ADU rules in Modesto

Detached ADUs are allowed with an existing or proposed single-family, duplex or multifamily dwelling: MMC 10-4.502(a) lets ADUs be created 'in any zone that allows single-family residential, multi-family residential, or mixed-use', the 10-3.101 land use table marks ADUs and JADUs 'P' in R-1, R-2, R-3, MU-P and MU-H, and 10-4.504(b) and 10-4.505(a)(2) allow a new-construction detached unit with 4-foot side and rear setbacks. On a single-family or duplex lot 10-4.505(a) caps the combination at 'two (2) additional units', but 10-4.511 requires ministerial approval of every Gov. Code 66323 unit 'notwithstanding anything in this article to the contrary', and 66323(a) allows 'any combination', so a converted ADU, a JADU and one new detached unit of up to 800 sq ft with 4-foot setbacks can share a lot (the point of HCD's November 27, 2023 finding on 10-4.505). Ord. No. 3797-C.S. (effective July 24, 2025) allows a duplex on any R-1 lot and gives duplex lots the same ADU options as single-family lots (10-3.217(b)); a lot with a proposed multifamily building may add up to two detached ADUs and one with an existing multifamily building up to eight (10-4.505(b)(3)). A lot with a 501(c)(3) place of worship in a residential or commercial zone may add up to two detached ADUs of up to 1,200 sq ft each (10-4.505(c)), a local Housing Element program; otherwise the P-O, C-1, C-2, C-3, C-M, M-1 and M-2 columns of the land use table do not list ADUs except with a multifamily project approved by conditional use permit.

Attached ADUs and conversions are allowed: an ADU may be attached to a proposed or existing single-family or multifamily dwelling (10-4.504(a)) or located inside the house, an attached garage or other living space, or an existing detached accessory structure (10-4.504(c)), and a conversion inside the house needs exterior access (10-4.505(a)(3)). Existing multifamily buildings may convert non-livable space such as storage rooms, attics, basements or garages into at least one ADU and up to 25 percent of the existing units (10-4.505(b)(1)-(2); Gov. Code 66323(a)(3)). One JADU of up to 500 sq ft is allowed within the walls of a single-family home (an attached garage counts) on a lot zoned for single-family use with one house, with a separate entrance, an efficiency kitchen and either its own or a shared bathroom (10-4.504(d), 10-4.505(a)(4), 10-4.509(a)-(e)); 10-4.502(b) confines JADUs to zones that allow single-family homes. Attached ADUs may reach 25 feet or the zone height limit, whichever is lower, and no more than two stories (10-4.507(a)(4)).

Section 10-4.506(b) caps a detached ADU at 1,200 sq ft of floor area and at the floor area of the existing or proposed house, and 10-4.506(a) caps an attached ADU at 50 percent of the primary dwelling (for a duplex, 50 percent of the unit or units it attaches to) while guaranteeing an attached unit of up to 800 sq ft; Ord. No. 3787-C.S. replaced the 2023 bedroom-based detached caps (850 and 1,200 sq ft) with these rules. The code measures 'floor area', a term Title 10 does not define, while Gov. Code 66321(b)(2)-(3) now sets the state floors in interior livable space: no maximum below 850 sq ft (1,000 sq ft with more than one bedroom), and no percentage-of-house limit that blocks an 800 sq ft unit with 4-foot side and rear setbacks; HCD's March 2026 handbook reads a percentage cap as also subject to the 850 and 1,000 sq ft floors, and 10-4.512 makes state law control any conflict. Converting an existing accessory structure allows a 150 sq ft expansion for ingress and egress (10-4.506(c)), conversions of existing space carry no size cap under Gov. Code 66323(a)(1), lot coverage does not apply to an ADU of 800 sq ft or less (10-4.503(k)), JADUs are capped at 500 sq ft (10-4.509(b); interior livable space under 66313(d)), and place-of-worship ADUs at 1,200 sq ft each (10-4.505(c)(3)). Heights track the state floors: 16 feet for a detached ADU, 18 feet within one-half mile walking distance of a major transit stop or high-quality transit corridor or beside a multistory multifamily building, plus 2 feet to match the main roof pitch, and 25 feet or the zone limit for an attached ADU (10-4.507; Gov. Code 66321(b)(4)).

No owner-occupancy requirement applies to ADUs: Article 5 contains none, and Gov. Code 66315 bars any standard beyond 66314, 'including an owner-occupant requirement'. For JADUs, 10-4.509(f) still requires a recorded deed restriction obliging the owner to live in the house or the JADU 'at all times' (a governmental agency, land trust or housing organization owner excepted), and the city's JADU deed-restriction instructions and form (last modified August 2023, citing former Gov. Code 65852.22 and 'Section 08') repeat that condition for every JADU. Since January 1, 2026, Gov. Code 66333(b) as amended by AB 1154 requires owner occupancy only when the JADU shares sanitation facilities with the house, and 10-4.512 and Gov. Code 66339.5 make the state rule control, so a JADU with its own bathroom carries no owner-occupancy duty even though the local text and form still say otherwise. HCD's May 11, 2026 letter to the city listed this change among those its ordinance on file may not reflect.

The Chapter 5 parking table (10-5.102) still requires '1 additional space' for a 'Second Unit (Granny Flat)', and 10-4.508(a) caps the requirement at one off-street space per ADU, which may be tandem in an existing driveway or uncovered on a paved existing driveway in the front setback (or in the rear setback on an alley lot). Section 10-4.508(c) waives parking for an ADU or JADU within one-half mile walking distance of public transit, in an architecturally and historically significant district, within the proposed or existing house or an accessory structure, where on-street permits are required but not offered to the occupant, within one block of a car share vehicle, or when filed with a permit for a new single-family or multifamily dwelling (Gov. Code 66322(a)); 'public transit' includes any fixed-route bus stop (Gov. Code 66313(m)). Garages, carports or covered spaces demolished or converted for an ADU need not be replaced (10-4.508(b)), and Gov. Code 66314(d)(11) extends that to uncovered spaces. JADUs sit inside the house and need none, Gov. Code 66323(b) and HCD's March 2026 handbook bar parking standards on state-exempt units, and HCD reads the state cap of one space 'per accessory dwelling unit or per bedroom, whichever is less' (66314(d)(10)(A)) to mean a studio needs none, which the local one-per-unit wording does not reflect. Place-of-worship ADUs need no new parking if one existing space is reserved for each unit (10-4.505(c)(7)).

ADUs and JADUs get a ministerial building permit: 10-4.503(a) ('Ministerial Approval Only') requires the city to approve or deny a complete application within 60 days or it is deemed approved, bars other local rules from delaying or denying the permit (10-4.503(f)), and 10-4.511 adds ministerial approval of every Gov. Code 66323 unit. Applications are filed through the city's eTRAKiT portal, and the city's ADU Express program (flyer dated March 2026) offers a one-hour Tuesday meeting with Building, Planning, Fire and Engineering staff and same-day approval for complete, professionally prepared plans that meet 'State and local ministerial standards'. Gov. Code 66317 applies directly where the 2024 local text is silent: a completeness decision within 15 business days, a list of missing items, and an appeal to the council or Planning Commission with a decision within 60 business days, points HCD's May 11, 2026 letter said the city's ordinance on file may not reflect. The city ADU page still warns that a second-story ADU over a garage next to single-story homes 'may need an administrative review', a reference to the 10-4.110 second-story development plan review with neighbor notice and appeal, but 10-4.503(a) and Gov. Code 66316 and 66317(a)(1) forbid discretionary review of an ADU, and the objective window-sill rule in 10-4.510(b) is the ADU-specific privacy standard.

Section 10-4.502(c) says 'ADUs and JADUs may only be rented in increments of thirty (30) days or more', matching the 'terms 30 days or longer' that Gov. Code 66315 allows, while state law requires JADU rentals and rentals of 66323 state-exempt units to run 'longer than 30 days' (66333(g), 66323(e)), so plan on 31 days or more for those. Modesto has no short-term rental ordinance or permit program (Municode searches for short-term, vacation rental and home sharing return nothing), so the 30-day rule is the whole short-term bar; the transient occupancy tax reaches lodging stays of 27 consecutive days or less (8-2.602). An ADU may be rented but not sold or conveyed separately except as state law provides (10-4.502(g)(5)), which leaves only the qualified-nonprofit path in Gov. Code 66341, since Modesto has adopted no Gov. Code 66342 ADU-condominium ordinance; a JADU's recorded deed restriction bars its separate sale (10-4.509(g)-(h)). A rented ADU is a 'Residential Rental Unit' that must be registered under the Rental Housing Safety Program (Ord. No. 3703-C.S., 2019) and may be inspected for a fee, unless it is a legal ADU 'where the owner lives on the same property' (9-13.201(j), 9-13.301, 9-13.401); new buildings are exempt from routine inspections for ten years but still register (9-13.501(a)(1)).