ADU Construction in Foster City CA — Accessory Dwelling Units | Quartz Construction
Quartz Construction & Remodeling builds ADUs (Accessory Dwelling Units) in Foster City, CA. We are a full-service ADU contractor serving San Mateo County — handling design, permits, construction, and final inspections for new detached ADUs, garage conversion ADUs, and attached ADUs. CSLB #1003436. Call (650) 749-7436 for a free ADU estimate in Foster City.
ADU Types We Build in Foster City, CA
Industry Resources & Standards:
- Detached ADU (Backyard Cottage): A fully independent new structure built in the backyard of your Foster City property. Detached ADUs in San Mateo County range from 300 to 1,200 sq ft and are the most valuable addition for rental income or multigenerational living. They require their own foundation, utility connections, and full compliance with Foster City Building Department requirements.
- Attached ADU (Home Addition): An addition connected to the main house but with a separate entrance, kitchen, and bathroom. Popular in Foster City neighborhoods where the backyard is limited. Shares foundation and utility connections with the main home, reducing cost.
- Garage Conversion ADU: Converting an existing attached or detached garage into a living unit. The most cost-effective ADU option in Foster City because the structure, foundation, and utilities already exist. Requires insulation, drywall, plumbing for a kitchen and bathroom, and HVAC upgrades.
- Junior ADU (JADU): A unit created within the existing primary dwelling (usually a bedroom suite) up to 500 sq ft with an efficiency kitchen. JADUs have a streamlined permit process and are the fastest and least expensive way to create an ADU in Foster City. Owner-occupancy is required on the property.
- ADU Over Garage: A second story unit built on top of an existing single-story detached garage. Common in San Mateo County neighborhoods with large garages. Requires structural engineering to ensure the existing garage can support the added load.
ADU Cost in Foster City 2025
| ADU Type | Size | Cost Range |
|---|---|---|
| JADU (Junior ADU within home) | Up to 500 sq ft | $60,000 – $120,000 |
| Garage Conversion ADU | 200 – 600 sq ft | $80,000 – $160,000 |
| Attached ADU (home addition) | 300 – 800 sq ft | $150,000 – $280,000 |
| Detached ADU (new construction) | 400 – 800 sq ft | $180,000 – $350,000 |
| Detached ADU (1,200 sq ft max) | 1,000 – 1,200 sq ft | $280,000 – $500,000 |
ADU Rules in Foster City
Foster City follows California state ADU law (AB 68, SB 9, AB 881), which requires local jurisdictions to allow ADUs on single-family and multi-family lots with ministerial approval for qualifying projects.
ADU Rental Income in Foster City
The average ADU rental income in San Mateo County is $2,000-$3,500/month for a studio or 1-bedroom unit and $2,500-$4,500/month for a 2-bedroom unit. Foster City’s strong rental market means a $200,000-$300,000 ADU investment can generate $25,000-$42,000 in annual rental income — a 8-14% cash-on-cost return. Many Foster City homeowners also use ADUs for multigenerational living, housing aging parents while adding home value.
Get a Free ADU Estimate in Foster City
Quartz Construction builds ADUs throughout Foster City and San Mateo County. We handle design, permits, construction, landscaping, and final inspections — a complete turnkey ADU solution. CSLB #1003436 | BBB A+ | 4.9 Stars. Call (650) 749-7436.
What Our Clients Say
Join hundreds of happy Bay Area homeowners who trusted Quartz Construction & Remodeling for their projects.
Professional Resources & Regulations
- California Contractors State License Board (CSLB) — Verify contractor licenses
- City of San Jose Building Permits — Permit requirements & inspections
- ENERGY STAR — Energy-efficient materials & standards
Trusted by San Jose Homeowners
4.9/5.0 • 65+ Google Reviews
“Amazing kitchen remodel — on time, on budget. Highly recommend Quartz Construction!”
— Maria S., San Jose
“Professional, communicative, and the bathroom renovation exceeded expectations.”
— David L., Santa Clara
“Built our ADU from scratch — permits, foundation, everything handled seamlessly.”
— Jennifer T., Sunnyvale
Get Your Free Estimate
Licensed • CSLB#1003436 • Serving San Jose & Bay Area
Or call us directly: (650) 749-7436
