A Costa Mesa ADU can be up to 1,200 square feet detached, up to two stories and 27 feet tall, with 4-foot side and rear setbacks and optional parking. Costa Mesa also allows efficiency ADUs as small as 150 square feet, which is unusual in Orange County. Those rules come from Costa Mesa Municipal Code § 13-35 and the city’s ADU page.
Costa Mesa is the most expensive ADU market I track, but it also has the highest ADU rents. Below are the rules, the costs, and the math I use with buyers who want to own here.
Costa Mesa ADU Rules at a Glance
| Rule | Single-family lot | Multifamily lot |
|---|---|---|
| How many | 1 detached ADU + 1 conversion ADU + 1 JADU | Conversions up to 25% of existing units (min. 1), plus up to 8 detached, not more than existing units |
| Detached size | Up to 1,200 sq ft | 850 sq ft, or 1,000 sq ft with 2+ bedrooms |
| Attached size | Greater of 50% of the home or 1,000 sq ft | Greater of 1,000 sq ft or 50% of average unit size |
| JADU | Up to 500 sq ft, inside the home or attached garage | Not allowed |
| Minimum size | 150 sq ft efficiency unit | 150 sq ft efficiency unit |
| Height | 27 ft or 2 stories for new construction | Same |
| Side and rear setbacks | 4 ft; none for conversions or same-footprint rebuilds | Same |
| Distance from main home | 6 ft minimum | 6 ft minimum |
| Parking | Optional; not replaced when a garage is converted | Same |
| Rental term | 31 days minimum | 31 days minimum |
| Approval | Ministerial, no public hearing | Ministerial |
Always confirm current standards with the Costa Mesa Planning Division at (714) 754-5245 before you buy. The state rules every city must follow are on the California HCD ADU page.
Costa Mesa Design Rules That Trip People Up
Costa Mesa’s objective design standards are stricter than many OC cities. These are the ones I flag on tours:
- Second-story ADUs must be at least 25 feet from the front property line.
- Second-floor side windows within 25 feet of a neighbor’s home need a sill at least 5 feet high.
- No new roof decks, and front-facing balconies need a 20-foot setback.
- Exterior stairs can’t sit on the front facade within 20 feet of the street.
- The ADU entrance can’t be on the same side as the main front door unless the director approves.
- Materials and colors must match the main home.
If you want a two-story ADU or one above the garage, these rules decide whether the design fits. I compare both layouts in Detached vs. Attached ADU in Costa Mesa.
What Lot Size Do You Need?
Many Costa Mesa single-family lots run 6,000 to 7,500 square feet, which fits a detached ADU of 800 square feet or more with room to spare. Corner lots, alley access and deep rear yards make it easier. I run the numbers by lot size in What Lot Size Do You Actually Need to Add an ADU in Costa Mesa?
Costa Mesa ADU Fees and Costs
- Impact fees: ADUs of 750 square feet or less pay none. Starting January 1, 2027, SB 1117 means larger ADUs only pay on the space above 750.
- School fees, plan check and permit fees still apply.
- Build cost: A 2-bed, 2-bath, 850 square foot detached ADU I’m working on in Orange County is budgeted at about $300,000. Costa Mesa builds often run higher because of design rules and two-story layouts.
- Garage conversions cost less. My OC garage conversion cost breakdown has the ranges.
What ADUs Rent For in Costa Mesa
In my October 2026 Costa Mesa ADU report, ADU rents on listed and sold homes ranged from $3,500 to $5,750 a month. A new 1,200 square foot ADU at 1822 Pomona is renting for $5,000. Those are the highest ADU rents I see anywhere in Orange County, driven by proximity to the beach, South Coast Plaza, and the jobs around John Wayne Airport.
Here’s how Costa Mesa compares with the other cities I track:
| City | Entry point for ADU homes | ADU rents |
|---|---|---|
| Costa Mesa | About $1.6M to $1.9M and up | $3,500 to $5,750 |
| Garden Grove | Closed $1.02M to $1.2M | $2,350 to $3,800 |
| Anaheim | From $968K | $2,800 to $3,450 |
| Westminster | From about $1.15M | $3,300 to $4,300 |
The full ranking is in Best Cities to Buy a Home and Add an ADU.
The Payment Math on a Costa Mesa ADU Home
I compare the total monthly payment (principal, interest, taxes and insurance) with total rent. No cap rates, just what comes in and what goes out.
Example: a $2,200,000 Costa Mesa home with a 1,200 square foot ADU renting for $5,000. I estimated the main house at about $6,500. Rate is 7.75% for an investment loan, taxes at about 1.15%, and $200 a month for insurance.
| 25% down ($550,000) | 50% down ($1,100,000) | |
|---|---|---|
| Monthly payment (PITI) | About $14,130 | About $10,190 |
| Total rent | $11,500 | $11,500 |
| Monthly difference | About -$2,630 | About +$1,310 |
At 25% down, Costa Mesa is an appreciation play, not a cash flow play. At 50% down it cash flows. Many of my Costa Mesa buyers live in the main house and rent the ADU, which turns a $14,000 payment into about $9,100 out of pocket. I show that approach in House Hacking vs. Investment Property.
Which Costa Mesa Neighborhoods Work Best?
- Eastside: Highest values and rents, often $2M+ for a teardown. See Eastside Costa Mesa.
- Westside: The value play, with more fixers and bigger price gaps.
- Halecrest and Mesa Verde: Family neighborhoods with larger lots. I explain why Halecrest wins on value in The Best Costa Mesa Neighborhoods for ADU Investing.
Frequently Asked Questions
How big can an ADU be in Costa Mesa?
Up to 1,200 square feet detached on a single-family lot. An attached ADU can be the greater of 50% of the home or 1,000 square feet. A JADU can be up to 500 square feet.
Does Costa Mesa require parking for an ADU?
No. Parking is optional, and the city can’t require you to replace garage parking you convert into an ADU.
Can I build a two-story ADU in Costa Mesa?
Yes, up to 27 feet or two stories, as long as the second story is at least 25 feet from the front property line and meets the window and balcony rules.
Can I rent my Costa Mesa ADU on Airbnb?
No. ADUs and JADUs must be rented for at least 31 days.
What does an ADU rent for in Costa Mesa?
On current listings and recent sales, $3,500 to $5,750 a month depending on size and age.
See Which Costa Mesa Homes Can Fit an ADU
Costa Mesa listings move fast when the price is right. I’ll show you which homes fit a permitted ADU, what it should rent for and what your payment looks like before you offer.
👉 Book a buyer ADU consult call to go through your options with Dylan
Prefer to talk now? Call Dylan Serna, ADU Realtor at 714-860-2868.
Dylan Serna | The ADU Realtor | eXp Realty | DRE #02217359
This post is general information, not legal, tax or financial advice. Rents and payments are estimates; confirm rules for any specific property with the city.
