Multifamily and Duplex Properties for Sale in Boonville: Zoning and Rent Rules

What multifamily in Boonville looks like

Boonville’s rental housing is small and scattered: a few duplexes and cottage courts near Highway 128, apartments over or behind downtown buildings, homes with an ADU, and housing for vineyard and farm workers. The Anderson Valley CSD’s project notice counts just 6 multifamily residential connections among about 139 in the planned sewer area, alongside 96 single-family homes. Rental housing is scarce, and new units are limited mostly by wells and septic. For financing, see financing 2-4 unit properties and apartment building loans.

Where the zoning allows it

Boonville is unincorporated and inland, so the county’s Inland Zoning Code applies. In almost every district, density depends on whether the parcel has public water, public sewer, both or neither. Today Boonville has neither.

The CSD’s notice summarizes what changes with service. Today’s minimum residential lot is 40,000 square feet. With water or sewer it could drop to 12,000 square feet, and with both to 6,000. A 2015 county analysis estimated that units could theoretically rise from 174 to about 813 or 1,441. The notice says that would require rezoning and is not expected. State funding rules also limit the system to about 10% extra capacity.

ADUs. The county’s ADU page allows detached ADUs up to 1,200 square feet and JADUs up to 500. JADUs require owner occupancy. ADUs cannot sit on a leach field or its replacement area, and new ADUs need solar panels. On a small Boonville lot, the replacement leach field area often decides whether an ADU fits. See our ADU investment guide.

SB 9. SB 9 applies only to qualifying urban single-family lots, and Boonville parcels generally do not qualify. Confirm with Planning and see our SB 9 lot split guide.

Rent rules: state law only

The Attorney General’s rent cap page, updated July 16, 2026, puts Mendocino County under All Other Counties. The cap for covered units is 7.7% for increases from August 1, 2025, and 8.6% from August 1, 2026 through July 31, 2027. No Mendocino County jurisdiction appears on the AG’s chart of local rent ordinances, and the county has no local just cause ordinance. Covered units follow state just cause rules. Many single-family homes and some duplexes are exempt if the owner gives the required notice. Read our AB 1482 guide and just cause eviction guide.

Boonville rental property rules

• Vacation rentals and TOT. Short-term stays of 30 days or less are taxed under the county’s Chapter 5.20. It sets an 8% base and authorizes an extra 2% approved by voters in 1997. Each parcel needs its own registration certificate. A Business Improvement District assessment is paid through the Treasurer-Tax Collector.
• Farmworker housing. On agricultural land, check whether any units are tied to a farm operation under a permit or recorded agreement before treating them as market rentals.
• Transfer tax. The county collects documentary transfer tax through the Recorder, which does not post the rate. Confirm it on the escrow estimate. There is no city tax.

Wells, septic and the planned public systems

Water quality is the central risk in Boonville rental property. The CSD’s notice reports E. coli in 70% of 23 wells sampled in winter 2016 and nitrate above the drinking water limit in 30%. It says many domestic wells sit too close to septic systems. County setbacks of 50 feet from a tank and 100 feet from a leach field are impractical on small lots. For a multi-unit property, a failed well test or septic system can stop a sale or a refinance.

The CSD’s project page describes the plan: about 14 wells feeding a public system with 300,000 gallons of storage, and a tertiary wastewater plant south of the airport. Planning was funded by state grants, and the state is expected to fund construction. A draft EIR was expected in spring 2026, and LAFCo approval and a Proposition 218 rate notice are still needed. No construction date is set. The General Plan’s Anderson Valley chapter supports the project but says systems should be sized only for planned growth.

Older wood-frame buildings may warrant a seismic retrofit review. Buildings of three or more units with elevated decks need SB 721 inspections.

Underwriting a Boonville multifamily purchase

1. Order a county Letter of Zoning ($152 initial deposit per parcel) to confirm the district and any violations.
2. Test the well for bacteria and nitrate, and get a septic inspection and the Environmental Health file.
3. Map the leach field and replacement area before planning any ADU.
4. Check whether the parcel sits in the planned CSD service areas, but do not count on future density.
5. Classify units under AB 1482, model increases at or below 8.6%, and collect estoppel certificates.
6. Rebuild expenses with reassessed taxes, water treatment and septic reserves in our rental property analysis.

Questions people ask

Sources & notes

1. AVCSD, Notice of Preparation and Water & Wastewater page
2. Mendocino County General Plan Ch. 6 (2009)
3. Mendocino County Inland Zoning Code Ch. 20.040, R-C and R-3 (GoCodebook)
4. County ADU page
5. California Attorney General, Rent Caps
6. Mendocino County Code Ch. 5.20, TOT and TOT payment page
7. County Recorder

Note: The R-3 summary shows conflicting density figures, so confirm with the county. We did not verify parcel zoning in Boonville.

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

Related: Boonville real estate guide · Commercial real estate in Boonville · Multifamily investing guide · Investor services