Accessory Dwelling Units

A second, self-contained home on a lot that already has a house. It needs its own entrance, its own kitchen and its own bathroom, and it can be detached, attached to the main house, converted from a garage, or carved out of the existing house as a junior ADU. People use them for rental income, for a parent or an adult child, or as a home office with a door that closes.

Most jurisdictions allow a detached ADU up to 1,200 square feet of interior livable space. Separately, an 800 square foot unit is protected: a city cannot use lot coverage limits, minimum lot size or open space rules to block one that meets the setback and height standards. What your particular parcel will carry still depends on the existing structures and the local objective standards.

Up to three on a single-family lot: one attached ADU, one detached ADU and one junior ADU. SB 543 made that combination explicit because some jurisdictions had been refusing it. Multi-family properties can qualify for more, depending on site conditions. What your particular parcel will physically carry is a separate question from what the law permits, which is what the feasibility visit settles.

For a new attached or detached unit, a city generally cannot require more than four feet from the side and rear property lines. If you convert inside an existing structure, or rebuild on the same footprint, no new zoning setback applies at all. Front setbacks and fire separation are handled separately and are worth checking on your specific lot.

A detached ADU tops out at twenty feet under state law, and it gets there in steps. Sixteen feet is the base. Eighteen feet if the property is within half a mile walking distance of a major transit stop or a high-quality transit corridor, or if the lot has an existing or proposed multistory multifamily dwelling. Then two feet on top of that if the ADU's roof pitch matches the main house, which is worth designing for because it is the difference between a cramped roof and a well-proportioned one.

An attached ADU can go to twenty-five feet, or to the primary dwelling's height limit where that is lower. A city is not obliged to allow more than two storeys. Those are state floors, not ceilings, so plenty of local ordinances permit more and it is worth checking yours rather than assuming sixteen.

Usually not. No parking can be required if the property is within half a mile of public transit, and no replacement parking can be demanded when you convert or demolish a garage to create the unit. Several other state exemptions also apply, so it is worth checking rather than assuming you need a new space.

Not for a standard ADU. The owner occupancy requirement was permanently removed by AB 976, so both the house and the ADU can be rented. For a junior ADU, AB 1154 narrowed the requirement from January 2026 so that it applies only where the JADU shares a bathroom with the primary dwelling.

Once an application is complete, the agency has sixty calendar days to approve or deny it, and a denial must itemise exactly what needs correcting. Since 1 January 2026 the agency also has fifteen business days to tell you whether the application is complete. The part that varies is how many rounds of corrections it takes to get there, which is a function of how well the submittal package anticipates the local comments.

Only where your city or county has opted in under AB 1033, and it is a condominium conversion rather than a lot split: the land stays one parcel as common area and each unit becomes a separate airspace unit. As of September 2026 the statewide list is short, and of the counties we work in San Francisco has opted in and San Jose has opted in for Santa Clara County. No city in Marin, Contra Costa, Alameda or San Mateo had adopted an ordinance as of the latest reporting we could find.

It is also a real process rather than a filing: a parcel map, a condominium plan and recorded CC&Rs, under the Subdivision Map Act and the Davis-Stirling Act, usually with lender consent and separated utilities. Worth confirming your city's current position before it affects a decision.

Possibly, and it is a different law. SB 9 lets an owner of a single-family lot divide it in two, with each new parcel at least 1,200 square feet and the split no more uneven than 60/40, and put two primary units on each. Four homes where one house stood, with ministerial approval, no hearing, no environmental review and a sixty day decision.

The conditions matter. A lot split requires an owner-occupancy affidavit committing you to live in one of the units as your principal residence for three years from approval, rentals have to be longer than thirty days, and rent-controlled, recently tenanted and historic-landmark properties are excluded. SB 9 also survived a court challenge rather than winning it outright: a 2024 ruling struck it down for five Southern California charter cities, the Court of Appeal reversed that in November 2025 and sent the question back, so it stands statewide in 2026 with the constitutional argument still open.

SB 9 units are primary dwellings, not ADUs, and whether an ADU can additionally go on a split lot is constrained. If your goal is rental income rather than selling land, an ADU is usually the simpler route.

Recorded values on Ecostep's residential building permits run from $25,000 to $210,000, which gives you the shape of it without pretending to be a quote. On an ADU the lot drives the number more than the finishes do: how far the unit sits from the existing service, whether the panel has capacity, the slope, the site access and the condition of the sewer lateral. A garage conversion is usually the cheapest permitted route because the footprint and slab already exist.

Not on a unit of 750 square feet or less of interior livable space. Above that, fees must be proportional to the primary dwelling rather than charged as though you had built a new house. Units under 500 square feet are also exempt from school district developer fees, which means every junior ADU falls inside both exemptions.

On a sloped or hillside lot, usually yes, and it is worth budgeting for early because the report dictates the foundation rather than the other way round. Flat lots often do not need one. Engineering reports from specialist consultants, geotechnical, soils, structural, civil or energy, are part of the soft cost side of an ADU budget rather than the construction side.

It depends on the unit, the site and whether the main house has them. Fire and life safety is one of the six things that genuinely move an ADU budget, along with the separation required from existing structures and the property line, so it belongs in the feasibility conversation rather than being discovered at plan check.

Yes, and it is normally the least expensive permitted route, because the footprint, the slab and often the shell are already there. No new zoning setback applies to a conversion inside the existing footprint, and no replacement parking can be required. The work that does need budgeting is the envelope, the insulation, the plumbing and whatever the demolition uncovers under the slab.

A junior ADU is up to 500 square feet and sits within the walls of the existing or proposed single-family house. It needs an independent exterior entrance and an efficiency kitchen, and it may share sanitation facilities with the main home. It is the smallest and lightest option, and it is the one where an owner occupancy requirement can still apply, but only where it shares a bathroom with the primary dwelling.

We do. Ecostep prepares the stamped construction drawings, submits them, answers the plan check comments and carries the application through to an issued permit before work starts. Our licence number goes on the application, which also means the work is inspected against our licence rather than yours. If you pull it yourself as an owner-builder, you accept responsibility for every permit, every development standard and code compliance across the whole project.

Contra Costa, Alameda, San Francisco, San Mateo, Santa Clara and Marin counties. Our permitted work on public record sits in Alameda, Berkeley, Concord, Fremont, San Francisco, Belmont and San Leandro.