ADUs in Costa Mesa
Backyard homes for Costa Mesa, priced before you sign.
We design, permit and build standard ADU models in Costa Mesa and across Orange County. You pick a model, we assess your lot, and you get one all-in price before you sign.
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The models
Four standard homes, each with a price up front.
Starting prices include design, permitting and construction with standard finishes. Site work and utilities generally run $15,000 to $35,000 and are priced after we see your lot, and city permit fees are billed at cost. All four are under the City's 1,200 sq ft limit for a detached ADU. See what's included.
Around Orange County
Buildoly projects across Orange County.
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Costa Mesa ADU rules in plain English
What you can build in the City of Costa Mesa.
The basics for a single-family lot in Costa Mesa. Every lot is different, so we confirm each of these for your address before we price anything.
How many
Two ADUs and a junior ADU
A lot with a single-family house can have one new ADU, attached or detached, one ADU converted from existing space, and one junior ADU.
CMMC 13-35(b)(4)(a)Size
Up to 1,200 sq ft detached
A detached ADU can be up to 1,200 sq ft. An attached ADU can be up to half the existing house or 1,000 sq ft, whichever is greater. A conversion inside existing space has no size cap, and an ADU can be as small as 150 sq ft.
CMMC 13-35(b)(5)(a), (c); (b)(6)Height
One story in most cases
A new ADU stays on the first floor, and nothing can top 27 ft or two stories. A second story is allowed on a lot with an existing house when five conditions are met, such as keeping every second-floor part at least 25 ft from the front property line, or when the ADU is built entirely inside the house.
CMMC 13-35(d)(9)Setbacks
4 ft from the side and rear
The front follows the zone's setback, unless that would rule out an 800 sq ft ADU under state law. A second-floor ADU follows the zone's setbacks, and parts of an ADU converted from existing space need none, as long as fire and life safety setbacks are met.
CMMC 13-35(d)(8)Parking
Parking is optional
The City's code lets you add a parking space but doesn't require one. Parking lost to a garage conversion, or to a garage torn down where the ADU is built, doesn't have to be replaced.
CMMC 13-35(d)(15)Renting and selling
Rent for 31 days or more
An ADU or junior ADU can't be rented for less than 31 days, and it can't be sold separately from the house except as the law allows.
CMMC 13-35(b)(8), (c)(4), (d)(3)Worth knowing in Costa Mesa
Units built under state law skip the City's development standards: a conversion, a converted junior ADU, or a new detached ADU of up to 800 sq ft with 4 ft side and rear setbacks and up to 18 ft tall, with two more feet allowed to match the house's roof pitch.
Costa Mesa's ADU code has no owner-occupancy rule for an ADU. A junior ADU can be up to 500 sq ft, and under state law it needs the owner living on site only when it shares a bathroom with the house. We confirm how the City applies that to your plan.
On a lot with an existing apartment building, the City allows up to eight detached ADUs, never more than the units already there.
Changing on January 1, 2027
SB 1117 limits the impact fee on an ADU over 750 sq ft to the area above 750 sq ft, in proportion to the main house, for up to two ADUs on a lot. ADUs of 750 sq ft or less still pay no impact fee.
AB 956 lets you build two new detached ADUs on a single-family lot through the state's approval path. On that path a city can cap each one at 800 sq ft.
Sources: Costa Mesa Municipal Code Section 13-35, as amended through Ordinance No. 2025-07 (adopted October 21, 2025); California Government Code sections 66310 to 66342; SB 1117 and AB 956 (2026). Checked November 2026.
How it works
From your lot to move-in.
Step 1
Pick a model
Choose a model and finishes online, or book a free consultation to talk it through.
Step 2
Site assessment
We visit your lot, check utilities, access and the City's rules, and give you one all-in price before you sign.
Step 3
Design and permits
We finish the plans and run the permit with the City of Costa Mesa from submittal to approval.
Step 4
Build and move in
We build on site and manage the job through final inspection and your certificate of occupancy.
Typical timeline: 8 to 12 months from start to finish. About 1 month of design, 2 to 4 months of permits and 3 to 5 months of construction. Your lot and the City's review times can move it.
Costa Mesa ADU questions
What Costa Mesa homeowners ask us.
Our models start at $165,000 for Model S, $195,000 for Model 1, $270,000 for Model 2, $360,000 for Model 3. Those are all-in starting prices for design, permitting and construction. Site work and utilities generally run $15,000 to $35,000, and we price yours after the site assessment, before you sign. City permit fees are separate: your city sets them and we bill them at cost. You start with $1,000, credited in full to your contract, and many of our clients finance the whole build through our lending partners, subject to credit approval. See what's included in the price.
A detached ADU can be up to 1,200 sq ft, and an attached ADU up to half the existing house or 1,000 sq ft, whichever is greater. A conversion inside existing space has no size cap. All four of our models are under 1,200 sq ft. We check the exact limit for your lot.
Our typical timeline is 8 to 12 months from start to finish: about 1 month of design, 2 to 4 months of permits and 3 to 5 months of construction. Your lot and the City's review times can move it either way, and we give you a schedule for your address after the site assessment.
No. Costa Mesa's code lets you add a parking space, at least 8.5 by 18 ft, but doesn't require one. Parking lost to a garage conversion, or to a garage torn down where the ADU is built, doesn't have to be replaced.
Sometimes. A new ADU stays on the first floor unless it's on a lot with an existing house and meets five conditions, such as keeping every second-floor part at least 25 ft from the front property line, or it's built entirely inside the house. No ADU can top 27 ft or two stories.
Yes. We build across Orange County, as well as Los Angeles County, San Diego and the Inland Empire. Each city has its own ADU rules and plan check, and we confirm them for your address. Enter your address in our quote tool to check that you're in our service area.
Where we build
We also build in these cities.
Each city page covers that city's ADU rules and who issues the permit. All cities.
Los Angeles County
Orange County
San Diego County
Inland Empire
See your price for a Costa Mesa ADU.
Pick a model and tell us your address. After the site assessment you get one all-in number, before you sign.