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County of Alameda ADU Guide

David Reyes
David Reyes

ADU Jurisdiction Guide: County of Alameda
Fees and Regulations (September 2022)

ADU-allowed Zones

ADUs are permitted in any zoning district that allows single-family or multifamily residential use. ADUs may be denied if their creation would have a negative impact on traffic flow or public safety, or if adequate water and sewer service are not available, as determined by the County.

JADUs are permitted only in R-1 (Single-Family Residence) zoning districts and only on properties that have only one existing primary dwelling unit in combination with one detached ADU of a maximum 800 sq. ft. size. JADUs may be denied if their creation would have a negative impact on traffic flow or public safety, or if adequate water and sewer service are not available, as determined by the County.

Properties containing one existing or proposed single-family dwelling are allowed one ADU and/or Junior ADU (JADU). The property owner may build a new detached ADU that does not exceed 800 sq. ft.

ADU Size Limitations

500 sq. ft. for JADU and 800 sq ft for Detached ADU (optional)

All new construction ADUs, attached and detached, are subject to the maximum unit sizes:  

  • 850 sq ft (one bedroom or less) or 
  • 1,000 sq ft (more than one bedroom) or 
  • 1,200 sq ft for properties in Ag zoning,  regardless of bedroom count 

*ALL attached ADUs over 800 sq ft on parcels with single-family dwellings shall not exceed 50% of the primary dwelling living area

However, maximum unit size requirements with one bedroom must be at least 850 square feet and 1,000 square feet for ADUs with more than one bedroom. If ADU exceeds the thresholds above, ADUs shall not exceed 50% of the total floor area of the primary dwelling or 1,200 sq ft whichever is less.

ADU Maximum Height

  • Attached ADUs: standard for the zone district (usually 28 feet)
  • Detached ADUs outside the Urban Services Line: standard for the zone district (usually 28 feet)
  • Detached ADUs inside the Urban Services Line: 16 feet (21 feet allowed with Zoning Administrator approval)
  • ADUs above Detached Garages inside the Urban Services Line: 24 feet at the roof peak, 20 feet at top of exterior wall

Lot Size

Single-family and multi-family residential parcels will not be subject to the minimum lot size of the underlying zoning district or by ordinance for Accessory Dwelling Unit projects.

ADU Setbacks

  • JADUs are contained entirely within the single-family dwelling (including the attached garage) and are subject to the same setbacks and development standards as the primary dwelling (including the attached garage).
  • Minimum setbacks of 4 ft from the side and rear lot lines are required for new construction ADU of any size, both attached and detached. 
  • Detached ADUs over 800 sq ft must provide a minimum 6 ft separation from the primary house, and 6 ft from accessory structures.
  • Newly constructed, detached ADUs must be located entirely behind the permitted primary dwelling, 75 ft away from the front property line or rear half of the lot. However, this requirement does not apply to ADUs located in the Agricultural (Ag) District.

Parking Space Requirements

a) No additional parking is required when: 

  • Property is within ½-mile walking distance of public transit 
  • Property is within an architecturally and historically significant historic district 
  • ADU is entirely within the existing interior space of the primary residence or accessory structure (e.g. sheds, garages, carports, covered parking structures, etc.) 
  • On-street parking permits are required but not offered to the ADU occupant 
  • A car share vehicle is located within one block of the ADU

b) For attached or detached ADUs requiring new construction (e.g. a new accessory building or an addition), one parking space is required per unit or per bedroom, whichever is less. These spaces may be located in setback areas or provided through tandem parking on a driveway.

Owner Occupancy Requirements

Owner occupancy of the property is not required for standard ADUs. However, JADUs are subject to owner-occupancy requirements described in Gov. Code Sec. 65852.22.(a)(2).

Other Regulations

  • ADUs and JADUs may not be rented for a period of fewer than 30 days. 
  • ADUs and JADUs may not be sold or conveyed separately from the main residence. 
  • ADUs shall match the exterior material, color, and roof form of the permitted primary dwelling structure. 
  • ADUs shall appear subordinate to the existing primary dwelling in terms of size and location.  
  • Exterior access is required for ADUs and JADUs. 
  • ADUs contained entirely within an existing permitted, detached accessory structure may be allowed an expansion of no more than 150 sq. ft. beyond the exact physical dimensions of the existing detached accessory structure. The expansion is limited to ingress and egress purposes only. 
  • Newly constructed ADUs over 800 sq. ft. are subject to additional development standards, such as lot coverage and building separation of 6 ft. from the primary dwelling for detached ADUs, as set by Alameda County Code Section 17.30.110. 
  • For ADUs contained entirely within an existing, permitted, single-family residence or related accessory structure, with no change in building volume, the minimum setbacks are whatever is required for fire and safety.

Resources

Resources

Contact

  • General Contact 5106705440 ext. 6 & 0 for permit inquiries; planninginfo@acgov.org
  • William  510-670-6519; william@acpwa.org
  • Christine 510-670-6020

Useful Information

We hope this quick reference guide helps you out in your ADU project. Part of our service if you build with us is the full service of our in-house permit technician who handle the processing of your permit on your behalf with no mark-ups. 

If you’re interested in building with ADU.Works, please don’t hesitate to book a consultation with us or give us a quick call through our number +1 (650) 227 4810. We’ll gladly discuss the specifications of your permit processing and accommodate any questions you may have. Our team looks forward to hearing from you.

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