San Joaquin County
Mountain House ADU rules
Research summary of local ADU rules on file for Mountain House in San Joaquin County. Not a determination for a specific lot.
City or county ADU page
Check any California address for freeMountain House rules on file
Local rules recorded in this project for this jurisdiction. Where a city figure on file is stricter than the state baseline on file, both are shown.
Local ADU rules
Mountain House incorporated as a city on July 1, 2024. Its ADU rules are in Title 9, Division 8, Chapter 13 of the Mountain House Municipal Code, as amended by Ordinance 2026-02. ADUs are allowed in the R-VL, R-L, R-M, R-MH and M-X zones, and in the AU-20 zone where the Master Plan designation is residential.
Source: City of Mountain House Ordinance No. 2026-02 (Municode ordinance library), § 9-8-1302, accessed 2026-10-24.
Building permit
ADUs are applied for with a building permit application to the Community Development Department; the owner must confirm City water and sewer service is available.
Source: City of Mountain House Ordinance No. 2026-02 (Municode ordinance library), § 9-8-1302(f), (k)-(l), accessed 2026-10-24.
Size
Detached ADU: up to 1,200 sq ft. JADU: 150 to 500 sq ft inside the existing walls of a single-family home.
Source: City of Mountain House Ordinance No. 2026-02 (Municode ordinance library), §§ 9-8-1303(a)(1), 9-8-1304(a), accessed 2026-10-24.
Height
Detached ADU: 16 ft; 18 ft within a half mile walk of a major transit stop or high-quality transit corridor (plus 2 ft to match the main home's roof pitch) or on a lot with a multistory multifamily building. Attached ADU, or an ADU mostly above a garage: 25 ft.
Source: City of Mountain House Ordinance No. 2026-02 (Municode ordinance library), § 9-8-1303(b), accessed 2026-10-24.
Setbacks
4 ft side and rear setbacks (none for a converted ADU). Front and street-side setbacks follow the main home, but an ADU up to 800 sq ft may go into those setbacks when there is no other place for it. Lot coverage limits cannot block an 800 sq ft, 16 ft ADU with 4 ft side and rear setbacks.
Source: City of Mountain House Ordinance No. 2026-02 (Municode ordinance library), § 9-8-1303(c)(1), (f), accessed 2026-10-24.
Parking
One space per ADU (tandem on a driveway or in a setback allowed), with none required within a half mile of public transit, in a City historic district, for units in the main home or an existing accessory structure, where on-street permits are not offered, or within one block of car share. Garage or carport parking lost to an ADU need not be replaced.
Source: City of Mountain House Ordinance No. 2026-02 (Municode ordinance library), § 9-8-1303(g), accessed 2026-10-24.
Sale, rental and surveys
An ADU may not be sold separately from the main home or rented for less than 30 days, and owners must answer the City's periodic ADU surveys for state reporting. A JADU needs a recorded deed restriction with the same terms.
Source: City of Mountain House Ordinance No. 2026-02 (Municode ordinance library), § 9-8-1302(j), accessed 2026-10-24.
Impact fees
No impact fee for an ADU under 750 sq ft.
Source: City of Mountain House Ordinance No. 2026-02 (Municode ordinance library), § 9-8-1302(g), accessed 2026-10-24.
Junior ADUs
A JADU must be inside the walls of a single-family home, with a separate exterior entry and an efficiency kitchen; it may share the main home's bathroom. No added parking is required.
Source: City of Mountain House Ordinance No. 2026-02 (Municode ordinance library), § 9-8-1304, accessed 2026-10-24.
California state rules on file
The statewide baseline recorded from state law and the HCD ADU Handbook.
Detached size cap described in state law
1,200 sq ft. Recorded in this rules file from Government Code section 66314.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.
Side and rear setback cap for a new ADU
4 ft. Recorded in this rules file from Government Code section 66314, which describes a cap on side and rear setbacks for a new ADU.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.
Setback for a conversion of an existing structure
No additional setback, as recorded in this rules file from Government Code section 66314 for a conversion of an existing structure.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.
Minimum lot size
A city may not impose a minimum lot size for an ADU, as recorded in this rules file from Government Code section 66314.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.
Minimum unit the handbook says cities must allow
At least an 800 sq ft ADU with 4 ft side and rear setbacks. The HCD ADU Handbook (updated March 2026) describes this under section 66321(b)(3).
Source: HCD ADU Handbook, 66321(b)(3); handbook updated March 2026, accessed 2026-10-16.
Detached height described in the handbook
16 ft, or 18 ft or 20 ft in some cases. The HCD ADU Handbook (updated March 2026) describes this under section 66321(b)(4).
Source: HCD ADU Handbook, 66321(b)(4); handbook updated March 2026, accessed 2026-10-16.
Permit decision deadline
60 days. Recorded in this rules file from Government Code section 66317(a)(3): the permitting agency must decide on (approve or deny) a completed ADU application within 60 days when an existing single-family or multifamily dwelling is on the lot. Section 66317(a)(1) says the application is considered ministerially, without discretionary review or a hearing.
Source: California Government Code section 66317, 66317(a)(1), (a)(3), accessed 2026-10-17.
Owner-occupancy requirement
None. Recorded in this rules file from Government Code section 66315: a local agency may not impose an owner-occupancy requirement on an ADU. It may require that the property be used for rentals of terms 30 days or longer. The HCD ADU Handbook (updated March 2026) describes the same rule.
Source: California Government Code section 66315, 66315; HCD ADU Handbook updated March 2026, accessed 2026-10-17.
Parking near public transit
No parking standards within one-half mile walking distance of public transit. Recorded in this rules file from Government Code section 66322(a)(1). Section 66322(a) lists several other cases where parking standards may not be imposed.
Source: California Government Code section 66322, 66322(a), accessed 2026-10-17.
Ministerial categories
The HCD ADU Handbook (updated March 2026) states that Government Code section 66323 sets four ministerial categories. This rules file does not restate those four categories.
Source: HCD ADU Handbook, 66323; handbook updated March 2026, accessed 2026-10-16.
Fire Hazard Severity Zones
If your lot is in a Fire Hazard Severity Zone, the CAL FIRE Office of the State Fire Marshal says in Information Bulletin 25-004 (issued March 3, 2025): “Fire Hazard Severity Zones (FHSZs) should not restrict the development of Accessory Dwelling Units (ADUs). However, additional building standards may be necessary, including but not limited to California Fire Code Chapter 49 and California Building Code Chapter 7A.” State ADU law does not itself bar an ADU there. A city or county may still apply its building code, including wildfire-resistant construction (Gov. Code 66314(d)(8)); require side and rear setbacks “sufficient for fire and safety” for a unit converted under 66323(a)(1); and decline parking in setbacks on specific fire and life safety findings (66314(d)(10)(B)). It may also consider public safety when it designates where ADUs may be permitted (66314(a)). Sprinklers can't be required in an ADU or JADU if the primary residence doesn't need them (66314(d)(12), 66323(d)). Some cities and counties still apply their own fire-zone limits. Where this summary has one, it is listed with the local rules. This summary does not look up your lot. Check the CAL FIRE Fire Hazard Severity Zone map viewer linked from the State Fire Marshal's Fire Hazard Severity Zones page: https://experience.arcgis.com/experience/5065c998b4b0462f9ec3c6c226c610a9
Source: CAL FIRE Office of the State Fire Marshal Information Bulletin 25-004, page 4, issued March 3, 2025; Gov. Code 66314, 66323, accessed 2026-10-20.
State baseline on the directory → · State ADU rules on the homepage →
Can I build an ADU in Mountain House?
State law sets a baseline for ADUs in Mountain House, and the local standards on file are listed on this page. Whether a specific lot can add an ADU depends on zoning, utilities, and other site facts this page does not measure. Confirm with the City of Mountain House Community Development Department, or run a free address summary.
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