Bay Area ADU Builder. We Design, Permit, and Build It All

We serve six Bay Area counties — Alameda, Contra Costa, Marin, Napa, San Mateo, and Santa Clara — covering 47 cities from Oakland to San Jose. Our crews are based in Castro Valley, putting us within 45 minutes of every city we serve. We handle permits and know the building codes for each jurisdiction, which matters because requirements and fees vary significantly between cities.

We offer $0-down financing with approved credit, 12-month same-as-cash plans, and extended terms up to 15 years for larger projects. You only pay for completed, inspected work — we never ask for full payment before the job is done. Your project manager will walk you through every option during your estimate appointment. We also accept checks, ACH, and all major credit cards.

Yes. CSLB License #1078806 — verify it anytime at cslb.ca.gov. We carry $2M general liability, full workers’ comp, and are bonded per California law. We hold GAF Master Elite, Owens Corning Preferred, CertainTeed ShingleMaster, and James Hardie Elite Preferred certifications — each requires annual training and quality audits to maintain. We’re happy to provide proof of insurance before any work begins.

Most exterior projects in the Bay Area require a building permit. Fees vary by city — Oakland runs $200–$600, Peninsula cities often more. We handle the entire permit process: application, plan submission, and scheduling inspections. Permit fees appear as a separate line item in your estimate so you’ll see every cost upfront. For homes in WUI fire zones (Berkeley Hills, Oakland Hills, parts of Marin), we ensure full compliance with local fire-rated material requirements.

Bay Area ADU costs range from $150,000 to $350,000 for a detached unit and $100,000 to $200,000 for a garage conversion. Junior ADUs (JADUs under 500 sq ft) start around $80,000. Key cost factors include foundation type, utility connections, interior finishes, and local permit fees — which range from $5,000 to $15,000 depending on the city. Detached ADUs with separate utility meters cost more upfront but allow independent billing for rental income. Site preparation including grading, trenching for utilities, and concrete work typically adds $20,000 to $40,000. Hamilton Exteriors provides fully itemized estimates at every phase — design, permitting, and construction costs are all broken out so you can plan your budget before committing.

Yes. California's ADU laws (AB 68, SB 13, AB 881) streamlined permitting statewide, but you still need building permits from your local jurisdiction. Most Bay Area cities must approve or deny ADU permits within 60 days under state law. Required permits typically include building, electrical, plumbing, and mechanical — and some jurisdictions require a separate grading permit if site work is involved. Cities cannot require owner-occupancy for ADUs built after January 2020, and impact fees have been eliminated for units under 750 sq ft. Hamilton Exteriors handles the full permitting process — architectural plans, structural engineering, Title 24 energy compliance calculations, plan check submittal, and inspection coordination throughout construction.

From permit approval, most detached ADUs complete in 4 to 6 months. Garage conversions are faster at 2 to 4 months since the shell already exists. The permitting phase adds 2 to 4 months depending on your city's plan review backlog — San Francisco and some Peninsula cities tend toward the longer end, while most East Bay cities process within 60 days. Total timeline from design to move-in is typically 8 to 12 months. The longest single phase is usually foundation and framing at 6 to 8 weeks for a detached unit. Your project manager provides a detailed construction schedule at contract signing with weekly milestone updates so you can plan your rental listing or family move-in date accordingly.

Under California law, most single-family residential lots can accommodate at least one ADU. Since January 2020, cities cannot impose minimum lot size requirements for ADUs under 800 sq ft. Setback requirements have been reduced to 4 feet for rear and side yards. You are also allowed to build one JADU (junior ADU up to 500 sq ft) inside your existing home in addition to a detached ADU, for a total of two accessory units on a single-family lot. The main factors that affect feasibility are utility access — sewer and water connections must reach the ADU location — and any easements or deed restrictions on your property. Hamilton Exteriors evaluates your lot for utility access, setbacks, fire department access, and local overlay zones during your initial consultation at no cost.

Yes. California law explicitly allows homeowners to rent ADUs as long-term rentals (30 days or more). Some Bay Area cities — including San Francisco, Oakland, and Berkeley — restrict or prohibit short-term rentals under 30 days in ADUs, so check your local ordinance before planning an Airbnb-style rental. An ADU in the Bay Area typically rents for $1,800 to $3,500 per month depending on size, location, and finishes. A well-built 600 sq ft one-bedroom ADU in Alameda County can generate $2,200 to $2,800 per month in rental income, often covering the construction loan payment within 3 to 5 years. Many homeowners also use ADUs for aging parents, adult children, or home offices rather than rental income — the flexibility adds long-term value regardless of how you use the space.

ADUs are one of the highest-ROI home improvements in the Bay Area. Freddie Mac research shows that accessory dwelling units add 20 to 35% to overall property value, translating to $150,000 to $300,000 in added equity depending on size and location. Beyond property value, rental income from a detached ADU typically ranges from $2,000 to $3,500 per month in Bay Area markets. At those rates, a detached ADU costing $200,000 to $275,000 pays for itself in 5 to 8 years through rental income alone. Garage conversions offer an even faster payback period of 3 to 5 years because construction costs are 30 to 40% lower than new builds. Many homeowners also benefit from reduced property tax impact, since California Proposition 13 limits reassessment on the existing home — only the ADU improvement is assessed at current value.

California AB 68 significantly reduced setback requirements for ADUs statewide. Rear and side yard setbacks are now limited to 4 feet for detached ADUs, regardless of what local zoning previously required. Garage conversions have no additional setback requirements since the structure already exists in its current location. Lot coverage limits have been relaxed since 2020 — cities cannot deny an ADU of up to 800 sq ft based on lot coverage, floor area ratio, or open space requirements. There is no minimum lot size for ADUs under 800 sq ft. Front setbacks still follow local zoning rules, and ADUs must maintain fire department access. Hamilton Exteriors evaluates all setback and zoning requirements during your initial lot assessment.

Yes. Under California SB 9 and existing ADU law, single-family lot owners can build one ADU plus one JADU (junior ADU up to 500 sq ft) on the same property. The JADU must be built within the existing home footprint — either a converted bedroom, attached garage, or other interior space. No additional parking is required for either unit. The JADU must include a separate entrance and an efficiency kitchen, though it can share a bathroom with the main house. This combination allows homeowners to add two income-producing units to a single-family lot, maximizing rental income potential.

Several financing options are available for ADU construction in the Bay Area. Home equity lines of credit (HELOCs) are the most common, allowing homeowners to borrow against existing equity at competitive rates. Cash-out refinancing replaces your current mortgage with a larger one that includes ADU construction costs. Construction-to-permanent loans fund the build and then convert to a standard mortgage upon completion. RenoFi loans are specifically designed for renovation and ADU projects, offering higher borrowing limits based on the after-renovation value of your home. Projected rental income from the completed ADU strengthens loan applications — lenders increasingly factor this into qualification. Hamilton Exteriors works with lending partners who specialize in ADU financing and can connect you during your consultation.

San Jose leads the Bay Area with a streamlined online permitting portal and pre-approved ADU plans that reduce design costs and approval times. Oakland offers reduced permit fees for ADUs designated as affordable housing units and has processed thousands of ADU applications since 2020. Fremont, San Mateo, and Berkeley have all adopted expedited review processes for ADUs under 750 sq ft. California state law (AB 68, SB 13) overrides restrictive local zoning since January 2020, meaning no Bay Area city can impose minimum lot sizes, owner-occupancy requirements, or parking mandates that effectively block ADU construction. Hamilton Exteriors has permitted ADUs across 30 Bay Area cities and knows the specific requirements and review timelines for each jurisdiction.