Multifamily and Duplex Properties for Sale in Port Costa: Zoning and Rent Rules

What multifamily in Port Costa looks like

Port Costa is a tiny unincorporated town on the Carquinez Strait, and its rental stock is small enough to count. CCSD’s FY 2023-24 sewer use charge report lists the 86 properties on the town sewer:

• 72 single-family homes
• 7 duplex or apartment properties
• 4 mixed-use properties holding 26 apartments
• 3 commercial customers

The same report counts 113 properties inside the sanitary boundary, but only 85 connected, plus one served from outside. Vacant or unconnected lots inside the boundary would need a new connection and a full per-unit charge before any rental could be added. The plant itself dates to 1984, when County Sanitation District No. 5 ran it; CCSD’s Port Costa Sanitary Department took it over in June 2008.

So "multifamily" here means a handful of duplexes and small apartment buildings, plus apartments above the old commercial buildings near the water. Listings are rare. Most buyers will finance as a 2-4 unit property; see financing 2-4 unit properties. For a mixed-use building with more units, see apartment building loans.

Where the zoning allows it

Port Costa follows Contra Costa County zoning under Division 84. We did not confirm the district on each parcel, so check the county map. These are the county districts that allow more than one unit:

Legal unit count. Many Port Costa buildings predate county zoning. Confirm the legal number of units with county records, and compare it with the count CCSD bills. CCSD charges mixed-use buildings at least one apartment rate for each unit present.

ADUs and SB 9. County Chapter 82-24 allows ADUs and JADUs. Chapter 88-36 allows two units on eligible single-family lots, but only inside a Census urbanized area. State law also excludes very high fire hazard areas and unstable sites. Port Costa may not qualify, so confirm before relying on it. In practice, each added unit also adds a full sewer charge. See our ADU investment guide and SB 9 lot split guide.

Growth limits. CCSD’s history page says the strait, hills and unstable soils hem in the town, and most surrounding land belongs to the Port Costa Conservation Society or the East Bay Regional Park District. It expects only very limited infill.

Rent rules: state law only

Unincorporated Contra Costa County has no local rent cap on the Attorney General’s chart. Covered Port Costa units follow the Tenant Protection Act. For the San Francisco area, the cap is 6.3% from August 1, 2025 through July 31, 2026, and 8.8% from August 1, 2026 through July 31, 2027. State just cause rules apply after 12 months.

Nearly all Port Costa buildings are well over 15 years old, so the new-construction exemption rarely applies. An owner-occupied duplex is exempt. A single-family home owned by an individual can be exempt with the required notice. See our AB 1482 guide and just cause eviction guide.

Port Costa rental property rules

• Fire-safe sale document. Port Costa is in the Crockett-Carquinez Fire Protection District. Under county Chapter 84-90, the seller must deliver the Fire Code Official’s fuel mitigation compliance document before escrow closes. Budget for brush clearance on canyon lots.
• Short-term rentals. County Chapter 88-32 requires a permit, excludes buildings of five or more units, and caps non-hosted stays at 90 days a year.
• No local registry or inspection program. We found none for Port Costa.

Sewer costs: the biggest line item

Port Costa’s sewer bill is unusually high for a rental owner:

• Per-unit charge. The FY 2023-24 charge was $2,345 per single-family home and per apartment for the year, plus $50.31 per 100 cubic feet of water above 4,662 cubic feet. A fourplex would pay at least $9,380 a year.
• Deficit. The same report shows FY 2023-24 costs of $432,138 against about $299,000 in charge revenue. CCSD held a July 2025 hearing on further FY 2025-26 increases. Underwrite rising charges.
• Small plant. The 1984 plant at the end of Canyon Lake Drive discharges to the strait. Under the Regional Board’s 2024 tentative permit, average dry-weather discharge is capped at 0.033 million gallons per day through July 2029.

Buildings. Port Costa’s old wood-frame buildings sit on steep or unstable ground. Get structural, drainage and seismic retrofit reviews. Buildings with three or more units and elevated balconies or stairs need SB 721 inspections.

Transfer tax. The Contra Costa Clerk-Recorder charges $0.55 per $500, with no city tax.

Underwriting a Port Costa multifamily purchase

1. Confirm the zoning and the legal unit count, and compare it with CCSD’s billed units.
2. Get the last two years of sewer and water bills and model the charge rising.
3. Get the Fire Code Official’s fuel mitigation document and price clearance work.
4. Classify units under AB 1482 and model increases at the state cap.
5. Collect leases and estoppel certificates.
6. Add structural repairs, SB 721 work and reassessed property tax to your rental property analysis.

Questions people ask

Sources & notes

1. Crockett Community Services District: Port Costa Sanitary history, FY 2023-24 Sewer Use Charge Report, July 2025 hearing notice
2. SF Bay Regional Water Quality Control Board, Port Costa tentative order (2024)
3. Contra Costa LAFCO, Fire Protection Districts (2025)
4. County Ordinance Code (GoCodebook): Chapters 84-22, 84-26, 84-50, 84-66, 84-90, 82-24, 88-32, 88-36
5. California Attorney General, Rent Caps
6. Contra Costa Clerk-Recorder, Documentary Transfer Tax

Note: Parcel zoning, SB 9 eligibility, current sewer rates and fire hazard zone mapping were not verified.

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

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