Building an Accessory Dwelling Unit (ADU) Home in San Francisco

Building a Backyard Home in San Francisco: What You Need to Know About ADUs

Imagine turning your backyard, basement, or unused garage into a modern, self-contained home — complete with a kitchen, bathroom, and private entrance. That’s exactly what an Accessory Dwelling Unit (ADU) allows you to do. In San Francisco, ADUs have become one of the most practical and in-demand ways to increase housing supply, generate rental income, and support multigenerational living — all without leaving your property.

But while California state law strongly supports ADU development, local codes add layers of detail you’ll need to follow closely. Here’s what you need to know before you start drafting blueprints or hiring a contractor.

The Basics: Where and What You Can Build

In California, ADUs are permitted on any lot zoned for single-family or multifamily use that contains an existing or proposed dwelling. Whether attached, within the primary house, or detached in the backyard, an ADU offers flexible design options — but only one ADU is typically allowed per lot under “state” standards. You cannot have both an ADU and a Junior ADU (JADU) on the same lot unless specific conditions are met.

Size and Design Limits

ADUs come with specific size and height restrictions to maintain neighborhood character and ensure safety:

• Detached ADUs:

  • Maximum 16 feet in height
  • Minimum 4-foot side and rear yard setbacks
  • Size limit of 850 sq ft (1 bedroom or less) or 1,000 sq ft (more than 1 bedroom)

• Attached ADUs:

  • Can occupy up to 50% of the primary dwelling’s floor area, or the size limits above — whichever is less

Additionally, no minimum lot size, open space, or lot coverage requirements apply if your ADU is up to 800 sq ft, 16 ft high, and within 4 ft setbacks, making small backyard units especially accessible for urban homeowners.

Parking, Access, and Use Restrictions

Building an ADU doesn’t mean you’ll lose parking — or need to add more. In fact:

• No parking is required for new ADUs
• No replacement parking is needed if you convert a garage or carport
• Independent exterior access is required for all ADUs

However, ADUs must not be used for short-term rentals (such as Airbnb) and cannot be sold separately from the primary residence. They must remain on the same parcel as the main dwelling and comply with all building, fire, and health safety codes.

For Homeowners and Developers: What This Means

These ADU rules reflect California’s push to ease the housing crisis by empowering property owners to add small, livable units to existing neighborhoods. For homeowners, that means new opportunities to:

• Create rental income or family housing without purchasing new land
• Increase property value through legal, compliant expansion
• Participate in sustainable urban growth that makes use of existing infrastructure

But it also means careful planning: exceeding height limits, removing key residential features, or skipping permits can lead to violations and costly corrections. Always consult local planning officials before construction begins.

References

1. San Francisco Planning Code, Sections 207.1–207.2, 2025 Edition.
2. California Government Code Section 65852.2 – Accessory Dwelling Units.
3. California Residential Code (CRC) and CalGreen 2025 – State Housing Standards.
4. City and County of San Francisco Department of Building Inspection (DBI) – ADU Guidelines and Permit Requirements. https://sfdbi.org
5. San Francisco Planning Department – ADU Program: https://sfplanning.org/project/accessory-dwelling-units
6. Reference Clause Section Codes:

• Section 207.2(e), Section 207.2(b), Section 207.1, Section 207.2(c), Section 207.2(d), Section 207.2(e)(1-9), Section 207.1(f), Section 207.2(b), Section 102, Section 414A.6(e); file: 2025 San Francisco CA Planning Code
• Government Code Section 65852.2; file: 2025 CRC Residential Code, 2025 CalGreen

Original Referenced Articles:

“An ADU located on a lot that is zoned for single-family or multifamily use and contains an existing or proposed dwelling…shall meet all of the following requirements…no minimum or maximum size for an ADU, any size based upon a percentage of the proposed or existing primary dwelling, or any limits on lot coverage, floor area ratio, open space, front setbacks, and minimum lot size, for either attached or detached dwellings, that does not permit construction of an ADU meeting all other requirements that is 800 square feet or less in Gross Floor Area, 16 feet or less in height, and with four foot side and rear yard setbacks...”
“Only one ADU will be constructed....The ADU is either (A) attached to or will be constructed entirely within the proposed or existing primary dwelling...or (C) detached...”
“No setback is required for an ADU located within an existing living area or an existing accessory structure, or an ADU that replaces an existing structure and is located in the same location...A setback of no more than four feet from the side and rear lot lines shall be required for an ADU that is not converted from either an existing structure or a new structure constructed in the same location and to the same dimensions as an existing structure…”
“No parking is required for the ADU...When a garage, carport, or covered parking structure is proposed to be demolished in conjunction with the construction of an ADU or converted to an ADU, replacement of those offstreet parking spaces is not required...” “The ADU must have independent exterior access from the existing or proposed primary dwelling or existing accessory structure...”
“An ADU shall not be used for Short-Term Residential Rentals under Chapter 41A of the Administrative Code...”
“A lot with an ADU authorized under this Section 207.1 shall not be subdivided in a manner that would allow for the ADU to be sold or separately financed ..” “ADUs shall include permanent provisions for living, sleeping, eating, cooking and sanitation on the same parcel as the singlefamily or multifamily dwelling is or will be situated. (See Government Code Section