Lancaster | ADUs and SB 9

Lancaster ADUs, Junior ADUs and SB 9 Two-Unit Projects

An accessory dwelling unit (ADU) is a second, complete home on a lot that already has, or will have, a main home. A junior ADU is a unit of up to 500 square feet inside a single-family home. An SB 9 two-unit project puts two main homes on one house lot under state law. Lancaster sets its rules for all three in Chapter 17.41 of the Municipal Code, rewritten by Ordinance No. 1106 on October 10, 2023. Several City rules are stricter than state law and yield to it; this page names each one. See the Lancaster overview for the whole guide.

ADU Approval

ADUs and junior ADUs are a P use, permitted, in every residential zone and in the MU-N, MU-C and MU-E mixed use zones. Section 17.41.014 says one that "complies with all of the applicable development standards of this article requires only a ministerial building permit." Ministerial means staff check the plans against fixed rules, with no hearing and no judgment calls. The City "shall approve or deny an application to create an ADU or JADU within 60 days from the date that the city receives a completed application," or the application "is deemed approved." A denial must come with a list of what is missing and how to fix it, within the same 60 days.

Two timing rules come from state law. When the ADU is filed together with a new house or apartment building, the City may wait to act on the ADU until it acts on the main building. And Government Code 66317 requires the City to say whether an ADU application is complete "not later than 15 business days after the permitting agency received the application," a step the City code does not repeat. A demolition permit for a detached garage that an ADU replaces is reviewed with the ADU and issued at the same time.

ADU Size, Height and Setbacks

Sources: Lancaster Municipal Code 17.41.012 and 17.41.016, Supplement 10 - 2025.

A setback is the open distance a building must keep from a property line. Lancaster uses the same three numbers for every new ADU: "Front-yard setback: 25 feet," and 4 feet from the side and rear lines. An interior ADU, made from space inside the house or an existing accessory building, keeps only the side and rear distances "sufficient for fire and safety." Lot coverage, the share of the lot under roofs, may not pass 40 percent in RR-2.5, RR-1, SRR, R-15,000 and R-10,000, 50 percent in R-7,000, MDR and HDR, and 60 percent in the mixed use zones. For attached ADUs, the code adds that the front setback and lot coverage may not force the ADU below 850 square feet.

On a lot with apartments, Section 17.41.016.B allows interior ADUs in unused space such as storage rooms, garages and attics, up to 25 percent of the number of homes in the building and at least one, plus "No more than 2 detached ADUs per lot." Every ADU must be at least the size of an efficiency unit as defined in Section 17958.1 of the Health and Safety Code. A garage that serves an existing home may be converted into an ADU or junior ADU under Section 17.08.220.

Parking, Rentals and Other ADU Rules

• Parking. "One off-street parking space is required for each ADU," 10 by 20 feet, in a garage, carport or driveway, and tandem parking (one car behind another) is allowed. None is needed for an interior ADU, an ADU "within one-half mile walking distance of public transit," or the small ADUs state law exempts.
• Garage replaced by an ADU. Lost garage spaces do not have to be replaced. When a junior ADU is made from an attached garage, the lost spaces "shall be replaced in kind."
• Rentals. "No ADU or JADU may be rented for a term that is shorter than 30 days."
• No separate sale. An ADU may be rented, but may not be sold apart from the lot and main home, except under Government Code Section 65852.26. A deed restriction saying so is recorded before the building permit.
• Fire sprinklers. Required in the ADU only if the main home needs them, and building an ADU "does not trigger a requirement for fire sprinklers to be installed in the existing primary dwelling."
• Septic. An ADU on a septic system needs a percolation test (a soil drainage test) from the last 5 years, or 10 years if recertified.
• Old problems on the lot. The City "will not deny an ADU or JADU application due to a nonconforming zoning condition, building code violation, or unpermitted structure" that is no threat to health and safety. An unpermitted ADU built before January 1, 2018 can be legalized, with health and safety exceptions.

Where State ADU Law Controls

A city ADU rule that is stricter than state ADU law has no effect to that extent. These Lancaster rules are in that group:

• Number of ADUs. Section 17.41.016.A.1.a allows "No more than one JADU and one ADU" per parcel. Government Code 66323 requires approval of "any combination" of a converted ADU and junior ADU plus one "detached, new construction" ADU of up to 800 square feet. The statute controls.
• Owner occupancy. Section 17.41.018.E requires an owner to live on site. Government Code 66315 bars any added standard for an ADU, "including an owner-occupant requirement." Government Code 66333 allows the rule only for a junior ADU that shares a bathroom with the house.
• Front setback for a detached ADU. The 25 foot front setback comes with an 850 square foot guarantee only for attached ADUs. Government Code 66321 bars front setbacks and lot coverage limits that do not permit an ADU "with at least 800 square feet of interior livable space and with four-foot side and rear yard setbacks." The statute protects a detached ADU too.
• Parking. Government Code 66322 also bars ADU parking within a historic district, where permit parking is not offered to the ADU, "When there is a car share vehicle located within one block," and when the ADU is filed with a new home. The City list is shorter; the statute controls.
• Impact fees. Government Code 66311.5 bars impact fees on an ADU of 750 square feet or less and a junior ADU of 500 square feet or less. Lancaster's impact fee chapter has no ADU rule, so the statute sets the limit.
• Old statute numbers. Section 17.41.010 cites Government Code 65852.2, 65852.22 and 65852.23. The first two are no longer in the Legislature's database of sections in force, and the state ADU rules now sit in Sections 66314 and following. Section 17.41.012 also points to "Section 21.78.050.D.1," a section that does not exist in Lancaster's code.

SB 9 Two-Unit Projects in Lancaster

Source: Lancaster Municipal Code 17.41.058, Supplement 10 - 2025.

Lancaster's two-unit rules are in Chapter 17.41, Article III. The applicant must be an "individual property owner," which leaves out companies but includes a community land trust and a qualified nonprofit, and must bring a certificate of compliance showing the lot was legally created. The Community Development Director decides "ministerially by the community development director without discretionary review," and the approval takes effect only once the deed restrictions and easements are recorded. Government Code 65852.21 adds the deadline the City code leaves out: a decision "within 60 days from the date the local agency receives a completed application," or the project is deemed approved.

The size limits come first. "The total floor area of each primary dwelling that is developed under this section must be less than or equal to 800 square feet and more than 500 square feet," the two units together may not pass 1,700 square feet, and "No new primary dwelling unit shall exceed a single story." An existing house over 800 square feet may stay but may not grow. State law allows such limits because it only guarantees that each unit can reach 800 square feet; the City code says any standard, other than side and rear setbacks, that would stop two homes of 800 square feet "shall be waived." By VanPlex arithmetic, 50 percent coverage on a 7,000 square foot lot is 3,500 square feet, so on most R-7,000 lots the 1,700 square foot total is the limit that binds.

Other conditions apply. Homes must not replace rent-restricted or rent-controlled housing, housing withdrawn under the Ellis Act in the last 15 years, or housing "occupied by a tenant in the last 3 years," and no more than 25 percent of the exterior walls may be torn down unless no tenant has lived there in 3 years. Each home needs its own utility connection and underground service. Rentals must be 30 days or longer, the lot may hold residential uses only, and the owner records a deed restriction that bars separate sale of a home and any further building beyond the two units. The objective design standards in Article IV apply, with Article III winning a conflict.

Duplex by Director's Review: the Other Route

SB 9 is one of two ways to get two main homes on a Lancaster house lot. The residential uses matrix in Section 17.08.050 lists a duplex as a director's review use in the RR, R, MDR and HDR zones. A duplex built that way follows the zone's ordinary height, lot coverage and setbacks (in the R zones, 35 feet and 40 to 50 percent coverage), with a corner duplex lot needing 10,000 square feet and 100 feet of width. Each duplex home also needs 2 garage spaces under Section 17.08.100, unless the lot is within one-half mile of a major transit stop. The SB 9 route caps each new home at 800 square feet and one story but asks for only 1 parking space per home. The parking, setbacks and height page compares the two sets of standards.

Where to Go Next

Related on VanPlex: the Lancaster overview, the California ADU law page and the SB 9 page.

Frequently Asked Questions

Official sources (checked October 2026)