Building an ADU in Antioch & Contra Costa County: Costs, Permits, and Rental Income

Most Antioch ADUs are garage conversions or detached backyard units, and permitting depends on whether you are inside city limits. Here is how costs, permits, and rental income actually work.

Accessory dwelling units (ADUs) are the fastest-growing type of residential construction in Contra Costa County, and for good reason: rents in East County stay strong, lots in Antioch are larger than in the inner Bay Area, and California law now requires cities to approve most ADU applications. But the difference between a smooth ADU project and a stalled one usually comes down to two things most owners discover too late - which agency actually issues your permit, and whether your budget covered the site work, not just the structure.

Otto & Sons Construction is a licensed California general contractor (CSLB #1104751) based in Antioch. We build ADUs and additions across Contra Costa County, and this guide reflects how these projects actually run here - not statewide averages.

What an ADU costs in Antioch in 2026

The short version:

• Garage conversion ADU: typically the least expensive path, because the foundation, walls, and roof already exist. Most of the budget goes to insulation, windows, a kitchenette, a bathroom, and utility connections.
• Attached ADU (addition-style): mid-range. You share at least one wall and often utility runs with the main house.
• Detached new-construction ADU: the most expensive and the most valuable for rental income and privacy. In the East Bay, all-in budgets for a detached unit commonly land in the low-to-mid six figures once site work, utilities, and fees are included.

Three cost drivers matter more than square footage:

1. Site work and utilities. Trenching for sewer, water, and electrical from the street or main house to the unit can swing a budget by tens of thousands of dollars. A long backyard run costs more than the extra bedroom you were debating.
2. Fees. Plan check, building permit, and school fees are calculated on project valuation. California law waives impact fees for ADUs under 750 square feet - one of the strongest arguments for keeping a unit under that line.
3. Kitchen and bath scope. These are the two most expensive rooms per square foot in any home. An ADU is essentially a small home that is mostly kitchen and bath.

Who issues your permit: City of Antioch vs. Contra Costa County

This is the single most common point of confusion, and getting it wrong costs weeks.

• If your property is inside Antioch city limits, the City of Antioch Community Development Department issues your ADU permit, through its online Enterprise Permitting & Licensing (EPL) system.
• If your property is in unincorporated Contra Costa County, the county Department of Conservation and Development (DCD) handles it instead.

The rules are similar - both must follow California ADU law - but the forms, fees, review queues, and inspectors are different. Your contractor should know which counter your parcel belongs to before drawing anything.

The shortcut: Antioch's pre-approved ADU plans

The City of Antioch maintains a pre-approved ADU plan program: complete construction drawings that city staff have already reviewed. Using one of these plans is the quickest and most cost-effective way through permitting - plan-check time drops dramatically because the structural review is already done, and you only need site-specific review (placement, utilities, setbacks).

The tradeoff is design flexibility. If a pre-approved layout fits your lot and your needs, take it. If you need a custom design, budget extra time for full plan check.

What California law guarantees you

A few statewide rules that apply in Antioch and everywhere in Contra Costa County:

• Cities must approve or deny a complete ADU application within 60 days.
• Setbacks for a new detached ADU are capped at 4 feet from side and rear property lines.
• No owner-occupancy requirement applies to ADUs permitted through 2026 under current state law.
• ADUs under 750 square feet are exempt from impact fees.
• You can generally rent an ADU for terms of 30 days or longer.

Does an ADU pay for itself in Antioch?

For many owners, yes - this is the part of the market our team sees driving demand. A one-bedroom ADU in Antioch rents in a range that can service a meaningful share of the construction loan, and East County's commuter demand keeps vacancy low. Owners doing it for family (aging parents, adult kids saving for their own place) get the same asset value without the landlord duties.

Run the numbers conservatively: realistic rent for your street, minus insurance, maintenance, and the property-tax increase on the added value (the existing house is not reassessed - only the new unit adds assessed value).

How Otto & Sons runs an ADU project

1. Feasibility first. We check your parcel's jurisdiction, utility locations, and setbacks before you spend money on design.
2. Fixed scope, written schedule. You get a line-item scope and a construction calendar before work starts.
3. Permits handled. We manage submittal, plan check corrections, and every inspection through final.
4. One point of contact. You deal with Otto directly - no layers of project managers.

Related reading

• Do You Need a Permit for a Room Addition in Antioch? (2026 Guide)
• Our residential construction services