ADU Constructionin Arcadia, CA
ADU construction in Arcadia is approved ministerially, and the City must act on a complete application within 60 days or it is deemed approved. A detached unit up to 800 square feet needs only a building permit, and larger units are capped at 850 or 1,000 square feet. Protected oaks and sycamores can still decide where it goes.
Arcadia's first residential subdivisions arrived in the 1930s, and the City's General Plan says the southern part of the old Baldwin Ranch turned from poultry and produce farms into moderately priced homes while the population more than quadrupled between 1940 and 1960. The Census Bureau's 2024 estimates show what that left behind: 11,941 of 20,992 homes are single-family detached houses, and 69 percent of all homes predate 1980. Many owners are now weighing a backyard unit against an aging house.
We handle an ADU as MDG Builders, CSLB #1040337. Our part is the design, the filing with Building Services and the schedule, and licensed concrete, framing, plumbing and electrical subcontractors do the building. The City keeps its own ADU page, our cost table and phases are on the ADU construction page, and the Arcadia overview lists our other trades here.
What Arcadia's ADU ordinance and state law say about building an ADU
Where adu construction permits come from in Arcadia: City of Arcadia Development Services Department, Building Services Division.
- The City's ADU ordinance (Development Code Section 9102.01.080, amended through Ordinance 2401) approves an ADU permit ministerially, without a hearing, and says the City must approve or deny a complete application within 60 days or it is deemed approved unless the applicant asks for a delay.
- The ordinance allows a conversion ADU and a JADU with only a building permit, and one new detached ADU of up to 800 square feet, at least 4 feet from side and rear lines and within the height limit, with only a building permit. A JADU may be no more than 500 square feet and must sit inside the existing or proposed single-family house.
- A detached ADU may be up to 16 feet high, up to 18 feet within one half mile walking distance of a major transit stop or high quality transit corridor, and up to 2 feet more to match the roof pitch of the house. An attached ADU may be up to 25 feet or the zone limit, whichever is lower, and no more than two stories.
- An ADU that needs the full ADU permit is limited to 850 square feet for a studio or one bedroom and 1,000 square feet for two or more bedrooms, but the rules say no FAR, coverage or open-space limit may force it below 800 square feet. The ordinance uses a 45 percent FAR cap, 45 percent lot coverage for a one-story house and 35 percent for a two-story house, and at least 50 percent open space, with 4 foot side and rear setbacks.
- The ordinance requires one off-street parking space per ADU, which may be in a setback or tandem, but none is required when the ADU is within one half mile walking distance of public transit, in a historic district, part of the existing home or an existing accessory structure, or when a new home and ADU are built together. Parking removed for the ADU does not have to be replaced.
- The ordinance requires an independent exterior entrance, down-lighted exterior lighting, matching exterior materials and roof slope, clerestory and obscure windows above nine feet, no loft or mezzanine, and the same driveway as the house unless the Fire Department requires otherwise. It bars rentals shorter than 28 days and separate sale of an ADU, and a JADU needs an owner on site and a recorded deed restriction.
- The ordinance charges no impact fee for an ADU under 750 square feet and a proportional fee above that, requires fire sprinklers in an ADU only if the main house has them, and says an ADU on a property listed in the California Register of Historic Resources must not be visible from a public right of way.
- The City's Planning Fees sheet (Resolution 7684, updated 7/1/26) lists an ADU Permit at $775 and a Zoning Clearance at $383. The fee sheet comes out of the PDF as scattered text, and these two amounts sit on the same line as their labels.
Sources for these rules
Which Arcadia ADU rules apply: the ordinance or the older Single Family standards?
The City posts two documents that both discuss ADUs, and they do not agree. The Accessory Dwelling Unit ordinance, Development Code Section 9102.01.080 as amended through Ordinance 2401, is the current text. The Single Family Developments Standards PDF still carries an older ADU section that refers to the rules in force before 2020. We built this page from the ordinance, and Planning can say which document the counter applies to your plans. Where either one conflicts with state law, state law controls.
State law sets the floor. Government Code sections 66310 to 66342 require ministerial approval, guarantee 800 square feet with side and rear setbacks of no more than 4 feet, and do not let a city demand that the owner live on site for an ADU. We did not open the City's ADU Development Standards sheet, revised May 2025, or its flyers, so check them before the plan set is final. Our explainer on ADU versus JADU in California covers the difference between the two unit types.
How big can an ADU be in Arcadia, and how tall?
Under the ordinance, one new detached ADU of up to 800 square feet, at least 4 feet from the side and rear lines and within the height limit, needs only a building permit. A bigger unit takes the full ADU permit and is capped at 850 square feet for a studio or one bedroom and 1,000 square feet for two or more bedrooms, although the text adds that no floor area, coverage or open space rule may push a unit below 800 square feet. Its yardsticks are a 45 percent floor area ratio, 45 percent lot coverage for a one-story house and 35 percent for a two-story house, and at least 50 percent open space.
A detached unit may stand 16 feet high, or 18 feet within a half mile walk of a major transit stop or high quality transit corridor, with up to 2 more feet to match the house's roof pitch. An attached unit may reach 25 feet or the zone limit, whichever is lower, in no more than two stories. The ordinance also wants a separate exterior entrance, down-lighted fixtures, materials and roof slope that match the house and clerestory or obscure glass on windows above nine feet, and it bans lofts and mezzanines.
Can you build an ADU near an oak or sycamore tree in Arcadia?
Often not within 15 feet. The City's Tree Preservation chapter protects coast live and Engelmann oaks over 4 inches in trunk diameter, other oaks over 12 inches, western sycamores over 6 inches and other mature trees over 12 inches that stand in a required yard setback. No building, structure, wall or impervious paving may sit inside a protected zone, and that zone is never less than 15 feet from the trunk.
That rule settles more Arcadia ADU sites than the size caps do, because the easiest spot for a backyard cottage is often the spot under the best tree. The slab, the walkway and a widened driveway all count as impervious paving or structure. We locate every qualifying tree on the survey before the first sketch and ask Planning what the chapter requires before any tree is cut or any trench is dug near one.
Can you convert a garage to an ADU in Arcadia, and what happens to the parking?
Yes. The ordinance allows a conversion ADU and a junior ADU with only a building permit, and it says parking removed for the ADU does not have to be replaced. That matters in a city whose single-family standards otherwise call for two garage spaces on a lot under 5,000 square feet with up to four bedrooms. A junior ADU may be no more than 500 square feet and must sit inside the existing or proposed house.
One off-street space is required for an ADU, in a setback or in tandem, but none is needed within a half mile walk of public transit, in a historic district, when the unit is part of the existing home or an existing accessory structure, or when a new house and ADU are built together. Sprinklers go in the ADU only if the main house has them. Our garage conversion permit steps show the checklist the City of Los Angeles uses, a useful comparison for what Arcadia's counter will ask.
Can you build two homes or split the lot under SB 9 in Arcadia?
Arcadia's SB 9 handout, dated December 2021, allows two units on a lot zoned R-M, R-0 or R-1, each no larger than 800 square feet and 16 feet high, with one off-street space per unit. An urban lot split needs both new lots to be over 1,200 square feet, with a split no more uneven than 60:40.
The handout's other conditions rule out much of the foothill. The lot may not be a historic landmark or in a historic district, and may not sit in a high fire hazard area, fault zone, wetland or 100-year flood zone, and the owner must be an individual. The handout was written in 2021, so confirm each condition with Planning before you count on it.
Fire, fault and HOA checks before you build an ADU above Orange Grove Avenue
The City's February 2025 newsletter says the areas above Orange Grove and Sycamore avenues are Very High fire hazard severity zone, and the City's own Wildland-Urban Interface rules apply there. Its handout, dated 2011 and still to be confirmed with Building Services, lists Class A roofing, one-hour eaves and mesh vents. The ordinance says an ADU uses the house's driveway unless the Fire Department requires otherwise. Whether the City has formally adopted the 2025 CAL FIRE map is unconfirmed, so ask which map governs your parcel.
The County's hazard layers also show an Alquist-Priolo fault zone and liquefaction areas along parts of the city, so a soils report and a parcel lookup decide, not a map glance. Five HOAs have Architectural Review Boards. HCD's ADU Handbook says covenants that effectively prohibit or unreasonably restrict an ADU on a single-family lot are void and that AB 130 bars HOA fees or financial requirements, but a board can still review design, so plan on submitting drawings.
How long does it take to build an ADU in Arcadia, and can you rent it?
The ordinance approves an ADU permit without a hearing and says the City must approve or deny a complete application within 60 days, or it is deemed approved unless the applicant asks for a delay. State law adds a written completeness check within 15 business days. Our planning ranges, which are estimates and not published data, run 4 to 10 weeks for design, 3 to 5 months of construction for a garage and 5 to 9 months for a detached unit.
The ordinance charges no impact fee for a unit under 750 square feet and a proportional fee above that. We print no fee amounts, because the City's fee sheet extracts from its PDF in scattered pieces and we could not tie amounts to labels with confidence, and utility connection charges sit outside the 750 square foot exemption in state law. Your proposal lists each fee once Planning confirms it.
The ordinance bars rentals under 28 days and separate sale of an ADU, and a JADU needs an owner on site with a recorded deed restriction. State rental language runs 30 days, and since January 1, 2026 a JADU needs the owner only when sanitation is shared, so ask Planning which wording the counter uses.
ADU scenes from Arcadia and nearby communities
Six ADU scenes tied to Arcadia and places close by such as Sierra Madre, Monrovia and Temple City, from slab and framing to finished backyard cottages. Tap a photo to enlarge it.
What adu construction costs around Arcadia
On our ADU cost table, a garage conversion runs about $100,000 to $200,000 and a new detached unit of 400 to 800 square feet about $200,000 to $400,000, ranges built from studies published between 2018 and 2026. The 2025 Cost vs. Value report prices a Los Angeles ADU at $178,536, of which 39.9 percent came back at resale. No Arcadia ADU price study turned up, so read these as regional benchmarks. The ADU cost guide names the sources, and choosing between a room addition and an ADU helps when a backyard unit competes with space in the house. Monrovia, Temple City and Duarte are separate cities with their own rules; to compare, see Pasadena, El Monte, Alhambra and Los Angeles. A lot with a unit often also needs a room addition plan or bathroom work in Arcadia in the main house. Our license check guide helps you vet any ADU builder, us included.
| 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 |
Arcadia ADU size, tree, parking and permit questions
How big can an ADU be in Arcadia?
One detached ADU up to 800 square feet needs only a building permit. A full ADU permit allows 850 square feet for a studio or one bedroom and 1,000 square feet for two or more bedrooms, and no coverage or floor area rule may force a unit below 800. A junior ADU tops out at 500 square feet.
Do I need to provide parking for my ADU in Arcadia if I convert a garage?
No. The ordinance says parking removed for the ADU does not have to be replaced. A new ADU generally needs one off-street space, but none is required near transit, in a historic district, inside the existing house or an existing accessory structure, or when a new home and ADU are built together.
Do I have to live on the property to build or rent an ADU in Arcadia?
Not for an ADU under state law, which does not let a city require owner occupancy. Arcadia's ordinance asks for an owner on site only for a JADU, while state law since January 2026 limits that to shared sanitation. Planning can say which wording the counter applies to your lot.
Can I build more than one ADU on my Arcadia property?
On a single-family lot the ordinance allows a conversion ADU, a JADU and one new detached ADU of up to 800 square feet, each with only a building permit. State law lists the same protected combinations. Tree zones, setbacks and the lot's coverage still limit the plan, so Planning should confirm the combination.
How long does it take to build an ADU and get a permit in Arcadia?
The City must decide a complete application within 60 days or it is deemed approved, after a written completeness check in 15 business days. Our planning estimates add 4 to 10 weeks of design and 3 to 5 months of construction for a garage or 5 to 9 months for a detached unit.
How much do ADU permits and fees cost in Arcadia?
We print no amounts, because the City's fee sheet came out of its PDF in pieces and we could not match numbers to labels reliably. The ordinance charges no impact fee under 750 square feet and a proportional one above. Your proposal lists every City fee once Planning confirms the schedule.
How much does it cost to build an ADU in Arcadia?
Regional studies put a garage conversion at about $100,000 to $200,000 and a new detached unit of 400 to 800 square feet at about $200,000 to $400,000. We found no Arcadia study. Trees, slope, utility runs and finish level move the number, and a written proposal for your lot sets it.
Can I build an ADU near an oak tree in Arcadia?
Only outside the protected zone, which is never less than 15 feet from the trunk. The City's tree chapter bars buildings, walls and impervious paving inside it for coast live and Engelmann oaks over 4 inches, other oaks over 12 inches and western sycamores over 6 inches, so the unit moves or the plan changes.
Can my HOA stop me from building an ADU in Arcadia?
Not outright. HCD's handbook says covenants that effectively prohibit or unreasonably restrict an ADU on a single-family lot are void, and AB 130 bars fees or financial requirements. An Architectural Review Board may still review the design, and Arcadia has five HOAs, so confirm yours before drawings are final.
Can I rent my Arcadia ADU on Airbnb?
Not for short stays. The ordinance bars rentals shorter than 28 days, and state law lets a city require terms longer than 30 days, so the two wordings differ slightly. Ask Planning which one the counter applies. Long-term rental and family use are the intended paths, and the unit cannot be sold separately.
Tell us where the ADU would go
Tell us the street, any oak or sycamore near the spot you have in mind, whether a garage could convert and whether the lot sits above Orange Grove Avenue, and we will check trees, fire map and HOA before the visit.