Merced | ADUs and SB 9
Merced, California ADUs and SB 9: Chapter 20.42, ADU Sizes, Heights, Setbacks, Parking, Junior ADUs, Tiny Homes on Wheels and Two-Unit Projects
This page covers the state routes to a second or third home on a Merced house lot. An accessory dwelling unit, or ADU, is a second home on a lot with a main home; a junior ADU, or JADU, is a unit of 500 square feet or less inside the house. Merced writes its ADU rules in Chapter 20.42 of the Zoning Ordinance, adopted by Ordinance 2555 on September 18, 2023. The chapter also sets rules for tiny homes on wheels used as ADUs. The chapter still cites Government Code 65852.2 and 65852.22, the section numbers state ADU law used before 2024, and several of its rules are stricter than today's statutes. Merced has no SB 9 chapter at all. This page sets the City text beside state law and says which controls. The Merced overview has the summary.
Merced ADUs and SB 9 in six facts
- A detached ADU may have up to 1,200 square feet of floor area, not counting a garage or carport. Source: Merced Municipal Code 20.42.030(E)(2)
- A detached ADU may be 16 feet tall, or 18 feet within one-half mile walking distance of a major transit stop or high-quality transit corridor. Source: Merced Municipal Code 20.42.030(G)(1)
- New detached ADUs need four-foot side and four-foot rear setbacks. Source: Merced Municipal Code 20.42.030(G)(3)(d)
- The City acts on an ADU or JADU application within 60 days of a complete application, or it is deemed approved. Source: Merced Municipal Code 20.42.020(B)
- A tiny home on wheels used as an ADU must be under 400 square feet, excluding lofts, and no taller than 14 feet. Source: Merced Municipal Code 20.42.060(D)
- An SB 9 two-unit project is decided ministerially within 60 days, and the City may not block two units of at least 800 square feet. Source: Government Code 65852.21(b) and (h)
How an ADU Is Approved in Merced
Section 20.42.020(A) says an application that meets Chapter 20.42 "shall be granted with ministerial review, approval, and issuance of a building permit." Ministerial means staff checks the plans against written rules with no hearing. Where a house or apartment building already exists, "the city shall act on the application to create an ADU or a JADU within sixty (60) days from the date a complete application is received", and an application not acted on in that time "shall be deemed approved." When a new house is proposed at the same time, the City may wait to approve the ADU until it approves the house. The City may not require the owner to fix nonconforming zoning conditions or install public improvements as a condition.
State law adds a step that the City text does not repeat. Government Code 66317 requires the permitting agency to tell the applicant whether the application is complete "not later than 15 business days after the permitting agency received the application," with a list of missing items. If it does not, the application is deemed complete. An applicant who is turned down may appeal in writing, and the agency must decide the appeal within 60 business days.
Section 20.42.020(C) lets an owner of an ADU built before January 1, 2020 ask for a five-year delay in enforcing a building standard, when the violation is not a health and safety issue. The City may not approve such requests after January 1, 2030.
How Many ADUs and of What Kind
Section 20.42.030 lists four kinds of ADU: attached to the house, detached, converted from existing space (an attached garage, a detached garage, a pool house, or unused space in an apartment building), and the junior ADU. A tiny home on wheels is a fifth kind, covered below. On a lot with a house, Merced allows "One (1) attached ADU (new or converted), one (1) detached ADU (new or converted), and one (1) JADU." That is up to four homes on one R-1 lot: the house and three accessory units.
On a lot with an apartment building, converted ADUs may not exceed 25 percent of the existing units, and "Not more than two (2) detached ADUs may be located on a parcel that contains an existing multi-family dwelling." Government Code 66323, as amended effective January 1, 2026, requires a city to approve "not more than eight detached accessory dwelling units" on a lot with an existing multifamily dwelling, limited to the number of existing units. A building with eight or more apartments can therefore add eight detached ADUs under the statute, against two in the City text. The statute controls, and Section 20.42.020(D) says so itself: where the chapter conflicts with state law, "state law shall supersede this chapter."
Section 20.42.030(D) sets "No Minimum Parcel Size" and says no lot coverage, floor area ratio, open space or minimum lot size rule may block an ADU of 800 square feet, 16 feet tall, with four-foot side and rear setbacks. An ADU may be built only when a house or apartment building exists on the lot or is built at the same time.
The City Rules Beside State Law
| Subject | Merced Chapter 20.42 | State law (Government Code section) |
|---|---|---|
| Number on a house lot | One attached ADU, one detached ADU and one JADU | At least one ADU and one JADU, or one detached ADU of up to 800 sq ft with a JADU (66323) |
| Detached ADUs, existing apartments | Not more than 2 | Up to 8, no more than the existing units; 2 with proposed apartments (66323) |
| Size, detached | 1,200 sq ft | Up to 1,200 sq ft; a cap may not be under 850, or 1,000 with more than one bedroom (66314, 66321) |
| Size, attached | 50 percent of the house or 1,200 sq ft, whichever is less | 50 percent of the existing house; 800 sq ft must still fit (66314, 66321) |
| Height, detached | 16 ft; 18 ft near transit, plus 2 ft for a matching roof; 18 ft on a multistory apartment lot | The same floors (66321) |
| Side and rear setbacks | 4 ft; none for a conversion or same-place rebuild | No more than 4 ft (66314) |
| Parking | One space per ADU or per bedroom, whichever is less; six exemptions | No more than one per ADU or per bedroom (66314) |
| Owner occupancy, ADU | In R-1 after January 1, 2025, the owner may rent the house or the ADU, not both | No owner-occupant requirement may be imposed (66315) |
| Owner occupancy, JADU | Required in every case | Only where the JADU shares sanitation facilities (66333) |
| Impact fees | None for ADUs "of less than seven hundred fifty (750) square feet" | None at 750 sq ft of interior livable space or less (66311.5) |
| Water and sewer | Only on parcels connected to public water and sewer | Areas may be set by adequacy of water and sewer; septic allowed with a percolation test (66314, 66323) |
Sources: Merced Municipal Code Chapter 20.42; Government Code 66311.5, 66314, 66315, 66321, 66323 and 66333.
Size, Height, Design and Parking
- Size. An attached or converted ADU may not exceed "fifty (50) percent of the living area of the existing primary dwelling on the parcel or one thousand two hundred (1,200) square feet, whichever is less." Up to 150 square feet added only for an entrance does not count.
- Height. An attached ADU may not exceed the height of the house or 25 feet, whichever is lower. A detached ADU may be 16 feet, or 18 feet within one-half mile walking distance of a major transit stop or a high-quality transit corridor, plus two feet to match the house's roof pitch, and 18 feet on a lot with a multistory apartment building.
- Design. The ADU or JADU entrance must have "the same exterior finish materials" as the main home, and the ADU "shall have the same roof form as the primary dwelling and shall not have a flat roof." Both addresses must be visible from the street.
- Other standards. Section 20.42.030(F)(1) applies the setbacks, coverage, height and design rules of the main home, "with certain exceptions, discussed in this chapter."
- Parking. "A maximum of one (1) additional off-street parking space shall be provided for an ADU or per bedroom, whichever is less", allowed as tandem parking on a driveway or in a setback, uncovered in a setback. A converted garage need not be replaced.
- No parking needed within one-half mile walking distance of public transit, in an architecturally and historically significant historic district, for an ADU inside the house or an accessory structure, where on-street permits are not offered to ADU occupants, within one block of a car share vehicle, or for any attached or converted ADU.
- Sprinklers. Fire sprinklers are not required in an ADU "if they are not required for the single-family dwelling."
Section 20.42.030(F)(4) says "An ADU or JADU shall only be allowed on parcels connected to public water and sewer service." State law lets a city designate ADU areas based on "the adequacy of water and sewer services" (Government Code 66314(a)), and Government Code 66323(f) lets a city ask for a percolation test for an ADU on a septic system. An owner of a Merced lot on a well or septic system should ask the City how it applies this rule to the ADUs state law requires it to approve.
Junior ADUs
Section 20.42.050 allows one JADU per lot, "entirely within the walls of an existing or proposed single-family dwelling," of no more than 500 square feet, with its own entrance and an efficiency kitchen with "Cooking facilities with appliances" and a counter and cabinets. A JADU may share the house's bathroom or have its own. Before a building permit, the owner records a deed restriction that limits the size, bars separate sale and requires owner occupancy.
Section 20.42.050(G) says "The owner shall reside on the property in either the newly created JADU or the remaining portion of the single-family dwelling", in every case. Government Code 66333, as amended by AB 1154 effective January 1, 2026, says "Owner-occupancy shall not be required if the junior accessory dwelling unit has separate sanitation facilities." The statute controls: a Merced JADU with its own bathroom should need no owner-occupancy restriction. Confirm the deed restriction wording with the City before recording it.
Owner Occupancy, Rentals and Fees
Section 20.42.040(A) says the City will not impose owner occupancy on ADUs permitted between January 1, 2020 and January 1, 2025, and then: "After January 1, 2025, the following section shall apply. The owner of a parcel with an ADU shall be permitted to rent either the primary unit or the ADU, but not both", in the R-1 district and equivalent planned development land. Government Code 66315 says no standard beyond Section 66314 "shall be used or imposed, including an owner-occupant requirement, except that a local agency may require that the property may be used for rentals of terms 30 days or longer." The statute controls, so a Merced owner should be able to rent both the house and the ADU. Ask the City in writing if a permit condition says otherwise.
An ADU or JADU may be rented for at least 30 continuous days, and an ADU may not be sold apart from the house except by a qualified nonprofit to a qualified buyer. ADUs are not a new residential use for water and sewer connection fees unless built with a new house. Section 20.42.040(D)(3) says "no impact fees shall be imposed on ADUs of less than seven hundred fifty (750) square feet", and larger ADUs pay in proportion to the house. Government Code 66311.5 sets the line at "750 square feet of interior livable space or less", so an ADU of exactly 750 square feet pays no impact fee under the statute. The City impact fee itself is on the permits and fees page.
Tiny Homes on Wheels
Section 20.42.060 lets a tiny home on wheels (THOW) serve as a detached ADU, "approved ministerially by the director" when it is registered with the California Department of Motor Vehicles and certified to the listed national standards. Motorhomes and other recreational vehicles do not qualify. If the wheels come off and the unit sits on a foundation, it "no longer qualifies as a THOW" and must meet the building code.
| Subject | Rule in Section 20.42.060 |
|---|---|
| Number | One per lot with a house; on an apartment lot, no more than two detached ADUs in total |
| Size | Under 400 sq ft excluding lofts; at least 100 sq ft of first floor living space |
| Height | 14 ft at most |
| Placement | 5 ft side and rear setbacks, 6 ft from any building, never in the front yard or the driveway |
| Certification | Registered with the DMV and certified to ANSI A119.2, NFPA 1192 and A119.5 |
| Design | No flat roof; pitch at least 2:12; wheels and hitch screened; built to look like a house |
| Utilities | Connected to electricity, water and sewer; one water and sewer connection per parcel |
| Renewal | Permit ends if the unit leaves, lapses with the DMV, or is not renewed with the City every five years |
Source: Merced Municipal Code 20.42.060.
A THOW must sit on a paved pad: asphalt at least two inches thick, concrete at least three inches thick, permeable pavers at least 80 millimeters thick, or another surface the City approves. No added parking is required, and in a flood zone the unit must meet the elevation rules for manufactured homes in Chapter 17.48.
SB 9 Two-Unit Projects in Merced
SB 9 lets an owner build two main homes on a lot in a single-family zone with staff approval. Merced has no SB 9 chapter, so Government Code 65852.21 applies directly; the statute covers a city "whether general law or chartered", which includes Merced, a charter city. In Merced's R-1 district, two homes on one lot are already a permitted use as a duplex under Section 20.08.020(B), when the lot meets the zone minimum size and the driveway and street rules. SB 9 adds a second path with state limits on what the City may require:
- Size and setbacks. The City may not apply standards that "would physically preclude either of the two units from being at least 800 square feet in floor area," and may require side and rear setbacks of "up to four feet."
- Parking. Up to one space per unit, with none within one-half mile walking distance of a high-quality transit corridor or major transit stop, or within one block of a car share vehicle.
- Timing. A complete application is approved or denied within 60 days, or it is deemed approved.
- Tenants. The project may not demolish or alter housing "that has been occupied by a tenant in the last three years", or rent-restricted housing.
- Rentals. Units may be rented only for terms longer than 30 days.
- With a lot split. On lots that use both SB 9 sections, the City need not allow ADUs or JADUs, and need not allow more than two units on each new lot.
Using SB 9 with an urban lot split, an owner can reach four homes on what was one R-1 lot: two on each new lot. Without a split, the ADU path in Chapter 20.42 allows the house plus an attached ADU, a detached ADU and a JADU. The lot size and subdivisions page covers the split.
Best For
- ✓ R-1 house lots on City water and sewer, where an attached ADU, a detached ADU of up to 1,200 square feet and a JADU can be added with no hearing.
- ✓ Owners who want a small rental unit fast, using a registered tiny home on wheels of under 400 square feet on a paved pad.
- ✓ Lots with existing apartment buildings, where state law allows up to eight detached ADUs, no more than the existing units.
Usually Fails When
- ✕ The lot uses a well or septic system, where Section 20.42.030(F)(4) limits ADUs to parcels on public water and sewer.
- ✕ A detached ADU is drawn taller than 16 feet on a lot more than one-half mile from a major transit stop or high-quality transit corridor.
- ✕ A plan relies on the City text alone and stops at two detached ADUs on an apartment lot.
What To Verify Before Spending Money
- → Whether the City still asks for owner occupancy on R-1 ADUs or on JADUs with their own bathroom.
- → Which form the City uses for an SB 9 two-unit project, since Title 20 has no SB 9 procedure.
- → Whether the lot is within one-half mile walking distance of public transit, which removes the ADU parking space.
Where to Go Next
Related on VanPlex: the Merced overview, the California ADU law page and the California SB 9 page.
Frequently Asked Questions
How big can an ADU be in Merced, California?
How tall can a detached ADU be in Merced?
How many ADUs can I build on a house lot in Merced?
Do I have to live on my Merced property to rent an ADU?
Does an ADU need a parking space in Merced?
How long does Merced take to approve an ADU?
Can I put a tiny home on wheels in my backyard in Merced?
Do I have to live in the house to add a junior ADU in Merced?
Are there impact fees on an ADU in Merced?
Can I build two homes on my Merced lot under SB 9?
Official sources (checked October 2026)
Planning a multiplex in Merced?
VanPlex builds multiplexes in British Columbia, Canada. For a lot in Merced, our team makes a custom proforma by hand and emails it to you in 24 to 48 hours.
Get a proforma for your address