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

What multifamily in Diablo looks like

Diablo is a small, unincorporated enclave of large single-family lots around the Diablo Country Club, northeast of downtown Danville. We found no apartment buildings or multifamily zoning there. For an investor, "multifamily" in Diablo realistically means:

• A main house with an ADU, a JADU, or both
• A two-unit project under the county’s SB 9 rules, where the lot qualifies
• Renting a single-family home, often to long-term tenants

If you want actual apartment buildings nearby, the Town of Danville’s new M-30 and M-35 districts are the closest option. For financing a house-plus-ADU or a small property, see financing 2-4 unit properties and apartment building loans.

Where the zoning allows it

Contra Costa County zones Diablo. Parcels the Diablo Municipal Advisory Council has reviewed were R-20 with the -UE overlay, according to its May 13, 2024 agenda. LAFCo’s 2011 report listed several parcels on upper Mt. Diablo Scenic Boulevard as A-2.

ADUs. The county rewrote its ADU chapter through Ordinance 2025-07, adopted May 13, 2025. Because most Diablo lots exceed 12,000 square feet, a detached ADU can generally reach 1,200 square feet, and an attached garage for it can be up to 500 square feet on lots of 20,000 square feet or less, or 600 on larger lots under five acres. A private septic or water system must be approved by the county health officer before the ADU is established. See our ADU investment guide.

SB 9. The county’s Chapter 88-36, from Ordinance 2022-14, allows two units in R-6 through R-100 districts. The lot must also sit inside a Census urbanized area or urban cluster and clear the state’s site exclusions, and historic resources are excluded. New units in yard areas are capped at 16 feet with 4-foot side and rear setbacks. ADUs are not allowed on a lot created by an urban lot split. Diablo’s hillside setting means fire hazard mapping should be checked before relying on SB 9. See our SB 9 lot split guide.

Lot sizes in practice. Many Diablo lots are older and below current standards. The DMAC’s 2024 agenda included a variance for a substandard El Centro lot and a lot line adjustment creating a parcel of about 18,266 square feet against a 20,000 minimum. Plan for variances on additions.

Rent rules: state law only

Unincorporated Contra Costa County has no local rent cap. The Attorney General’s rent cap chart lists only Antioch, Concord and Richmond with local programs in the county. Covered Diablo units follow the state cap for the San Francisco area: 6.3% for increases from August 1, 2025, and 8.8% from August 1, 2026 through July 31, 2027. Most Diablo rentals are single-family homes owned by individuals, which can be exempt from the state cap and just cause rules if the lease gives the required notice. A rented ADU on the same lot as an owner-occupied home may also fall under an exemption; review each unit. See our AB 1482 guide and just cause eviction guide.

Diablo rental property rules

• 30-day minimum on ADUs and SB 9 units. County ADU rules bar renting an ADU or JADU for less than 30 days, and Chapter 88-36 does the same for two-unit developments.
• Short-term rentals of the main house. These need a county permit under Chapter 88-32. The requirements allow one per lot and cap non-hosted rentals at 90 days a year. They also ban events and require a responsible party within 30 miles.
• No local registry. We did not find a county rental registry or inspection program for Diablo.
• Security and roads. The Diablo Community Services District provides security services and maintains roads. Ask whether district rules affect construction traffic or tenant parking.

Roads, hillside and utility issues

• Private roads. LAFCo’s 2011 report says most Diablo roads are private. A construction project for an ADU or second unit should check access, encroachment and repair obligations with DCSD.
• Fire and insurance. Diablo sits at the base of Mount Diablo’s wooded slopes, so insurance cost and fire hazard mapping belong in underwriting.
• Sewer or septic. We did not verify Diablo’s sewer provider. Confirm whether the lot is sewered, because septic capacity and health officer approval can limit ADUs.
• Older homes. Older houses converted to two units may warrant a seismic retrofit review. SB 721 applies only to buildings of three or more units.
• Transfer tax. Only the county’s $0.55 per $500 applies.

Underwriting a Diablo income property purchase

1. Confirm R-20 or other zoning, the lot’s area and width, and any past variances.
2. Check SB 9 eligibility, including fire hazard mapping, before paying for a two-unit plan.
3. Confirm sewer or septic, and get health officer sign-off for any ADU on septic.
4. Classify each unit under the state Tenant Protection Act.
5. Request leases and estoppel certificates for any existing tenants.
6. Model long-term rents only for ADUs, with our rental property analysis guide, and include insurance.

Questions people ask

Sources & notes

1. Contra Costa County, Diablo and Diablo MAC agenda, May 13, 2024
2. Contra Costa LAFCO, LAFCO 11-02 staff report (May 2011)
3. County Ordinance Code via GoCodebook: R-20, R-6 uses, Chapter 82-24, Chapter 88-36, Chapter 88-32
4. California Attorney General, rent caps
5. Contra Costa Clerk-Recorder, Documentary Transfer Tax

Note: Fire hazard severity zones, the sewer provider and SB 9 urbanized-area status for Diablo were not verified.

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

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