ADU rules in San Carlos

Detached ADUs are a permitted (P) use in every residential district (RS-3, RS-6, RM-20, RM-59 and RM-100), in all ten mixed-use districts when the lot has a proposed or existing dwelling, and in planned development districts that allow homes. SCMC 18.23.210(D)(2)(c) lists a freestanding unit, a unit within or added onto an existing accessory structure, and a unit over or below a detached garage among the allowed locations. The local count (one ADU per single-family lot, two detached ADUs beside a multifamily building) is overridden by Gov. Code 66323(a), which allows a converted ADU, a detached new-construction ADU and a JADU together on a single-family lot and up to eight detached ADUs (never more than the existing unit count) beside an existing multifamily building, as HCD's 5-6-2026 findings 9 and 11 say.

Attached additions and conversions of space inside the house (including all or part of an attached garage) or inside an existing accessory structure are permitted wherever detached units are. A newly built attached ADU needs 4 ft side and rear setbacks on the first floor and must meet the main home's front setback and the district's second-story setbacks unless it qualifies as a statewide-exemption ADU (the Gov. Code 66323 unit); it may rise 25 ft, and conversions need no setbacks. JADUs of up to 500 sq ft inside the walls of a single-family home are permitted in RS-3 and RS-6 with a recorded deed restriction.

Ord. 1604 (2023) struck the earlier 850/1,000 sq ft caps, so SCMC 18.23.210(F)(4)(f) now limits an ADU to the maximum floor area (MFA) the district allows on the lot and, where a home already exists, to 50 percent of that home for attached and detached units alike, with 800 sq ft as the ceiling for a unit that cannot meet every district standard. Up to 800 sq ft of any ADU, and all of a statewide-exemption ADU, is excluded from floor area (18.03.080(B)); only RS-6 has an MFA (lots up to 7,500 sq ft: the greater of 1,100 sq ft plus 35 percent of the lot or 50 percent of the lot; larger lots 50 percent) and RS-3 has none, so there is no fixed square-foot cap, which HCD's handbook says a less stringent local ordinance may do. HCD's March 2026 handbook reads Gov. Code 66321(b)(2) to bar a percentage cap that holds a unit below 850 sq ft of interior livable space (1,000 with more than one bedroom), and 66321(b)(3) and 66323(a)(2) guarantee an 800 sq ft detached unit with 4 ft setbacks. Conversions of existing space are uncapped under 66323(a)(1) even though the local table prints 50 percent or 800 sq ft for them. Heights: detached 18 ft plus 2 ft to match the main roof pitch, 20 ft over a detached garage, attached 25 ft.

San Carlos imposes no owner-occupancy requirement on ADUs. SCMC 18.23.210(E)(2)(d) still carries the pre-2025 state sunset wording ('For applications received prior to January 1, 2025, there is no owner-occupancy requirement'), but nothing in the code imposes one on later applications, Gov. Code 66315 bars it (HCD finding 16 asks the city to delete the clause), and the city ADU page says ADUs have no owner-occupancy requirement and need no deed restriction. For JADUs, local (E)(1)(d) requires the owner to live in the house or the JADU whenever a lot has both an ADU and a JADU; since 1-1-2026 Gov. Code 66333(b) requires owner occupancy only where the JADU shares sanitation with the home and forbids it where the JADU has its own bathroom or the owner is a government agency, land trust or housing organization, so the local clause yields in those cases (HCD finding 15). A JADU deed restriction barring separate sale and fixing size and attributes is still recorded with the County.

SCMC 18.23.210(G)(1) and Table 18.20.040-A(3) require one space per ADU on top of the home's two spaces; it may be tandem on a driveway or in a setback area. No space is required for an ADU inside an existing or proposed home or accessory structure, within one-half mile walking distance of public transit (locally any fixed-route bus stop or train station), in an architecturally and historically significant district, where on-street permits are not offered to the occupant, or within one block of a car-share space, and garage or carport spaces lost to an ADU need not be replaced. State law also exempts an ADU applied for together with a new home (66322(a)(6)), lets uncovered spaces go unreplaced (66314(d)(11)) and caps parking at one space per unit or per bedroom, whichever is less (66314(d)(10)(A)), so on a plain reading a studio owes no space. The local rule that a JADU must replace displaced garage parking yields to 66334(a), which bars any additional JADU parking (HCD findings 31 to 33).

ADUs and JADUs are permitted (P) uses filed directly with the Building Division and checked by Planning against objective standards; the city ADU page says they 'are reviewed ministerially, meaning a discretionary review or a public hearing is not required', and SCMC 18.29.030(B)(5) says an ADU filed with a new home 'shall be considered without discretionary review or hearing.' Gov. Code 66317 adds the 15-business-day completeness check and the 60-day decision clock. Two local mechanics deserve watching: 18.29.030(B)(2) lets adjacent or facing neighbors ask the Director, within 10 days, to verify compliance with the objective design standards that 18.04.070(B) applies to ADUs, and deviations from objective standards or Hillside Overlay grading rules route to discretionary design review, which state law does not allow for a unit that meets 66314 or 66323. The code's test that an applicant must show no alternative before statewide-exemption waivers apply conflicts with 66323 (HCD findings 21, 23 and 27); the zoning code has no coastal-zone provisions.

SCMC 18.23.210(E) lets ADUs and JADUs be rented separately from the home but bars rentals of fewer than 30 consecutive calendar days, a floor Gov. Code 66315 allows for ordinary ADUs; units built under the state-exempt pathway and JADUs must be rented for longer than 30 days (66323(e), 66333(g)), a day stricter than the local text and the city page. ADUs cannot be sold separately 'unless permitted by State law' (the 66341 qualified-nonprofit route; San Carlos has adopted no 66342 condominium opt-in), and JADUs never. There is no short-term rental permit program: stays under 30 consecutive days are the commercial Lodging use (18.40.040(K)), which Table 18.04.020 does not list in any residential district, and any short-term rental booked online owes transient occupancy tax under Chapter 3.24. The code has no rental registration or local rent stabilization chapter (Titles 3, 5, 8, 9 and 18 searched).