Contra Costa County
Hercules ADU rules
Research summary of local ADU rules on file for Hercules in Contra Costa County. Not a determination for a specific lot.
City or county ADU page
Check any California address for freeHercules 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.
How many units (single-family lot)
One converted ADU, one newly built ADU and one junior ADU.
Source: City of Hercules, Accessory Dwelling Units page, Single-Family Lot Development Standards, accessed 2026-10-23.
Attached and detached ADU size
850 sq ft with one bedroom, 1,000 sq ft with two or more bedrooms. No size limit for converting an existing accessory structure or part of the main home.
Source: City of Hercules, Accessory Dwelling Units page, Single-Family Lot Development Standards, accessed 2026-10-23.
Side and rear setbacks
4 feet minimum for new attached and detached ADUs. None for an ADU in existing space or a new structure in the same place as an existing one. Front setbacks follow the zoning district but may not prevent a statewide exempt ADU, which may also sit 4 feet from a corner lot line.
Source: City of Hercules, Accessory Dwelling Units page, Single-Family Lot Development Standards, accessed 2026-10-23.
Height
Detached: 16 feet, plus 2 feet if needed to match the existing home's roof pitch. Attached: 25 feet or the zoning district limit, whichever is lower. Detached ADUs on multifamily lots: 18 feet plus 2 feet for roof pitch.
Source: City of Hercules, Accessory Dwelling Units page, Development Standards tables, accessed 2026-10-23.
Parking
Conversions: none. New attached or detached ADUs: none for a studio, one space for one or more bedrooms. No parking for statewide exempt ADUs or near transit, in a historic district, for ADUs that are part of the home or an accessory structure, where on-street permits are not offered, near car share, or when built with a new home.
Source: City of Hercules, Accessory Dwelling Units page, Single-Family Lot Development Standards, accessed 2026-10-23.
Multifamily lots
Interior conversion ADUs must use non-livable space: at least one and up to 25% of existing units. Detached: up to 8 (not more than the existing units) on a lot with an existing multifamily building, or 2 with a proposed one. Detached multifamily ADUs need no parking.
Source: City of Hercules, Accessory Dwelling Units page, Multi-Family Lot Development Standards, accessed 2026-10-23.
Impact fees
None for ADUs under 750 sq ft. ADUs of 750 sq ft or more pay City fees in proportion to the main home (for example, a 1,000 sq ft ADU next to a 2,000 sq ft home pays 50%).
Source: City of Hercules, Accessory Dwelling Units page, General ADU/JADU Information: Impact Fees, accessed 2026-10-23.
Address
Detached ADUs need a new address.
Source: City of Hercules, Accessory Dwelling Units page, General ADU/JADU Information, accessed 2026-10-23.
Rentals
All ADUs may only be rented for terms of more than 30 days.
Source: City of Hercules, Accessory Dwelling Units page, General ADU/JADU Information, accessed 2026-10-23.
Permit route and review
A building permit is required, reviewed by the Community Development Department. Allow up to 30 days for completeness review after submittal.
Source: City of Hercules, Accessory Dwelling Units page, Application Process, accessed 2026-10-23.
Pre-approving ADU plans
Hercules gets building inspection services from Contra Costa County, so a plan needs City planning sign-off before it goes to the County for pre-“approval”. Users of these plans still need planning and building permits, a site plan, foundation plan and details and, if needed, a geotechnical report.
Source: City of Hercules, Accessory Dwelling Units page, ADU Preapproval Application Process, accessed 2026-10-23.
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 Hercules?
State law sets a baseline for ADUs in Hercules, 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 City of Hercules Planning Division, or run a free address summary.
Check any California address for free