20.060, which shapes where a kitchen and bath can sit on a new slab. ADU circuits need AFCI and GFCI protection, tamper-resistant receptacles, a bathroom circuit and a laundry circuit if there is a laundry area, and the CalGreen recycling rate of 65 percent is why a waste management form comes with the plans.
Detached ADU or an apartment lot: yards, the 10 foot gap and up to eight units
A detached ADU under the code text sits at least 10 feet from the house, or 20 feet if its entrance faces the rear of the house, and follows the zone's height, setback, coverage and design rules. In R-1 and R-A that means 40 percent lot coverage including accessory structures and patio covers, a 35-foot height limit, 5-foot side yards and 10-foot rear yards for habitable buildings. State law caps side and rear setbacks for a new ADU at 4 feet, so those local numbers are another place we check which rule controls.
A detached unit also needs grading and drainage plans, since the Plan Review page requires them for a new detached building. A permit valued above $7,500 can be conditioned on curbs, gutters and sidewalks along the street frontage, and half-street dedication can be required at $15,000 where the street is narrow (Municipal Code 15.04.070 and 17.10.270). Ask Planning before the budget is final.
With 42.1 percent of Montebello's homes in buildings of two or more units, many lots already hold an apartment building. HCD's letter says state law allows up to eight detached ADUs on a lot with an existing multifamily building, and no more than the number of existing units.
ADU in the Montebello Hills: hillside plan, fire zone and the viewshed ordinance
Properties on the City's Hillside Areas map, dated December 1, 1971, need a hillside plan before a grading permit, heard by the Planning Commission and City Council, and hillside building has its own rules for eave gutters, footing clearance and driveways no steeper than 20 percent (Municipal Code 17.42 and 15.04.100). The hills sit over the Montebello Oil Field, and grading tied to oil and gas needs an engineer's supervision.
In our point sample of the County's CAL FIRE layer, the 8 Very High and 22 High points among 232 all fell in the northern hills, so check the address on the CAL FIRE viewer before promising a hillside ADU. Since October 8, 2025, the viewshed ordinance (Municipal Code 17.41) bars reflective roof coverings, deck surfaces and glazing in designated areas, and caps fences and hedges at 3 feet without a visual analysis. The area map could not be read, so we cannot say which streets it covers. Ask Planning, and plan on matte roofing and non-reflective glass until it answers.
Garage and backyard ADU details near Montebello
Three views of the framing, the slab and the finished interior of ADU projects around Montebello. Tap a photo to enlarge it.
What adu construction costs around Montebello
No Montebello ADU cost study with a stated method turned up, so the numbers on our ADU construction page are regional and carry medium-low confidence: about $100,000 to $200,000 for a garage conversion and $200,000 to $400,000 for a new detached unit. They lean on a UC Berkeley survey of 2018 and 2019 projects that found a Los Angeles County median of $100,000, with garage ADUs at $90,000 and detached units at $180,000, a Terner Center estimate of $148,000 for Los Angeles and a Cost vs. Value 2025 job cost of $178,536. Footings of the size the City's handout lists, a sewer connection and slab repair sit on top of averages like these. The City charges plan check and permit fees by the value of the work, with electrical, plumbing and mechanical permits priced per fixture, and we could not read its fee schedule, so no permit figure is printed. Rules differ next door, so compare Alhambra, Whittier, El Monte and Los Angeles, or read the Montebello overview. Other work on the same lot includes bathroom remodeling in Montebello and a room addition in Montebello. Our posts on ADU versus JADU, garage conversion permit steps, ADU costs and room addition versus ADU go further.
| 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 |
Questions about ADUs and garage conversions in Montebello
Do I need a permit to build an ADU in California?
Yes, a building permit, and in Montebello it comes from Building and Safety after plans go in through the GoPost portal. State law requires the City to approve an ADU permit without a hearing and to act within 60 days of a complete application, or it is deemed approved. Planning handles zoning at 323-887-1478.
How big can an ADU be in California?
State law lets a detached ADU reach 1,200 square feet and bars a city from setting a maximum below 850 square feet, or 1,000 if it has more than one bedroom. Montebello's code text still caps detached units at 1,200 and attached ones at 35 percent of the house, and state law controls where the two differ.
Do I need to provide parking for my ADU?
Not when you convert a garage. State law bars replacement parking when a garage or carport is converted, and HCD's letter lists that rule among the ones Montebello's older ordinance may conflict with. The code text still asks for an enclosed space per ADU, so get Planning's current position in writing.
Do I need to live on the property to build or rent an ADU?
No. State law bars cities from requiring owner occupancy for an ADU, though a city may require rentals of 30 days or longer. Montebello's older code text requires an owner of at least 50 percent to live in one of the units, and HCD's October 2025 letter says owner occupancy cannot be required.
Can I convert an unpermitted garage conversion into a legal ADU?
Often yes. HCD's letter says an unpermitted ADU built before January 1, 2020 cannot be denied a permit for building code violations unless correction is needed to avoid a substandard finding, and Montebello's code lets an owner apply to legalize one. Check first for a recorded 1988 deferment certificate, which must be cleared before title transfers.
How long does it take to get an ADU permit?
State law gives the City 15 business days to say whether an application is complete and 60 days after that to approve or deny, or it is deemed approved. Montebello's pages list no plan check turnaround, so we quote none. Design takes about 4 to 10 weeks first, and construction runs 3 to 5 months for a garage.
What is the difference between an ADU and a JADU?
A junior ADU is limited to 500 square feet and sits inside the existing single-family house, with an efficiency kitchen and a separate entrance. An ADU can be larger and can be detached. HCD's letter says state law approves one ADU and one JADU per lot without a hearing, and Montebello's handout requires separate heating for each unit.
How much does it cost to build an ADU in California?
Our ranges are about $100,000 to $200,000 for a garage conversion and $200,000 to $400,000 for a new detached ADU of 400 to 800 square feet, with medium-low confidence. They rest on Los Angeles County data, since we found no Montebello study with a method. Your written proposal is the real price.
How do I build an ADU on my Montebello lot?
Start with Planning at 323-887-1478 to confirm which rules apply, then design, file plans through GoPost and build once the permit issues. A garage conversion, a detached unit and a JADU follow different paths. We design, permit and manage the job, and licensed subcontractors do the field work. The counter is open Monday to Thursday.
Tell us about your Montebello ADU
Tell us the lot's neighborhood, whether you are thinking of a garage conversion, a detached unit or a JADU, the year the house was built and whether the garage is permitted, and we will plan the first visit around that.