Multifamily and Duplex Properties for Sale in Solano County: Zoning and Rent Rules

What multifamily in Solano County looks like

Apartment buildings in Solano County are almost all inside its seven cities: older courtyard and garden complexes in Vallejo and Fairfield, newer communities along I-80 in Vacaville and Dixon, and small duplex and fourplex stock in Benicia, Suisun City and Rio Vista. In unincorporated areas, multifamily is narrow. A handful of duplex districts sit in the unincorporated pockets near Fairfield and Vallejo, one multifamily district is mapped near Vallejo, and the rest of the county is farmland and rural homes where an ADU or secondary dwelling is the realistic way to add a rental. For financing, see financing 2-4 unit properties and apartment building loans.

Start by confirming jurisdiction. The county’s zoning FAQ says county zoning reaches only unincorporated land. Inside Benicia, Dixon, Fairfield, Rio Vista, Suisun City, Vacaville or Vallejo, the city’s code governs.

Jurisdiction	Who sets zoning and rental rules	Our guide
Benicia, Dixon, Fairfield, Rio Vista, Suisun City, Vacaville, Vallejo	Each city	Benicia · Dixon · Fairfield · Rio Vista · Suisun City · Vacaville · Vallejo
Unincorporated (Elmira, Cordelia, Green Valley, Birds Landing, Collinsville, rural areas)	Solano County Planning Services, Chapter 28	Elmira · Green Valley

Where the zoning allows it

Unincorporated residential districts are in Section 28.32, the Residential-Traditional Community (R-TC) districts. Most R-TC districts allow only single-family homes. These are the ones that allow two or more units:

District	Where it is mapped	Units allowed	Key standards
R-TC-D-4 Duplex (replaced R-D, Starr Subdivision)	Unincorporated Fairfield and Vallejo	Duplex or two houses	4,000 sq ft lot; one unit per 2,000 sq ft; 35 ft
R-TC-D-6 Duplex (replaced R-D, Homeacres)	Unincorporated Fairfield and Vallejo	Duplex or two houses	6,000 sq ft lot; one unit per 3,000 sq ft; 35 ft
R-TC-MF Multifamily (replaced R-M)	Unincorporated Vallejo	Duplexes, apartments, dwelling groups, rooming houses, SROs	5,000 sq ft lot; 50 ft height; 30 ft front setback
R-TC-MU Mixed Use	Unincorporated Vallejo, Birds Landing, Cordelia, Elmira	Multifamily with retail	4,000 sq ft lot; 35 ft; zero front and rear setbacks

In the duplex districts, a second house counts as a second main building, not an ADU. Outside R-TC-MF, a duplex and a single-family home generally cannot share a lot unless they form an approved dwelling group.

ADUs and secondary dwellings. Section 28.72 sets the county’s ADU rules. Detached ADUs are capped at 850 square feet in R-TC districts (1,000 with more than one bedroom) and 1,500 in rural residential districts; JADUs at 500. A lot can have both an ADU and a JADU only if the ADU is detached and no more than 800 square feet. Farm parcels use a different tool, the secondary dwelling, up to 2,400 square feet, and only one per lot. A secondary dwelling is not allowed on a lot that already has a duplex or multifamily building. The county’s ADU page confirms that ADUs are limited to residential lots. See our ADU investment guide.

SB 9. SB 9 applies to single-family residential zones in urbanized areas. Most unincorporated Solano land is agricultural or rural and will not qualify; test any parcel with the county first. See our SB 9 lot split guide. We did not find county SB 9 procedures in the sections we reviewed.

Rent rules: state law only

The Attorney General’s rent cap page lists no city or county in Solano with a local rent increase cap, and does not place Solano in a named region, so it falls under All Other Counties. The cap there is 7.7% for increases from August 1, 2025 and 8.6% for increases from August 1, 2026 through July 31, 2027. The Tenant Protection Act generally covers housing more than 15 years old, plus corporate-owned single-family homes and condos, and requires just cause for terminations after the qualifying tenancy period. Mobilehome space rent ordinances are separate from the AG chart, so check a park’s city or the county. Our guides cover AB 1482 and just cause eviction.

Solano County rental property rules

ADU and JADU rentals. An ADU may be rented separately but never sold separately. A JADU, and any ADU on a lot that also has a JADU, cannot be rented for 30 days or less. A JADU requires owner occupancy of either the main house or the JADU, backed by a recorded deed restriction.
Secondary dwellings on farm parcels must be rented for more than 30 days and cannot be sold apart from the main home.
Short-term rentals. The county short-term rental page treats stays of 30 days or less as transient. Operators need a planning permit, a business license and a TOT certificate.
Travis AFB. The 2024 amendments to the Travis compatibility plan, filed on CEQAnet, adjusted densities and added a Low Altitude Maneuvering Zone and updated deed notice requirements. The Airport Land Use Commission also administers plans for Nut Tree and Rio Vista airports. Density near any of the three can be capped.

Septic, water and building issues

Outside sewer districts, septic capacity is the usual limit on adding units. Under Section 28.72, an ADU or JADU on a private septic system cannot be occupied until county Environmental Health approves the system, and a secondary dwelling application must show enough water and wastewater capacity for both units. Older stock in unincorporated Vallejo and the Fairfield pockets may need a seismic retrofit review, and buildings of three or more units with elevated balconies need SB 721 inspections.

Underwriting a Solano County multifamily purchase

Confirm whether the parcel is in a city or unincorporated, and pull the zoning district from the county or city.
In unincorporated duplex districts, check the lot against the 2,000 or 3,000 square feet per unit standard before counting any extra unit.
Model rent increases at the AB 1482 cap for covered units, since no Solano jurisdiction is on the AG chart’s local list.
For ADU upside, apply the Section 28.72 size limits and the JADU owner-occupancy rule.
On septic, get Environmental Health records and test capacity; near Travis, Nut Tree or Rio Vista, check the airport compatibility zone.
Collect leases, rent roll, two years of statements and estoppel certificates, then run our rental property analysis.

Questions people ask

Does any city in Solano County have rent control?

The Attorney General’s chart lists no local rent cap in Solano County. The statewide cap of 8.6% applies to covered units from August 1, 2026.

Where can I build a duplex in unincorporated Solano County?

In R-TC-D-4 and R-TC-D-6, mapped in unincorporated Fairfield and Vallejo, and in R-TC-MF near Vallejo.

Is there apartment zoning outside the cities?

Only R-TC-MF, in unincorporated Vallejo, and the R-TC-MU mixed-use district in a few communities such as Elmira and Cordelia.

How big can an ADU be in unincorporated Solano?

Detached ADUs can be 850 square feet in R-TC districts (1,000 with more than one bedroom) and 1,500 in rural residential districts. JADUs are capped at 500.

Can I add a second home on Solano farmland?

On agricultural land the county uses secondary dwellings, up to 2,400 square feet, one per lot, generally on parcels meeting the district minimum size.

Can I rent an ADU as a vacation rental?

Not if the lot also has a JADU, and never a JADU itself. Other transient rentals need a county permit, business license and TOT certificate.

Does Travis Air Force Base limit housing density nearby?

Yes. The Travis compatibility plan sets density limits by zone, adjusted in 2024.

Sources & notes

Note: City rental programs (registration, inspection, mobilehome rent rules) are covered on each city page.

General information, not legal or tax advice. Last reviewed October 2026 by Matthew Martinez.

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