Accessory Dwelling Units

ADU, Accessory Dwelling Unit or JADU, Junior Accessory Dwelling Unit is a way to provide additional housing on the same lot. Its intention is to provide affordable housing options for renters, family members, the elderly, or caregivers. It can also help 1st time buyers to increase income to afford a new home. The following are some key points on ADU regulations:

Review period – An application for ADU or JADU for lots with a single-family dwelling shall be deemed approved, or denied, within 60 days from a completed application. The prior regulation was 120 days.
In San Francisco, if the ADU requires an expansion, neighbors within 150 feet radius and neighborhood groups will be notified for a 30-day public review period. During the 30-day period, a discretionary review process will apply.

Number of ADU allow – Under state law, you can provide (1) ADU and (1) JADU per lot, within the proposed or existing single-family dwelling, if certain conditions are met. For multi-family buildings, state law allows up to 25% of the unit count;

In San Francisco, if (4) or less legal dwelling units are on a lot, only (1) ADU is allowed. If (5) or more legal dwelling units are on a lot, then an unlimited number of ADUs are permitted under S.F. Planning Code. Then, the determination on the number of ADUs will depend on the S.F. Building Code for the abilities to provide natural light, ventilation, and two exits.

Size of the ADU – Under state law, the maximum size of a detached ADU is 1200 sq.ft. for more than one bedroom; local city requirements may vary. For example, the City of Millbrae is 1000 sq.ft. maximum for a detached ADU of more than one bedroom;

Size of the JADU – Under state law, the maximum size of a JADU within the primary dwelling is 500 sq.ft.; local city requirements may vary.

Density Limit – Adding an ADU does not change the allowable density limit of the lot.

Short-term Rentals – ADU cannot be used as short-term rentals.

Owner occupancy requirement – Owner occupancy is not required on ADUs until 2025, which means both the primary dwelling and the ADU can be rented.

For JADUs, owner occupancy is required at the primary dwelling or the JADU.

Rent Controlled – ADUs need to provide affordable rent to very low, low, or moderate-income households.

Unit Legalization – In San Francisco, there is also a unit legalization program to help you legalize an in-law unit without the ADU process. In that process, the unit needs to be existed prior to January 1, 2013.

The information provided is for general reference; Every site has unique challenges, we have the training and experience to help you decide if an ADU makes sense for you from a cost, benefit & lifestyle point-of-view. If you think an ADU makes sense, we can help you to explore design options.

Statewide ADU exemption

A statewide ADU exemption is an ADU of up to 800 s.f. , 16’ in height, as potentially limited by a local agency, and with 4’ side & rear yard setbacks. State ADU Law requires that no lot coverage, floor area ratio, open space, or minimum lot size will preclude the construction of a statewide exemption ADU.

1,000 s.f. ADU in Millbrae.
1,000 s.f. ADU in Alameda.
1,000 s.f. ADU in Millbrae

Senate Bill 9, SB 9

California State Senate Bill 9 (SB 9) would allow a 2 residential units development in a single-family residential zone to be considered ministerially, without discretionary review, public hearing & CEQA, if the development meets certain requirements by the local agency. SB 9 said this development approach does not require owner occupancy.  Neither unit would be subject to rent control & the units could be sold as condominiums. 

If your property is located within an urban area & your property was not established through a lot split prior to Senate Bill 9; Then you can subdivide your lot into 2 parcels and build 2 homes on each of the 2 new lots allowing for a total of 4 units. SB 9 said the lot split approach requires the applicant to occupy one of the units as their principal residence for a minimum of 3 years. SB 9 allows the local agency to ministerially approve a parcel map for an Urban Lot Split that meets certain requirements by the local agency.

The review & approval process under SB 9 will be “objective” in terms of zoning standards, subdivision standards, and design standards.

Key points to consider:

  1. Qualify Projects must be zoned in a single-family lot; Not a historic site or district; Cannot alter or demolish rent-controlled housing, housing that was Ellis Act in the last 15 years, or housing occupied by a tenant in the last 3 years.
  2. Minimum unit area 800 square feet.
  3. Minimum 4 feet setback at rear & side. (5 feet would be better in some situation)
  4. San Francisco requires minimum 25 feet separation between existing & new buildings.
  5. One parking per unit; Some local agency may consider no parking if parcel located ½ mile walking distance of transit.
  6. Cannot be use as short-term rentals.
  7. If units are attached, it must be design as condominium to allow for separate sale.
  8. Urban Lot Split should result in 2 approximately equal-sized lots (60-40 split). Each new lot is at least 1,200 square feet. Urbanized area is an area with 50,000 or more people. See US Census Bureau for urban area reference maps.

San Francisco projects without public notifications

Many projects in San Francisco require public notifications which include demolition, new construction, change in use, exterior alteration or building expansions in residential and some commercial zone areas but there are some exceptions as indicated below.


Dormers can be added to the attic to create additional usable space without public notifications under the following conditions.
(1) The dormer and all other roof features combined area is less than 20% of the roof area.
(2) The maximum plan dimension of the dormer is 8 feet by 8 feet and setback minimum 3 feet from the side property line.
(3) The dormer setback 10 feet from the front building wall.
(4) The dormer cannot be higher than the peak of the roof.


When the lightwell at the property line is against your neighbor’s wall and the lightwell is not visible from any offsite location, and the lightwell is not higher than 10 feet above the neighbor’s ground, then the lightwell infill can be approve without public notification. The Planning Department will need a set of reduced drawings signed by the adjacent owner or tenant. If the proposed lightwell infill is visible from any off-site location, then the public notification is required.


When you have a floor supported by columns and the open space below does not exceed one story or twelve feet, you can enclose the open space below without public notifications assuming that the floor above is legal.


An open deck can be built without notifications in RH and RM districts under the following conditions:
(1) The deck is less than 3 feet from existing grade.
(2) Some decks higher than 3 feet on a hill with slope over 15% (1 vertical & 6.6 horizontal)
(3) Decks on posts less than 10 feet above grade, within buildable area, no firewall required & setback 3 to 5 feet from property lines.
(4) Cantilevered decks within buildable area, no firewall required & setback 3 to 5 feet from property lines. Cantilevered deck can be more than 10 feet high as long as it is within the height limit.
(5) Deck on existing structure when the deck is within buildable area, no firewall required & setback 3 to 5 feet from property lines.
(6) Roof decks that are setback 5 feet from all building edges & within buildable area.
Please Click Here for more information on Deck


Older building may have the Building Code required egress stair located in the required open space. If the replacement stair is within the same footprint and height of the old stair or minimum required by the current Building Code, then the replacement stair can be exempt from notification. The exemption does not apply to new fire wall when the original stair did not have a fire wall, unless the entire firewall is located next to a blank wall of the adjacent neighbor.

The above 5 exceptions summarizes most but not all the rules, for a complete list of rules and regulations please see San Francisco Planning Department Zoning Administrator Bulletin Number 4.