Costa Mesa ADU Regulations.

Ministerial review, and one city-built shortcut that is open today. Existing unpermitted units can enter the Safe ADU Legalization Program and come to code under a defined process instead of an enforcement case. New units go through standard ADU plan check in TESSA — the city's Pre-Approved ADU program is set up, but no approved plans are published yet. Complete applications run the 60-day state clock.

• Submission is portal-only: every Costa Mesa ADU application and plan set uploads through the TESSA online portal (Tyler EnerGov) — Building & Safety states plainly, “All applications are submitted through TESSA.” Avorino files and tracks it there.
• Pre-Approved ADU program: set up, but no plans published yet — plan on standard review
• Safe ADU Legalization Program: a defined amnesty path for unpermitted units
• Units allowed down to 150 sqft — micro-studios are legal here
• ADUs carry a recorded 31-day minimum rental restriction — no short-term rental
• Costa Mesa runs a Safe ADU Legalization Program (AB 2533) for units built without permits before 2020 — optional confidential pre-inspection, no penalties for prior work.
• A Pre-/Pre-Engineered ADU program is set up but has no published plans yet; the city also allows unusually small units (as little as 150 sq ft).

Costa Mesa's three sub-markets read differently on a site plan. Eastside lots run deep with mature trees and detached garages; Mesa Verde curves through wide ranch parcels; the Westside mixes small bungalow lots with R2 pockets where a second unit has often existed — permitted or not — for decades.

• Eastside: deep lots off 17th — the city's strongest detached-ADU geometry
• Mesa Verde: wide 1960s ranch parcels with generous side yards
• Westside: older small lots + R2 pockets — frequent legalization candidates
• No Coastal Zone overhead anywhere in the city — coastal-adjacent without the CDP
• Retail water is Mesa Water District — a special district, not the city — so the meter, will-serve, and connection all go through Mesa Water, whose specifications meter each separately-owned unit on its own service; we confirm meter sizing with the district rather than assuming the ADU shares the house's.
• Costa Mesa's low western and southern flats along the channelized Santa Ana River sit in a State liquefaction Zone of Required Investigation, so those parcels usually need a geotech study and engineered foundation.
• Local sewer is the Costa Mesa Sanitary District, but regional treatment is OC San — so OC San's capacity charge is in play on a new detached ADU.

Verify with the city

All Costa Mesa ADU, custom-home, and construction details on this page, including fees, setbacks, timelines, and zoning, are based on the City of Costa Mesa Building & Safety Division's published information and are subject to change. This information may not be fully up to date or accurate. For the latest requirements, visit the city's building and planning department directly, and confirm anything that affects your budget or schedule before you build.

Building authority

City of Costa Mesa Building & Safety Division

Official website

costamesaca.gov

See how every OC city & community takes ADU permits — the Permit Index ↗

Official Costa Mesa ADU resources this page draws from

• City of Costa Mesa — Accessory Dwelling Units (ADU) landing page
• City of Costa Mesa — Building & Safety: Permit Types & Submittal Requirements ('All applications are submitted through TESSA')
• TESSA online permit portal (Tyler EnerGov / Civic Access) — Costa Mesa
• City of Costa Mesa — TESSA FAQs / Videos (portal overview, launched Summer 2023)
• City of Costa Mesa — Pre-Engineered / Pre-Approved ADU Program (plans 'COMING SOON')
• City of Costa Mesa — Safe ADU Legalization Program

Costa Mesa sits between the coastal premium and inland averages. Eastside Costa Mesa parcels skew higher; Westside and Mesa Verde stay closer to OC averages. Detached unit budgets typically land in the $245K–$440K+ range; attached and Junior ADU options trim significantly. Rental income tracks $3,000–$4,500+/mo.

Detached ADU:

$245K–$440K+ typical — Eastside parcels trend toward the top

Attached ADU:

$200K–$350K+ typical

Junior ADU:

$135K–$220K+ typical

Permit + design fees:

$6K–$15K+ typical

Rental income (Costa Mesa range):

$3,000–$4,500+/mo long-term lease

How an Orange County ADU budget is built up, line by line — the same breakdown behind the Costa Mesa range above.

Run your numbers

What would an ADU pencil out to in Costa Mesa?

Two free tools, calibrated to 2026 Orange County bids — a cost range in about a minute, and the rental return behind it.

Costa Mesa is one of 37 cities covered by our Orange County ADU construction service.