In short
- 151 ADU permits on record, nearly all since 2020.
- 27 in 2025, down 34% from 2024.
- Most common kind: detached ADUs (52% of permits with a known type).
About this data
- Source: California HCD Housing Element Annual Progress Reports. The state data does not say whether an ADU is detached, attached, a garage conversion or a JADU, so most records show as 'unknown'. Permit valuations are not published in this source. Timelines use the application date the city reported to the state, matched to the permit where possible; some cities report their planning application date, so timelines can include planning review. Each project is counted once, in its building-permit year (or completion year when no permit date was reported).
Permits by year
Permits by year and type (table)
| Year | All ADU permits | Garage conversions | Detached |
|---|---|---|---|
| 2018 | 4 | — | — |
| 2019 | 3 | — | — |
| 2020 | 6 | 0 | 1 |
| 2021 | 17 | 0 | 1 |
| 2022 | 23 | 2 | 8 |
| 2023 | 30 | 4 | 15 |
| 2024 | 41 | 8 | 20 |
| 2025 | 27 | 2 | 10 |
“—” means the source for that year does not report ADU type.
Read more: ADU types explained
ADU rules in San Marcos
State law check: No HCD review letter on file; that is not a finding of compliance.
| Rule | San Marcos' code | State law guarantees |
|---|---|---|
| Maximum size | 50% of the house, but at least 850 sq ft; 850 sq ft with one bedroom, 1,000 sq ft with more. Detached on lots of an acre or more: 1,000 sq ft. Junior ADU 500 sq ft. | At least 850 sq ft (1,000 sq ft with 2+ bedrooms) must be allowed; an 800 sq ft detached ADU always is. Conversions of existing space have no size cap. Junior ADU: up to 500 sq ft. |
| Height | Detached: 16 ft (18 ft near a major transit stop, plus 2 ft for a matching roof; 18 ft on a multistory multifamily lot). Attached: 25 ft or the house's limit, whichever is lower, 2 stories. | Detached: at least 16 ft (18 ft within ½ mile of transit, plus 2 ft for a matching roof). Attached: 25 ft or the zone's limit, whichever is lower. |
| Setbacks | 4 ft side and rear; none for conversions. A detached ADU goes in the rear half of the lot, 10 ft from other buildings, unless an 800 sq ft ADU has no other spot. | No more than 4 ft side and rear; none for conversions of existing space. |
| Parking | 1 paved space unless exempt (transit, historic district, inside existing space, car share, built with a new home). No replacement for a converted garage, but driveway spaces stay. | At most 1 space; none within ½ mile of transit and in several other cases. No replacement parking when a garage is converted. |
| Owner living on site | None for ADUs. Junior ADU: the house or the junior ADU is owner-occupied (deed restriction). | None for ADUs. A junior ADU that shares a bathroom with the house requires the owner to live in one of them. |
| How many on a single-family lot | The minimum state law requires (Gov. Code 66323). | One ADU converted from existing space, one junior ADU and one new detached ADU (up to 800 sq ft). From January 1, 2027: two new detached ADUs (a city then needn't allow a junior ADU on that lot). |
| Short-term rentals | 30 days or longer, for ADUs and junior ADUs. | No guarantee: cities may require rentals of more than 30 days, and junior ADUs can't be short-term rentals. |
- Permit-ready ADU program: city-provided detached one-story plans, decided within 30 days of a complete application.
- Ridgeline Overlay Zone: some placement, height and fire-fuel rules apply.
- ADUs must match the house's materials, colors and roof design.
Where local rules conflict with state law, state law controls. Sources: San Marcos Municipal Code Chapter 20.410 · City of San Marcos Permit-Ready ADUs page (checked October 5, 2026); HCD's list of review letters; California ADU rules. Verified Oct 5, 2026. Rules change and every lot is different: confirm with the city before you design.
How long permits take
Application and issue dates are not published for this area in these sources.
Read more: How long ADUs take to build
From permit to completion
Of the 75 ADU permits San Marcos reported issuing from 2020 through 2023, 63 (84%) have a completion reported to the state. For those, the median time from permit to completion was 8 months.
The state reports don't record expired or cancelled permits, and a completion the city hasn't reported looks the same as an unfinished project, so the share is a floor. Years with incomplete reports are left out.
What neighbors declared as cost
Cost data (declared permit valuations) is not published for this area in these sources.
Read more: How to choose an ADU builder
An ADU builder serving San Marcos? Feature your business on this page.
Questions about ADUs in San Marcos
How many ADUs have been permitted in San Marcos?
Public records show 151 ADU permits in San Marcos through 2025, nearly all since 2020. 27 in 2025, down 34% from 2024. These counts come from the city's reports to the state, which place a project in its completion year when no permit date was reported.
Can I build an ADU on my property in San Marcos?
California law allows ADUs on most lots with a house, including in San Marcos, but what you can build depends on your lot, local rules and utilities. This site shows permits, not eligibility. See California ADU rules.
Where do I apply for an ADU permit in San Marcos?
Apply with the City of San Marcos Building Division for a building permit; its permit-ready ADU plans are decided within 30 days of a complete application. Official ADU page.
How current is this?
- State annual progress reports: through 2025 (published once a year)
How our data works · Download the data · Free to reuse with credit to SoCal ADU Tracker (socaladutracker.com), under CC BY 4.0.