Our ADU Design and Build Process

A detached ADU is a separate structure positioned at least six horizontal feet from your primary residence. Detached ADUs may be up to 1,200 square feet and must maintain four-foot setbacks from the side and rear lot lines. Height is limited to 16 feet if the unit does not meet standard single-family setback requirements.

An attached ADU shares a roof, foundation and common wall of at least eight horizontal feet with your main home. Size limits for attached ADUs depend on the size of your primary residence:

• Homes that are 1,600 square feet or smaller can add an attached ADU of up to 800 square feet. 
• Homes between 1,601 and 2,400 square feet can add an attached ADU up to 50% of the primary residence’s floor area.
• Homes larger than 2,400 square feet can add an attached ADU of up to 1,200 square feet.

Yes, Santa Clara County allows one standard ADU and one junior accessory dwelling unit (JADU) per legal lot with a single-family home. A JADU must be contained entirely within your primary residence and cannot exceed 500 square feet. It requires its own entrance and an efficiency kitchen, but may share a bathroom. As your ADU contractor, Valley Home Builders handles the planning and permitting to maximize your property within zoning guidelines.

Requirements for ADUs on multifamily properties can differ, so our team can review your lot and explain what is allowed.

A conversion ADU transforms existing space, such as a garage, basement or attic, into a complete living unit. Conversion ADUs can maintain the existing structure’s setbacks, and a limited expansion of up to 150 square feet may be allowed to accommodate ingress and egress. Conversion ADUs offer a cost-effective option since you work with an existing foundation and roof. Our local ADU contractors evaluate your current structure to determine if conversion makes sense for your property.

ADUs generate income by creating additional, separate living space on your lot for short-term or long-term tenants. You can pursue long-term leases for stable monthly income or, where local regulations permit, list the space on short-term rental platforms like Airbnb or Vrbo for higher nightly rates.

Valley Home Builders can design the ideal backyard apartments and granny apartments to fit the space on your lot. We handle design, permitting and construction in-house so you can focus on the return, not the build.

When you need a contractor for ADU construction in San Mateo County, start by looking for these key attributes:

• Licensed: Verify your contractor has a valid California Contractors State License Board (CSLB) license via the CSLB website. Hiring a licensed contractor protects you legally if the work does not meet standards or the project stalls.

• ADU-specific experience: ADUs require specialized knowledge of factors like local setback rules, size limits and utility connection standards. A specialized ADU construction company helps you plan around these requirements from the start.

• Design-build capability: You can avoid coordination gaps by hiring one firm for design and construction. By choosing partners who can design and support the building process, you can streamline permitting, as one team handles the whole process. 

• Local permit knowledge: California laws like AB 68 have helped streamline ADU permitting statewide, while SB 9 has expanded certain housing options on single-family lots. Even with these state-level changes, property owners still need to follow local building permit processes, zoning standards and review requirements. In San Mateo County, a contractor with local permit knowledge can help you plan around these requirements and minimize avoidable delays.

Valley Home Builders is a licensed design and building services provider, registered with the CSLB as 889458. We have in-house ADU architects, designers and project managers who know all the ins and outs of the ADU construction process, including local permits and HOA requirements. 

When comparing garage conversions to ADUs for rental income, consider these factors:

• Up-front cost: Garage conversions can cost less than ADUs since the structure already exists. 
• Rental income potential: A custom ADU with more living space and purpose-built amenities may command higher rent than a converted garage.
• Design flexibility: ADUs can be designed with attractive features like natural light, separate entrances and full kitchens. Garages may have less design flexibility because you have to work around the existing structure. 
• Property value: Both options can add value to your property, though the amount varies by property and market. 
• Timeline: Garage conversions can move faster, but the scope of work and local permitting will ultimately determine which option gets your tenants in the door first.