California ADU Design, Plans & Structural Engineering

In most cases, yes. Since 2020, California state law requires cities and counties to allow ADUs on most single-family and multifamily residential lots. Generally, you must own a residential property with sufficient space to meet basic setback and zoning requirements. The fastest way to know for sure is to submit your property address. We provide a free ADU feasibility review within 24 hours, based on your lot, zoning, and local regulations.

The total cost to build an ADU in California typically ranges from $150,000 to $350,000, depending on factors such as size, construction type (detached ADU vs. garage conversion), finish level, and city requirements. Permit fees usually range from $5,000 to $15,000, with design and engineering costs calculated separately. We offer transparent, phased pricing so you understand costs before committing to each stage.

From initial design to move-in, most ADU projects take 10–14 months: Design & engineering (4–8 weeks), Permit approval (8–16 weeks, varies by city), and Construction (4–6 months). Delays most often occur during the permit phase. Our team actively coordinates with local planning departments to reduce back-and-forth comments and keep approvals moving.

California allows detached ADUs up to 1,200 square feet. Garage conversions and internal ADUs are typically limited to 800–1,000 square feet, depending on layout and local rules. In practice, lot size, setbacks, existing structures, and utility constraints often matter more than the statewide maximum. We evaluate these factors to help you maximize buildable area legally and efficiently.

Yes, in many cases. Studies and market data suggest ADUs can increase property value by 20%–30%, while also generating monthly rental income—often $1,500–$3,000 in major California markets. Actual value impact depends on location, design quality, and rental demand. A well-designed, fully permitted ADU typically delivers the strongest return.

Yes. All ADUs in California require approved building permits. Permit costs vary by city but generally fall between $5,000 and $15,000, including plan check, impact fees, and inspections. Most project delays and rejections happen during the permit process—not construction. We handle permit documentation and city coordination to help avoid costly mistakes and unnecessary delays.

Pre-approved ADU plans are architectural drawings that a city has reviewed for a repeatable design, but they still may need site, foundation, utility, and structural adaptation. Cecilia Home creates custom, permit-ready ADU plans for your property and can help you compare a city template with a site-specific design before you choose a path.

ADU legalization is the process of bringing an unpermitted dwelling unit into full compliance with California building codes and local ordinances. If you have a converted garage, basement, or addition that was built without permits, ADU legalization allows you to make it official. This typically involves submitting as-built architectural drawings, passing inspections, and addressing any code violations. Legalizing your ADU protects your property value, avoids fines, and allows you to rent the unit legally. We provide full ADU legalization support in California, from initial assessment to final permit approval.

Most detached, attached, second-story, and garage conversion ADUs need structural engineering for foundations, framing, shear walls, connections, or changes to the existing building. We coordinate the architectural and structural drawings in one plan set so the design, calculations, and permit application describe the same project.

Once you share your full project details — lot size, desired ADU type, city — we provide a complete, fixed-price quote. No hourly rates, no 'it depends' estimates. You get the full scope and full cost upfront, so you can plan your budget with confidence before committing.

Cecilia Home provides ADU design, structural engineering, and permit support rather than construction labor. You choose and contract with your builder directly; if helpful, we can share contractor referrals and coordinate plan questions, but homeowners should independently review licensing, insurance, scope, and pricing.