ADU Constructionin Monrovia, CA
Yes, ADU construction is allowed on Monrovia single-family lots through a ministerial building permit under Municipal Code 17.44.005. A lot can hold a converted ADU, a new detached unit of up to 800 square feet and one junior ADU. Foothill fire rules, landmark status and the state's 60 day clock decide the details.
Monrovia was laid out in 1886, so many of its backyards belong to houses a century old. The Monrovia Historic Preservation Group counts more than 2,500 homes built before 1940, and the Census Bureau's 2024 estimates count 14,855 housing units in the city with a median owner-occupied value of $909,400. At that price, a second small home behind the main one is a serious option for family, rent or both.
This page covers the unit types and sizes Monrovia's code allows, what a landmark or historic district lot adds, how the foothill fire zone and hillside rules touch a backyard unit, how garage conversions work, and what the state's clocks mean for your permit. Our statewide guide and cost table sit on the ADU construction page. The City's ADU pages and fee schedule were not readable because monroviaca.gov blocked automated access, so every local rule below comes from Municipal Code text on a third-party mirror, and the Building and Safety Division has the final word.
Monrovia Municipal Code rules that apply to ADUs
Where adu construction permits come from in Monrovia: City of Monrovia Community Development Department, Building and Safety Division.
- Monrovia's ADU rules sit in Municipal Code 17.44.005, sourced to Ord. 2020-10 and Ord. 2025-01, and are ministerial through a building permit. A single-family lot can have a converted ADU, a new detached ADU of up to 800 square feet with 4 foot side and rear setbacks, plus one junior ADU of 150 to 500 square feet.
- Under Monrovia's ADU code, a streamlined detached ADU is limited to 16 feet in height (18 feet within one half mile of a major transit stop, plus 2 feet for roof pitch), the alternative standards allow an attached ADU up to the lesser of 50 percent of the house or 1,000 square feet, and short term rentals are prohibited, with rentals of 30 consecutive days or more.
- Monrovia requires one parking space for an ADU but waives it for junior ADUs, streamlined ADUs, ADUs within one half mile of transit, ADUs in a historic district and garage or carport conversions. Owner occupancy is required only for junior ADUs, and a junior ADU needs a recorded deed restriction before final inspection.
- For an ADU on a designated Monrovia historic landmark or contributing property using the alternative standards, the code bars changes to the street facing facade, requires exterior changes to exactly match existing architectural features, and bars approval if a historic landmark must be demolished (Municipal Code 17.44.005(E)(5)).
Sources for these rules
Can you build an ADU behind a Craftsman or on a Monrovia landmark lot?
Often yes, with conditions. Monrovia has 169 local landmarks and two historic districts, the Wild Rose Tract and North Encinitas, and Municipal Code 17.44.005(E)(5) sets terms for an ADU on a designated landmark or contributing property that uses the alternative standards. The code bars changes to the street-facing facade, requires exterior changes to match existing architectural features exactly, and bars approval if a historic landmark would have to be demolished.
The code also waives the ADU parking space for lots in a historic district, which helps on narrow older parcels. Whether the simpler streamlined unit types need a certificate of appropriateness is a question we could not settle from the text we read, so ask Planning before the first drawing. Properties that are 70 years old are Historic Review Sites, which can be a lot in a town where the first lot sale was in 1886.
An older lot may also have a detached garage from the original house. That structure is often the cheapest place to start, and the garage section below covers how.
- Is this lot a landmark parcel, a district contributor or merely a Historic Review Site?
- Does the plan touch the street-facing facade at all?
How big can an ADU be in Monrovia, and which type fits your lot?
A single-family lot can hold three things under the code: a converted ADU inside existing space, a new detached ADU of up to 800 square feet with 4 foot side and rear setbacks, and one junior ADU of 150 to 500 square feet. The alternative standards allow an attached ADU of up to the lesser of 50 percent of the house or 1,000 square feet. State law sets a floor under local caps: a city must allow at least 800 square feet with 4 foot setbacks (Government Code 66321).
Height follows the same pattern. A streamlined detached unit tops out at 16 feet, or 18 feet within half a mile of a major transit stop, with 2 more feet to match a roof pitch. State law lets a detached ADU reach 1,200 square feet, but the City's own summary table could not be opened, so ask which standard governs a unit above 800.
The junior ADU is the one type with an owner-occupancy rule, and it needs a recorded deed restriction before final inspection. Our blog post on ADU versus JADU compares the two in plain terms.
Is your Monrovia lot inside the Very High fire zone?
The City Council held a public hearing on July 1, 2025 and adopted an ordinance updating the local fire hazard map to CAL FIRE's March 2025 recommendations, according to a press account from July 3. MonroviaNow reported on June 14 that the map would put 1,481 properties in the Very High zone, 619 in High and 754 in Moderate, almost all near the mountains. That article ran before the vote and named no neighborhoods.
The fire code sets defensible space by zone: 30 feet in Moderate, 100 in High and 200 in Very High, with 10 feet of clearance to bare mineral soil around outbuildings. In the Very High zone, a fence within 10 feet of a structure must be noncombustible, which matters if the ADU plan includes a new gate or privacy fence.
Here is what we could not confirm: the parcel-level map, how the code's Wildland Urban Interface Fire Area relates to the new zones, and whether a Class A roof rule applies to Very High parcels. The Fire Department page was blocked to us. Confirm your address with the Monrovia Fire Department before you pay for plans.
- Is the parcel in the Very High, High or Moderate zone on the adopted map?
- How much defensible space does that zone require around the new unit?
Hillside permits, grading and driveway grades for a Monrovia backyard unit
A Hillside Development permit applies to new single-family dwellings in the hillside zones and to earth movement that makes a footprint over 500 square feet, involves 5 feet or more of cut or fill, or builds a retaining wall 6 feet or taller (Municipal Code 17.12.010(H) and 17.12.020(H)). Swimming pools and basements are excluded from that trigger. The text does not name an ADU, so a pad cut into a slope is the case to raise with Planning.
Grading has its own chapter. Section 15.28.060 exempts excavation for footings, basements and pools covered by a valid building permit, and in flood hazard areas section 15.48.080 says a permit is required for all construction. Fire access is separate: the Fire Code limits road and driveway grades to 10 percent and calls for two ten foot traffic lanes (15.20.280).
Ember-resistant exterior rules, called Chapter 7A, apply to new buildings and additions in the Wildland Urban Interface Fire Area, with exceptions such as a Group U accessory building at least 50 feet from a house. Whether a given ADU falls inside that area is the same parcel question as above.
Can you convert a garage into an ADU in Monrovia, and what happens to the parking?
Yes, and it is the lowest-friction path. State law lets a city's building permit cover an ADU and a JADU made from existing space in a house or accessory structure, with up to 150 square feet added only for entry and exit, and a conversion needs no new setback. Monrovia requires one parking space for an ADU but waives it for garage and carport conversions, and state law bars cities from demanding replacement parking when a garage is converted.
On cost, UC Berkeley's 2021 survey of Los Angeles County households that finished or applied for an ADU in 2018 and 2019 found a median of about $90,000 for garage conversions, with response bias flagged by the authors. Our planning range for a garage conversion into a studio or one-bedroom today is $100,000 to $200,000, and the detached range is further down this page.
Units built without permits before January 1, 2020 have their own path under Government Code 66311.7, which limits a city's ability to deny them over building code violations unless the unit is substandard under Health and Safety Code 17920.3. A permit issued in Monrovia to legalize a structure expires 90 days after issue, so corrections should be ready to go.
- Does the old garage slab and footing need an engineer's review?
- Was the garage already converted without a permit?
Renting, selling and financing a Monrovia ADU
Short-term rentals are prohibited in Monrovia's ADU code, and rentals must run 30 consecutive days or longer. Owner occupancy is required only for a junior ADU. State law does not let a separate ADU be sold apart from the house, except to a qualified low-income buyer through a qualifying nonprofit or where a city has adopted a condominium ordinance. We have not read whether Monrovia has one, so ask before planning around a sale.
On financing, HCD's ADU handbook says FHA, Freddie Mac and Fannie Mae loans can count ADU rental income, FHA 203(k) works only for an attached unit, and the CalHFA ADU grant of up to $40,000 had run out of funds when the handbook was written. The same handbook says state law voids community rules that effectively bar an ADU on a single-family lot, which answers most HOA questions.
Property tax follows a simple idea from the State Board of Equalization: new construction is reassessed only on the new portion, at the market value it adds and not at its cost.
How long does an ADU permit take in Monrovia, and what do the fees cover?
Under state law the City must say within 15 business days whether an application is complete, then approve or deny within 60 days of a complete application, and silence means the permit is deemed approved. A denial has to list every defect, and an appeal gets an answer within 60 business days. These clocks took effect January 1, 2026 through SB 543.
Local timing rules still apply after approval. A Monrovia permit becomes invalid if work does not start within 12 months, plan review is treated as abandoned 180 days after filing, and fees are set by City Council resolution, so no dollar amounts appear here. State law adds that an ADU of 750 square feet or less pays no impact fee, while utility connection charges are separate.
Two build rules deserve a question to the Division. Monrovia's building code requires sprinklers in new buildings, while state law says an ADU needs none if the main house has none, and a new detached ADU needs solar under the Energy Code according to HCD's handbook. MDG Builders, CSLB #1040337, does the design, permits and project management, and licensed subcontractors build the unit.
ADU scenes from Monrovia and nearby foothill lots
Garage conversions, backyard units and site work from Monrovia and nearby communities. Tap a photo to enlarge it.
What adu construction costs around Monrovia
Our planning ranges are $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, and the ADU construction page explains each line. They lean on three sources that do not agree: UC Berkeley's survey put the Los Angeles County median at $100,000, or $197.22 a square foot, for projects from 2018 and 2019; Remodeling magazine's 2025 Cost vs. Value report lists $178,536 for Los Angeles with $71,300 of resale value, about 39.9 percent; and builders quote far more. We found no Monrovia price survey, and the City's permit and plan check fees could not be read, so they stay separate lines in a written proposal. Our ADU cost guide breaks down what moves the total, and the contractor license guide shows how to verify a builder. Compare rules in Arcadia, Azusa, Pasadena and El Monte. A unit that needs a new bath belongs with bathroom remodeling in Monrovia, new windows with Monrovia window replacement, and an attached unit may be a Monrovia room addition. See the Monrovia area page for everything else we do in town.
| 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 |
Monrovia ADU questions, answered
Can I build an ADU in a Monrovia fire zone?
Probably, with extra requirements. The City adopted an updated fire hazard map on July 1, 2025, and Monrovia's code applies ember-resistant exterior rules and defensible space of 30, 100 or 200 feet by zone. We could not confirm your parcel's zone or any roof class rule, so ask the Monrovia Fire Department for your address before paying for plans.
How big can an ADU be in Monrovia?
A new detached ADU under the streamlined standards can be up to 800 square feet with 4 foot side and rear setbacks. The alternative standards allow an attached ADU up to the lesser of 50 percent of the house or 1,000 square feet. State law lets a detached ADU reach 1,200 square feet, so ask which standard governs your plan.
Do I need to provide parking for my ADU?
Usually not. Monrovia requires one space for an ADU but waives it for junior ADUs, streamlined ADUs, units within half a mile of transit, lots in a historic district and garage or carport conversions. State law also bars replacement parking when a garage is converted. Check which waiver your lot qualifies for.
How much does it cost to convert a garage to an ADU?
Our planning range is $100,000 to $200,000 for a studio or one-bedroom garage conversion. UC Berkeley's survey of 2018 and 2019 Los Angeles County projects found a median near $90,000 for garage ADUs, but costs have risen since. Permit fees are extra, and Monrovia's amounts were not readable, so ask for them as separate lines.
What is the difference between an ADU and a JADU?
A junior ADU is smaller, 150 to 500 square feet in Monrovia, and it is carved from inside the existing single-family house. A regular ADU can be converted, attached or detached and is larger. Only the junior unit has owner occupancy and a recorded deed restriction before final inspection, and it needs no extra parking.
Can I rent my ADU on Airbnb or only for long-term rentals?
Only long-term. Monrovia's ADU code prohibits short-term rentals and requires rentals of 30 consecutive days or longer. State law lets a city set that 30 day floor. Owner occupancy is required only for a junior ADU, so you can rent a regular ADU while living elsewhere, if the lot's other rules allow it.
Can I sell my ADU separately from my main house?
Generally no. State law allows a separate sale only to a qualified low-income buyer through a qualifying nonprofit, or where the city has adopted a condominium ordinance. We have not read whether Monrovia has adopted one, so ask Planning before buying or financing on that assumption. Most owners keep both homes on one title.
How long does it take to get an ADU permit in Monrovia?
State law gives the City 15 business days to call an application complete and 60 days after that to decide, or the permit is deemed approved. That is the review clock, not the build time. We could not verify Monrovia's actual turnaround, so ask the Division, and note that its permits lapse after 12 months without a start.
Do I need a permit to convert my garage into an ADU?
Yes. Monrovia reviews ADUs ministerially through a building permit, and a garage conversion is a change of use that needs one, with plans showing the infilled door, egress and utilities. State law keeps that review limited to objective standards. Work done without a permit draws an investigation fee equal to the permit fee.
Can I build an ADU on a Monrovia landmark property?
Yes, under conditions. For an ADU using the alternative standards on a landmark or contributing property, the code bars changes to the street-facing facade, requires exterior changes to match existing architectural features exactly, and bars approval if a landmark must be demolished. Ask Planning whether the streamlined types need a certificate of appropriateness.
Tell us about your Monrovia ADU idea
Tell us the street, whether the lot sits north toward the foothills or in a historic area, what the garage or backyard looks like today and whether you want to rent or house family, and we will shape the first visit around that.