Accessory Dwelling Units

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The City of Hercules supports development of Accessory Dwelling Units (ADUs) and Junior Accessory Dwelling Units (JADUs), also known as granny or in-law units as an innovative, affordable, and effective option for adding much-needed missing middle housing. Please review the information below to learn more about our local requirements for developing an ADU or JADU.

Download the ADU Mini-Guide for Homeowners to learn more about building an ADU in Hercules.

General ADU/JADU Information

Location. ADUs are permitted in any residential or mixed-use zone that permits residential uses with a proposed or existing dwelling.

ADU. An ADU may be attached or detached from the primary dwelling, must have an exterior entrance separate from the primary dwelling, and must have complete independent living facilities for 1 or more persons, including permanent provisions for sleeping, living, eating, cooking, and sanitation.

JADU. A JADU is no more than 500 square feet in size and contained entirely within the walls of an existing or proposed single-family residence; it must include a cooking facility with appliances, and a food preparation counter and storage cabinets that are of reasonable size in relation to the size of the JADU. JADUs may share a bathroom with the primary residence and have an internal connection.

Statewide Exempt ADU (66323 Units). On single-family parcels, Statewide Exempt ADUs, also known as 66323 Units, are created from the conversion of existing space within a dwelling or accessory structure, a new construction detached ADU that has a total floor area of no more than 800 square feet, with 4 feet rear and side setbacks, and a maximum height of 16 ft, or a JADU. On Multi-family parcels, Statewide Exempt ADUs include internal conversions and detached ADUs. See tables below for more details.

Fire Sprinklers. Accessory dwelling units shall not be required to provide fire sprinklers if fire sprinklers are not required for the primary residence.

No Short Term Vacation Rentals. All accessory dwelling units shall only be rented for a term of more than 30 days.

Owner Occupancy Requirements for JADUs. The owner of a parcel with a single-family dwelling and a proposed junior accessory dwelling unit shall occupy as a principal residence either the primary dwelling or the accessory dwelling.

Design. An ADU must conform to the design characteristics of the existing residence or residences. Not applicable to Statewide Exempt ADUs.

ADU Address. Detached ADUs require a new address.

Impact Fees. ADUs of less than 750 square feet are not subject to impact fees. ADUs of 750 or more square feet are subject to any fees for residential units required by the City’s Master Fee Schedule. Fees shall be charged in proportion to the square footage of the primary dwelling (e.g., a 1,000-square-foot ADU would be charged 50 percent of the applicable fee if the primary dwelling is 2,000 square feet).

Development Standards

Single-Family Lot Development Standards Number of Units - One converted ADU, one newly constructed ADU, and one JADU
	Conversion of Structure	Attached ADU	Detached ADU
Maximum Square Footage	No unit size limits apply to conversion of an existing accessory structure or a portion of the existing primary residence to an ADU	1 bedroom 850 sq ft 2+ bedrooms 1,000 sq ft	1 bedroom 850 sq ft 2+ bedrooms 1,000 sq ft
Front Setback	N/A	As required by applicable zoning district.	As required by applicable zoning district, except that front setbacks may not preclude the construction of a Statewide Exempt ADU
Side - Interior Setback	No setback is required for an ADU created within an existing living area or accessory structure or an ADU created in a new structure in the same location as an existing structure	4 feet minimum	4 feet minimum
Side - Corner Setback	No setback is required for an ADU created within an existing living area or accessory structure or an ADU created in a new structure in the same location as an existing structure	As required by applicable zoning district.	As required by applicable zoning district, except Statewide Exempt ADU may be located 4 feet from a corner lot line
Rear Setback	No setback is required for an ADU created within an existing living area or accessory structure or an ADU created in a new structure in the same location as an existing structure	4 feet minimum	4 feet minimum
Height Limit	N/A	25 ft or the applicable height limits of the zoning district, whichever is lower	16 feet + plus 2 feet if needed to match roof pitch of existing dwelling
Parking	0 parking	Studio – 0 parking 1+ bedrooms – 1 parking	Studio – 0 parking 1+ bedrooms – 1 parking No parking required for Statewide Exempt ADUs
Parking is not required if the ADU is located (1) within one-half of one mile walking distance of public transit, (2) within an architecturally and historically significant historic district, (3) is part of the proposed or existing primary residence or an accessory structure, (4) when on-street parking permits are required but not offered to the occupant of the ADU, (5) there is a car share vehicle located within one block of the ADU, or (6) when a permit application for an accessory dwelling unit is submitted with a permit application to create a new single-family dwelling or a new multifamily dwelling on the same lot, provided that the accessory dwelling unit or the parcel satisfies any other criteria in ths list.

Multi-Family Lot Development Standards (Multi-family means a lot containing two or more attached dwellings) Number of Units - At least 1 ADU and up to 25 percent of the number of existing multifamily dwellings shall be allowed within an existing multifamily dwelling. On a lot with an existing multifamily dwelling, not more than 8 ADUs or up to the number of existing units on the lot, whichever is less. On a lot with a proposed multifamily dwelling not more than 2 detached ADUs. Internal conversion ADUs and Detached ADUs may be combined.
	Interior Conversion	Detached ADU
Maximum Square Footage	Must be converted from non-livable space, if no such area exists an interior ADU can’t be approved	1 bedroom 850 sq ft 2+ bedrooms 1000 sq ft
Front Setback	N/A	As required by applicable zoning district, except that front setbacks may not preclude the construction of a Statewide Exempt ADU
Side - Interior Setback	N/A	4 feet
Side - Corner Setback	N/A	As required by applicable zoning district, except Statewide Exempt ADU may be located 4 feet from a corner lot line
Rear Setback	N/A	4 feet
Height Limit	N/A	18 feet + plus 2 feet to match roof pitch of existing dwelling
Parking	0 parking	0 parking

Application Process

A Building Permit is required for the development of an ADU/JADU, with review by the Community Development Department.

Once the building permit application is ready and submitted to the Planning Department, please allow up to 30 days for completeness review. For more information, please contact the Community Development Specialist.

Download the ADU Mini-Guide for Homeowners to learn more about building an ADU or JADU in Hercules.

ADU Preapproval Application Process

Preapproval of ADU plans for use within the City of Hercules, which receives building inspection services from Contra Costa County, requires planning approval by the City prior to the applicant submitting to the County building department for preapproval. The City's Preapproval Checklist, as well as more information, can be found on the County's ADU Preapproval webpage (click on "Preapproval Process - Cities.")

Applicants using preapproved ADU plans will still need to secure planning and building permits, including, but not limited to; site plan, foundation plan and details, geotechnical investigation report (if needed), and compliance with fire and other safety requirements. For more information, please contact the Community Development Specialist.

ADU Amnesty (AB 2533) Information

If your unpermitted ADU or JADU was constructed before January 1, 2020, the following applies, as provided in Assembly Bill (AB) 2533:

The City of Hercules may deny a permit to legalize an ADU or JADU constructed before January 1, 2020, if the unit is not in compliance with California Health and Safety Code Section 17920.3.
The property owner applying for a permit to legalize an unpermitted ADU or JADU shall not be required to pay impact fees or connection or capacity charges except where utility infrastructure is required to comply with Health and Safety Code section 17920.3.
Before submitting an application for a permit, a homeowner may obtain a confidential third-party code inspection from a licensed contractor to determine that unit's existing condition or potential scope of building improvements before submitting an application for a Building Permit.

California Health and Safety Code Section 17920.3 - Checklist