Modesto | ADUs, Duplexes and SB 9

Modesto ADUs, R-1 Duplexes and SB 9: Sizes, Heights, Parking and Permits

A Modesto house lot can hold more than one home in three ways. The City's own rule allows a duplex on any R-1 lot. An accessory dwelling unit (ADU) is a second, smaller home on a lot with a main home, and a junior ADU is a unit of 500 square feet or less inside the house. State law, SB 9, adds a two-home route on single-family lots. This page sets out each route under the Municipal Code, read in Supplement 72, and says where state law overrides the City text. The Modesto overview sums up the whole guide.

The R-1 Duplex Rule

Section 10-3.217 reads: "A duplex is allowed on any lot in the R-1 Zone subject to all other standards of the R-1 zone including but not limited to lot coverage, setbacks, height, parking, and fencing but not including density. If the lot has access to an alley, one (1) or both driveways may access the alley." Section 10-2.121(B) defines a duplex as "Two (2) primary single-family dwelling units on a single parcel that have separate entrances." Joined or apart, the pair counts as a duplex, and the definition adds that a duplex "is not a multiple-family dwelling."

This rule is new. The old Section 10-3.217 dealt with "Corner Lots in the R-1 Zone," and the Housing Element the City adopted on December 5, 2023 says: "The City currently allows duplexes on corner lots in the R-1 Zone by right." Each home on a corner had to take driveway "access from different streets." City Council Ordinance No. 3797-C.S., printed in the City's 2025 ordinance book, opened the rule to every R-1 lot. Its findings say the duplex changes "build on the existing Code's allowance for duplexes on corner lots in the R-1 zone, extending the permission to all lots." The ordinance takes effect 30 days after final passage, and its pages print an effective date of July 24, 2025. It carries out Program A-11 of the Housing Element.

Because a duplex is "subject to all other standards of the R-1 zone," it must fit within R-1 lot coverage (50 percent on an interior lot), setbacks (15 feet in front, 5 feet on the sides for one story, 10 feet in the rear) and the height limit of two stories or 30 feet. Table 5.1-1 requires "2 per unit" for single-family residential parking, and a duplex is two single-family units, so four spaces is the plain reading; confirm the count with the Planning Division. The parking, setbacks and height page has every number. Table 3.1-1 marks the use "P," permitted, so no use permit is needed. A two-story duplex still goes through the second-story review in Section 10-4.110.

ADU Types and How Many Are Allowed

Modesto's ADU rules are in Article 5 of Chapter 4, Sections 10-4.501 to 10-4.512, adopted by Ordinance No. 3762-C.S. in 2023 and amended in 2025. Section 10-4.502 allows ADUs "in any zone that allows single-family residential, multi-family residential, or mixed-use." Section 10-4.504 names four types: attached, detached, converted (inside an existing house or accessory building) and the junior ADU.

Section 10-4.505(a) sets the count on a lot with a house or duplex: an owner may choose ADU types "or a combination thereof not to exceed two (2) additional units beyond the primary single-family residence." A detached ADU needs "Minimum four (4) foot side and four (4) foot rear yard setbacks." One junior ADU is allowed per single-family lot.

That cap is stricter than state law. Government Code 66323(a) requires a city to approve "any combination" of one ADU and one junior ADU within the space of the house, plus "One detached, new construction, accessory dwelling unit." That is three units beyond the house. Modesto's own Section 10-4.511 says the City "shall ministerially approve the types of ADUs specified in Government Code Section 66323," and Section 10-4.512 says state law applies "In the event of a conflict." So the state combination controls.

On a lot with apartments, Section 10-4.505(b) allows ADUs in non-living space such as storage rooms and garages, up to 25 percent of the existing units with at least one, and detached ADUs: up to two with a proposed apartment building and up to eight with an existing one. Section 10-4.505(c) also allows up to two detached ADUs of 1,200 square feet each on the lot of a tax-exempt place of worship in a residential or commercial zone. ADUs and junior ADUs "may only be rented in increments of thirty (30) days or more" (Section 10-4.502(c)).

ADU Size, Height and Parking

Sources: Modesto Municipal Code 10-4.505, 10-4.506, 10-4.507, 10-4.508 and 10-4.509, Supplement 72; Government Code 66321, 66322 and 66323.

Section 10-4.506(b) caps a detached ADU at 1,200 square feet and adds that it "shall not exceed the square footage (in floor area) of the existing or proposed single-family dwelling." On a lot with a small house, that second rule could hold an ADU under 800 square feet. Government Code 66321(b)(3) bars any size limit "that does not permit an accessory dwelling unit with at least 800 square feet of interior livable space," so an ADU of 800 square feet must be allowed whatever the size of the house.

Section 10-4.507 allows 16 feet for a detached ADU, 18 feet within "one-half (0.5) of one (1) mile walking distance of a major transit stop or a high-quality transit corridor," and two more feet for a roof that matches the slope of the main home. An attached ADU may reach 25 feet or the zone's limit, whichever is lower, and no ADU may exceed two stories. Section 10-4.503(k) adds that for an ADU of 800 square feet or less, the lot coverage limits in Table 4.1-1 do not apply.

Parking is light. Section 10-4.508 allows "No more than one (1) off-street parking space," which may be tandem in the driveway or in the front setback on the existing driveway. A garage torn down or converted for an ADU need not be replaced. No space is required within one-half mile walking distance of public transit, in an architecturally and historically significant district, for an ADU inside the house, where a car share vehicle is within one block, or when the ADU is applied for with a new home.

A second-story ADU next to the yard of a one-story home must have no windows facing it, or windows with a sill "at least five (5) feet eight (8) inches from the finished floor" (Section 10-4.510(b)). Section 10-4.510(c) lets the City Council bar ADUs by resolution in areas with water and sewer limits.

Junior ADUs

Section 10-4.509 requires a junior ADU to be "contained within the walls of the existing or proposed single-family residence," which includes a converted attached garage. It may not exceed 500 square feet, needs its own entrance, and needs an efficiency kitchen with a cooking appliance, a food preparation counter and storage cabinets. It may share a bathroom with the house.

Before final inspection, the owner records a deed restriction requiring the owner to live in either the junior ADU or the rest of the house, barring sale of the junior ADU apart from the house, and limiting its size. The City's Planning Forms page posts a Junior Accessory Dwelling Unit Deed Restriction form for this. Section 10-4.505(a)(4) allows a junior ADU only "where there is only one (1) existing or one (1) proposed single-family dwelling," so a duplex lot gets ADUs but no junior ADU.

Permits, Review Time and Fees for ADUs

Section 10-4.503 says the City "shall ministerially approve an application for a building permit for an ADU or JADU." Ministerially means staff check the plans against fixed rules, with no hearing. Where a home already stands on the lot, the City must act within 60 days of a complete application, or the application "shall be deemed approved." A denial must come with a full list of what is wrong and how to fix it. The City may not require fire sprinklers in the ADU if the main home has none, and may not require fixes to existing nonconforming conditions.

The City ADU page lays out eight steps and adds two programs. Stanislaus County and six cities, Modesto among them, have "developed seven (7) different ADU plans for detached new construction," and an ADU Express program offers a one-hour meeting with Building, Fire, Planning and Engineering staff where "Projects that meet State and local ministerial standards may receive same-day approval." Applications go in through the City's eTRAKiT online permit system.

On fees, Section 10-4.503(i) bars impact fees on an ADU "less than seven hundred fifty (750) square feet." The state rule, Government Code 66311.5, covers an ADU "that has 750 square feet of interior livable space or less." The statute controls, so an ADU of exactly 750 square feet pays no impact fee. Section 10-4.503(h) adds that an ADU is not a new residential use for water and sewer connection fees "unless the ADU was constructed in conjunction with a new single-family dwelling." The City's Building Safety fee page lists a permit fee of $2,429 for "Accessory Dwelling Units and small dwellings up to 1,200 sf," and a new address for the ADU costs $197.

SB 9 Two-Unit Projects

SB 9 requires staff approval of up to two homes on a lot in a single-family zone under Government Code 65852.21. Modesto has written no rules for it. Section 10-4.502(f) only says the ADU rules "may be applicable to lots zoned for single-family residential uses and built with units in compliance with Senate Bill No. 9." So the statute governs:

• The City may not apply standards that would "physically preclude either of the two units from being at least 800 square feet."
• Side and rear setbacks may be no more than four feet, and none for an existing building rebuilt in place.
• Parking may be one space per home at most, and none within one-half mile walking distance of a high-quality transit corridor or major transit stop.
• The project may not remove housing "occupied by a tenant in the last three years," and may not be on a City landmark site or a contributing building in a historic district.
• The City must decide within 60 days of a complete application, or the project is deemed approved.

Modesto is a charter city, and the statute applies to every city "whether general law or chartered." For most R-1 owners the City's duplex rule does the same job with fewer limits: it has no tenant test, no landmark bar, and ADUs can be added. SB 9 helps where the four-foot setbacks or the guaranteed 800 square feet per home make a plan fit that R-1 standards would block.

Where City Text and State Law Disagree

• Number of ADUs. Section 10-4.505(a) caps added units at two; Government Code 66323(a) requires the converted ADU, junior ADU and detached ADU combination. The statute controls.
• Detached ADU no larger than the house. Section 10-4.506(b)(2) could keep an ADU under 800 square feet; Government Code 66321(b)(3) guarantees 800. The statute controls.
• The 750 square foot fee line. Section 10-4.503(i) uses "less than" 750 square feet; Government Code 66311.5 uses "750 square feet ... or less." The statute controls.
• Transit definition. Section 10-4.507 cites "Section 2115 of the Public Resources Code"; the state height rule in Government Code 66321 cites Section 21155.
• City ADU page. It tells readers to check "Modesto Municipal Code section 10-4.115," which was repealed in 2023. The rules in force are Sections 10-4.501 to 10-4.512.

Where to Go Next

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

Frequently Asked Questions

Official sources (checked October 2026)