What does an 8,386-square-foot lot on East 18th Street turn into when the owners use California's lot-split law? According to the City of Costa Mesa's approval file for 378 E. 18th St., the parcel already held two houses. It will be divided into lots of 4,760 and 3,626 square feet, both houses will be torn down, and each new lot will get a two-story home with a first-floor accessory dwelling unit and a one-car garage.
That is about as complete as an SB 9 split gets on the Eastside, and very few happen. Costa Mesa's own reports to the state show that the extra capacity on these lots almost always becomes a backyard unit, not a new parcel. For anyone weighing an Eastside home on the strength of its lot, that difference shapes what the extra land is actually worth.
The paperwork that comes before the second house
The friction in a lot split shows up long before construction. Under state law, anyone applying for an urban lot split signs an affidavit saying they intend to live in one of the units as their principal residence for at least three years after approval. Qualifying community land trusts and nonprofits are exempt, and cities can't add other owner-occupancy rules on top.
Next comes the map. Costa Mesa's approval letter for 378 E. 18th St. tells the applicants to file a final map with Public Works and the County of Orange. It also says building permits can't be issued until the city has evidence that the final map was recorded with the county. So a split is a subdivision first and a building project second. The owner pays for surveying, mapping and recording before anyone can pull a permit for the house the split was meant to allow.
The city process itself has gotten faster and more predictable. SB 450 took effect on January 1, 2025. It requires cities to decide on a complete application within 60 days, and an application not decided in time is deemed approved. It also limits cities to the same standards they apply to other housing development, not stricter rules written just for SB 9 projects. SB 450 also removed the old limit on demolishing more than 25 percent of exterior walls for a two-unit project, and it narrowed the grounds for denial to a specific, unmitigable adverse impact on public health and safety.
Costa Mesa adopted its SB 9 rules through Urgency Ordinance 2021-21, dated December 7, 2021. The 2025 approval for 378 E. 18th St. still cites that ordinance's standards. After SB 450, review times aren't the main obstacle. The occupancy commitment and the subdivision step that has to come first are.
What the city's count shows
Every year, Costa Mesa files a Housing Element Annual Progress Report with the state. These counts are citywide, not Eastside only, but they show how owners actually use their single-family lots.
| Reporting year | ADU permits | ADUs completed | SB 9 duplex projects permitted | SB 9 lot-split projects permitted | All units permitted citywide |
|---|---|---|---|---|---|
| 2024 | 92 | 66 | 3 | 2 | 123 |
| 2025 | 89 | 73 | 0 | 0 | 230 |
In 2024, ADUs made up 92 of the 123 units Costa Mesa permitted, about three of every four. In 2025, the city permitted 89 ADUs and 13 detached single-family homes, and no units under either SB 9 path.
The 2025 applications table adds detail. It records approved SB 9 lot splits at 2197 Santa Ana Ave. on January 21, 2025, at 910 Dogwood St. on March 20, 2025, and at 378 E. 18th St. on May 5, 2025. The first two are listed as "SB9 subdivision only, no additional units proposed at this time." The 378 E. 18th St. approval letter is dated June 12, 2025, so the city's two records give different dates for the same case. Over that same year, the applications table is full of ADU entries at addresses like 1911 Orange Ave. and 290 Walnut St.
What a split actually leaves on the ground
The city's SB 9 decisions page lists seven approvals, and nearly all of them are on Eastside streets. The files show that the Eastside's "big lot" turns into two fairly small ones:
- 278 Broadway: approved November 21, 2023. A 6,560-square-foot lot becomes parcels of about 3,456 and 3,104 square feet, each with a two-story home and an attached two-car garage.
- 316 22nd St.: A 6,489-square-foot lot becomes parcels of 3,780 and 2,709 square feet, with detached homes of 2,054 and 2,002 square feet.
- 212 Del Mar: A 7,197-square-foot lot becomes parcels of 3,909 and 2,794 square feet, each with a single-family home and an attached garage.
- 910 Dogwood St.: A 72-by-100-foot site becomes two 3,600-square-foot parcels. The existing house and garage are marked for demolition, and each new lot shows a two-story home with an "ADU or JADU."
- 212 Broadway: This one isn't a split. The September 28, 2023 staff report describes a 6,560-square-foot lot that keeps its existing house and adds a 1,102-square-foot second-story dwelling over new garages.
The pattern holds across most of these files. On the Eastside, a split usually means demolition, two new houses, and parcels under 4,000 square feet. That makes it a ground-up development project. The 212 Broadway file shows the other way to add a unit, where the original house stays and a second dwelling goes in beside or above it.
Why the backyard unit wins
Costa Mesa's ADU page lays out a much lighter path. ADUs and junior ADUs are allowed in every residential district, the minimum size is 150 square feet, parking is optional, and design is governed by objective standards such as second-story setbacks. No subdivision map is involved.
UC Berkeley's Terner Center studied SB 9's first year in a sample of 13 jurisdictions and found little to no activity. Through November 2022, Los Angeles had 28 lot-split applications, and San Diego had three. The Terner Center's analysis lists the barriers as high construction costs, homeowners without building experience, restrictive local rules, and lots that are too small or oddly shaped. Planners it interviewed said ADUs' clearer state rules and flexibility pull homeowners in that direction. They also named SB 9's owner-occupancy condition and the costs of subdivision and construction-defect exposure as obstacles.
That research is statewide and dates from early 2023, so it doesn't explain Costa Mesa specifically. But Costa Mesa's filings match it closely. Planners also told the Terner Center they got many inquiries that never became applications. Interest in splitting a lot doesn't mean a split is likely.
Where the larger change is actually coming from
State law has changed the Eastside in a bigger way through a different route. As the Daily Pilot reported on September 30, 2026, the planning commission approved Eira, 21 single-family ownership homes on a 1.49-acre former mobile home park at 396 E. 21st St., near Tustin Avenue. Because one unit is set aside for a very low-income household, the project qualifies for a state density bonus. That bonus requires the city to waive standards on density, height, setbacks, open space and parking, and the homes will rise to 31 feet, 6 inches. The Daily Pilot quoted one commissioner as saying, "Because of state housing law, we really don't have any other option but to approve this project." Irvine-based Modern Mobile Ventures is redeveloping the site, and construction could take 18 to 24 months.
Eira also needed a 1.49-acre site that had been vacant for several years, which is far larger than any of the single-family lots on the city's SB 9 list.
Reading "lot potential" in an Eastside listing
For Eastside buyers, this changes what a large lot's upside means in practice. The filings point to three questions:
- Can the lot fit an ADU under the city's objective standards, given where the existing house sits?
- Does the property already have permitted ADU work, or approved plans, on file with the city?
- If a split is part of the appeal, how would the resulting parcels compare with the 2,700-to-3,900-square-foot lots in the approvals above, and does the owner-occupancy affidavit fit the buyer's plans?
The wider market gives buyers some room to look into these questions. The California Association of Realtors reported an Orange County median of $1,452,500 for existing single-family homes in August 2026, up 4.9 percent from a year earlier, with a median of 27 days on market. Citywide in Costa Mesa, one closed-sales tracker put the median sale price at $1.4 million over the six months through its October 3, 2026 update, at $789 per square foot. Neither figure isolates the Eastside. A citywide price per square foot measures the house, and the lot's value depends on what the city will actually let you build on it. Our Eastside Costa Mesa neighborhood page and buyer resources cover the rest of the purchase process.
A short FAQ
Do the ADU and SB 9 counts cover only the Eastside? No. The Annual Progress Report figures are citywide. The Eastside link comes from the addresses on the city's SB 9 decisions page, which are almost all on Eastside streets.
How long does Costa Mesa have to decide on an SB 9 application? Since January 1, 2025, state law has required a decision within 60 days of a complete application. An application not decided in that window is deemed approved.
Can construction start as soon as a split is approved? Not under the terms of the 378 E. 18th St. approval. That letter says building permits can't be issued until the final map has been recorded with the County of Orange.
If an Eastside property's appeal depends on its lot, the useful question is what can actually be permitted and built there. Mason | Taylor Associates can review a specific lot's permit history and the city's current ADU and SB 9 standards with you before you write an offer. Schedule a confidential buying or selling consultation.
This article is general information, not legal, tax or financial advice. Consult a qualified professional about your situation.
