ADU Additions & Garage Conversions in Contra Costa County
If you already have a garage, a finished or unfinished space, or an attached structure, there's a good chance you're sitting on your next ADU without knowing it. DevCo Building Enterprises converts existing structures and builds attached additions for Contra Costa homeowners who want rental income, in-law space, or long-term investment value without starting from scratch.
What Kind of ADU Addition Is Right for Your Property?
Not every ADU is a new build. Here are the four most common addition types DevCo builds in Contra Costa County:
Garage Conversion ADU
Your attached or detached garage already has a foundation, walls, and often basic utilities. Converting it into a permitted ADU is the fastest and most cost-effective path for most Contra Costa homeowners. Most garage conversion projects run $90,000-$200,000 depending on current condition, finish level, and plumbing configuration.
Attached ADU
An attached ADU shares one or more walls with your primary residence, often added as a home addition. These projects typically run $180,000-$320,000 and can be designed to feel completely separate with a private entrance, kitchen, and bathroom.
Junior ADU (JADU)
California law allows JADUs up to 500 square feet within an existing single-family home's footprint. These are typically the lowest-cost ADU option, ranging from $40,000-$120,000, and have a simplified permit pathway under state law. A great option for in-law suites or rental income in smaller spaces.
Home Addition / Living Space Expansion
Not ready for a full separate unit? We also build traditional home additions that expand your primary square footage: new bedrooms, bathrooms, or full wing additions for homeowners across Contra Costa and Alameda Counties.
Garage Conversion ADU: What to Expect
Garage conversions are the highest-volume addition type DevCo builds. Here's what the process looks like from start to finish:
Feasibility Assessment
We evaluate your garage's current condition, setbacks, utility access, and ceiling height. Most standard two-car garages in Contra Costa are viable ADU candidates.
Design & Permitting
We prepare architectural drawings and submit to Contra Costa County Building Inspection. State law (AB 2221) has streamlined garage conversion permitting, but permits are still required and we handle everything.
Construction
Foundation work (slab leveling or raising), framing, insulation, drywall, plumbing, electrical, HVAC, and finishing to your selected package.
Final Inspection & CO
DevCo coordinates all required county inspections and walks you through the unit before project closeout.
ADU Addition Costs in Contra Costa County
Most ADU projects in Contra Costa County run between $190,000 and $400,000 depending on unit size, design complexity, site conditions, and finish selections. Here's a rough breakdown by type:
6-10 months
5-9 months
These are estimates based on typical Contra Costa projects. Your number depends on your specific site, structure condition, and finish selections. The fastest way to get a real number is a site visit.
Frequently Asked Questions: ADU Additions
Garage conversions in Contra Costa County typically run $90,000-$200,000 depending on the garage's current condition, size, plumbing requirements, and finish selections. An unfinished two-car garage in good structural condition will run less than a one-car garage that needs significant foundation or utility work.
Yes. All garage conversions require permits in Contra Costa County. However, California state law (AB 2221) has significantly streamlined the garage conversion permit process, the county now has ministerial approval for qualifying conversions, which means faster turnaround. DevCo manages the entire permit process.
It depends on your lot and zoning. California law has eliminated off-street parking replacement requirements for many ADU conversions, meaning you can often convert your garage without being required to add new parking spaces. DevCo confirms this during your site assessment.
A Junior ADU (JADU) is a living unit up to 500 square feet created within the existing footprint of a single-family home. JADUs must include an efficiency kitchen but can share a bathroom with the primary residence. They have a simplified permit pathway and are a cost-effective way to add rental income or in-law space.
Most garage conversions in Contra Costa take 3-6 months from permit submission to certificate of occupancy. Permitting typically takes 4-8 weeks; construction runs 6-14 weeks depending on scope. DevCo gives you a realistic timeline during your initial site visit.
Ready to Find Out What Your Space Could Become?
If you have a garage, an attached space, or unused square footage, there's a good chance we can turn it into a permitted ADU. Schedule a site visit and we'll tell you exactly what's possible and what it'll cost.