ADU rules in San Marcos

Detached ADUs are allowed on any lot zoned for single-family, multifamily or mixed-use residential use with an existing or proposed home (SMMC 20.410.020.D), and the use tables list 'Accessory Dwelling Unit/Accessory Structure' as an accessory use (A) in every residential zone (R-1-20 through R-3-10) and in the A-1, A-2 and A-3 agricultural zones. The December 2025 Title 20 update (Ord. No. 2025-1568) changed the R-2, R-3-6 and R-3-10 cells from 'use not allowed' to A, ending a conflict with 20.410.020.D. The count follows the Gov. Code 66323 minimum (20.410.020.D.1): on a single-family lot one new detached ADU in addition to a conversion ADU and a JADU, and on a lot with an existing multifamily building up to eight detached ADUs, never more than the existing unit count. Local-standard detached units must sit in the rear half of the lot, no closer to the street than the house, with 4 ft side and rear setbacks and 10 ft from other buildings (20.410.060.E), while a state-exempt detached unit of up to 800 sq ft needs only the 4 ft setbacks and the state height limits (Gov. Code 66323(a)(2) and (b)).

ADUs attached to or inside the existing or proposed home, including an attached garage, and conversions of an accessory structure are allowed (SMMC 20.410.020.D.2.a); a converted accessory structure may grow up to 150 sq ft to accommodate ingress and egress. An attached ADU may be 25 ft tall or the height limit of the main home, whichever is lower, and no more than two stories (20.410.060.C.4.a). JADUs of up to 500 sq ft must sit entirely within a single-family home (attached garages count), with an efficiency kitchen and a separate exterior entrance, plus an interior door to the home if the JADU has no bathroom of its own (20.410.020.C, 20.410.060.D). On a single-family lot a conversion ADU and a JADU can be combined with a new detached ADU (Gov. Code 66323(a)).

SMMC 20.410.060.C caps every ADU at the smallest applicable figure: 50 percent of the main home's living area but at least 850 sq ft, 850 sq ft with one bedroom, 1,000 sq ft with more than one bedroom, and 1,000 sq ft for a detached unit on a lot of one acre or more; JADUs are capped at 500 sq ft. The code states these caps in gross floor area (20.600.080: the square footage within the surrounding walls), while Gov. Code 66321(b)(2), as amended by SB 543 effective 1-1-2026, sets the 850 and 1,000 sq ft floors in interior livable space, and 20.410.020 says state law prevails over conflicting local text. A Gov. Code 66323 state-exempt detached unit may be held only to 800 sq ft of livable space and the state heights, and a conversion within existing space has no size cap. Heights: 16 ft for a detached unit, 18 ft within one-half mile walking distance of a major transit stop or high-quality transit corridor (plus 2 ft for a roof pitch matching the house) or on a lot with a multistory multifamily building, and 25 ft or the home's height limit, at most two stories, for attached units.

San Marcos imposes no owner-occupancy requirement on ADUs, and Gov. Code 66315 bars one. The 2024 text still says 'Either the primary dwelling or the Junior ADU must be owner occupied' and writes that into the JADU deed restriction (20.410.020.C, 20.410.060.O.5), but Gov. Code 66333(b), as amended by AB 1154 effective 1-1-2026, allows owner occupancy only when the JADU shares sanitation facilities with the main home, so a JADU with its own bathroom carries no owner-occupancy duty. A deed restriction barring separate sale, limiting the JADU to 500 sq ft and keeping its exterior access must still be recorded before the JADU building permit issues (20.410.060.O; Gov. Code 66333(c)).

SMMC 20.410.060.J.1 requires one paved on-site space (9 by 18 ft) for a new ADU in addition to the main home's parking; it may be tandem, on an existing driveway or in a setback area (J.2), and a JADU needs none (J.5). No parking may be required within one-half mile walking distance of public transit (any bus stop or train station), in a historic district, for an ADU inside an existing home or accessory structure, where on-street permits are not offered, within one block of a car share vehicle, or when the ADU is permitted together with a new home (J.4; Gov. Code 66322(a)). State law also overrides the local text for studios, since Gov. Code 66314(d)(10)(A) caps parking at one space per unit or per bedroom, whichever is less, and for Gov. Code 66323 state-exempt units, which take no standard the section does not authorize (66323(b)). A garage, carport or covered space converted or demolished for an ADU need not be replaced (J.3), and Gov. Code 66314(d)(11) extends that to uncovered spaces, overriding the local rule that pre-existing driveway spaces must be maintained.

ADUs and JADUs need only a building permit, and SMMC 20.410.030.A makes approval 'ministerial in nature' with a decision within 60 days of a complete application, matching Gov. Code 66317; the zone use tables treat ADUs as an accessory use (A) with no planning permit. The Permit Ready ADU program offers city pre-approved detached one-story plans decided within 30 days, and the fee schedule effective 9-1-2026 lists flat plan check ($1,203) and permit ($2,667) fees for city standard ADU plans up to 500, 749 and 1,000 sq ft. Projects that exceed the ADU standards need a discretionary variance before the Planning Commission (for example V24-0001, a 1,964 sq ft two-story detached ADU in front of the home on a four-acre R-1-20 lot, heard 6-16-2025).

SMMC 20.410.060.H lets an ADU be rented for terms of 30 days or longer, the minimum Gov. Code 66315 allows, but Gov. Code 66323(e) and 66333(g) require terms longer than 30 days for state-exempt units and JADUs, overriding the 30-day JADU wording in 20.410.020.C and 20.410.060.O.4. The Finance Department's Transient Occupancy Tax page states that short-term rentals are not permitted in residential zones, consistent with the use tables and the rule that unlisted uses are prohibited (20.215.040.D); the only short-term lodging allowed is an owner-operated Bed & Breakfast approved by CUP, whose guest rooms may not have cooking facilities (20.400.120). An ADU may not be sold separately except through a qualified nonprofit under Gov. Code 66341, and the city declined the 66342 ADU-condominium opt-in in 2024. The municipal code has no residential rental registration or ADU rental licensing program.