What Is ADU Construction?

What is an Accessory Dwelling Unit (ADU)?

An Accessory Dwelling Unit (ADU) is a self-contained residential structure built on the same property as a primary single-family dwelling. Each ADU includes a kitchen, bathroom, and separate entrance, making it a fully functional living space that meets California’s zoning laws and building code requirements (American Planning Association). Often known as granny flats, in-law suites, or backyard cottages, these Accessory Structures help homeowners expand livable space, accommodate family, or generate passive income through long-term rentals.

Understanding what qualifies as an ADU is essential for homeowners, lenders, and permitting agencies, as it determines how projects are financed, appraised, and approved during the permitting process. Proper site preparation, utility connections, and construction documents ensure the ADU meets local codes and avoids potential building code violations.

In California, ADUs have become a cornerstone of multigenerational housing and sustainable community growth—offering an affordable, space-efficient solution that adds value to the property while supporting the state’s broader housing goals.

Expert Insight: Always verify your ADU plans meet California’s building codes for independent living spaces—kitchen, bathroom, and separate entrance are key compliance factors.

What are the Different Types of ADUs?

Homeowners can choose from several ADU types, each designed to match specific property layouts, budgets, and goals:

• Detached ADU – A standalone accessory structure, usually in the backyard. Offers the most privacy and design flexibility, often up to 1,200 sq. ft. under state zoning regulations.
• Attached ADU – A home addition that shares one wall with the primary dwelling. Ideal for expanding living space while keeping family close.
• Garage Conversion ADU – A converted garage that transforms existing square footage into an apartment. One of the most cost-effective ADU construction options, requiring minimal new site work.
• Junior ADU (JADU) – A smaller, 500 sq. ft. unit built within the main home, often converted from a bedroom or attic. This Junior Accessory Dwelling Unit shares utilities and offers a streamlined approval process.

Each option provides a path to expand usable space, enhance energy efficiency, and increase property value—whether you’re planning a rental unit, a guest suite, or a family living space for aging relatives.

Read more: Explore ADU Types »

Tip: Choose your ADU type based on space and budget—garage conversions are most cost-effective, while detached units offer maximum privacy and flexibility.

The ADU Construction Process Overview

The ADU construction process is a structured, multi-step approach that guides homeowners from site evaluation and design through permitting and construction. Whether you’re building a garage ADU, attached unit, or detached Ancillary Dwelling Unit, the process ensures compliance with local zoning districts and California Department of Housing and Community Development standards. In Southern California, most ADU projects take 10–18 months to complete, depending on site conditions, city review timelines, and overall building costs (Maxable). By understanding this process, homeowners can better plan their budget, construction contracts, and inspection schedule, avoiding costly delays or building code issues.

Step-by-Step: Build an ADU in California

1. Property & Feasibility Evaluation Begin by determining if your property qualifies for an ADU based on zoning laws, setbacks, and lot coverage. Conduct a cost evaluation and geotechnical engineering report if necessary to ensure your site can support new construction. California law allows most single-family lots to include at least one ADU (California HCD).
2. Design & Plan Development Collaborate with a licensed ADU designer or construction company to create architectural drawings, 3D modeling, and site plans that reflect your project’s goals. Choosing pre-approved ADU plans (available in cities like San Diego or Los Angeles) can shorten approval timelines and reduce design costs (City of San Diego).
3. Permit Submission & Approval Submit your construction documents to the planning and building departments for review. Under California law, cities must approve or deny ADU applications within 60 days, ensuring faster turnaround compared to traditional additions (CalMatters).
4. Construction & Inspections Once permits are issued, the construction phase begins — including site preparation, foundation work, plumbing rough-in, electrical setup, and interior finishes. Local inspectors will perform housing quality inspections to ensure compliance with building codes, stormwater flood regulations, and fire safety standards.
5. Move-In & Occupancy After final inspection, your ADU is move-in ready. Many homeowners choose to rent their units long-term for passive income or use them for multigenerational housing. California regulations permit ADUs to be rented for 30 days or longer, promoting sustainable community growth (Maxable).

Read more: Detailed ADU Construction Process »

Pro Insight: Plan for a 10–18 month build timeline. Early permit submission and pre-approved plans can significantly reduce project delays.

Permits, Approvals & Regulations for California Homeowners

Building an Accessory Dwelling Unit (ADU) in California involves several critical steps in the permitting process before construction begins. Homeowners must secure a building permit, confirm zoning compliance, and provide detailed floor plans and site drawings to their local planning and building departments. These departments review the project for structural safety, energy efficiency, and environmental standards, ensuring it aligns with both state and local building codes. Thanks to state housing reforms, California cities are now required to approve or deny ADU applications within 60 days, significantly streamlining development timelines (CalMatters).

During review, city planners verify that your ADU meets zoning district requirements, such as lot coverage, height limits, and setbacks, while the building department checks for compliance with energy-efficient features, solar panel mandates, and erosion control standards. Once all approvals are granted, you’ll receive your building permit and can begin site work and construction.

Key Regulatory Highlights (Especially for San Diego)

San Diego has some of the most ADU-friendly zoning laws in California, allowing for faster, more affordable development.

• 60-Day Permit Review: All ADU applications must be processed within 60 days of a complete submittal.
• No Parking Requirement: No additional parking is needed if the ADU is within a half-mile of transit or part of an existing structure.
• Size Limits: Detached ADUs can reach up to 1,200 sq. ft., while attached ADUs are capped at 50% of the main home’s floor area.
• Multiple Units Allowed: Homeowners may add both a full ADU and a Junior ADU (JADU) on the same property.
• Plus One ADU Program: Under San Diego’s ADU Bonus Program, homeowners can build additional ADUs if one or more are designated as affordable housing (City of San Diego).

Understanding permit history, design constraints, and local flood zone or stormwater regulations is essential before breaking ground. Partnering with an experienced construction company or technical assistance team can help you navigate San Diego’s evolving housing policies efficiently while avoiding costly delays.

Read more: California & San Diego ADU Regulations »

Tip: Secure all zoning and building permits before construction begins—California law mandates cities review ADU applications within 60 days.

ADU Design Options for Homeowners

California homeowners have exceptional flexibility when it comes to ADU design and home planning, whether they’re creating a compact studio, a rental-ready apartment, or a private suite for family. An ADU can take the form of a modern detached cottage, an attached unit, or a garage or attic conversion, depending on your lot, budget, and intended use. Homeowners can customize every detail—from floor layout and interior finishes to eco-conscious design features like solar panels, upgraded insulation materials, and water-saving fixtures—to comply with the zoning code and Title 24 energy standards.

Your ADU’s purpose will also influence the design. For example, a long-term rental may prioritize privacy and full amenities, while a family suite or multigenerational housing layout may focus on accessibility and comfort. Working with an experienced ADU design and construction company ensures compliance with local building codes, efficient site planning, and seamless integration with your existing property.

Popular Design Strategies

• Open-plan kitchenette + living area – Creates a spacious and modern feel, perfect for smaller ADUs.
• Lofted ceiling or mezzanine in detached ADUs – Adds height and natural light without expanding the footprint.
• Garage or attic conversions – Turn unused space into a cozy, compliant Accessory Dwelling Unit at a fraction of the cost of new construction.
• Multi-generational layout (bedroom & bath only) – Ideal for aging parents or adult children, balancing comfort and independence.

Many San Diego homeowners choose pre-approved ADU floor plans from the city to accelerate approval and simplify compliance with local zoning requirements (City of San Diego).

See ADU Design Examples » Better Place Design & Build ADU Projects

Expert Tip: Design your ADU for versatility — open layouts and natural light improve livability and boost long-term property value.

How Long Will ADU Construction Take and What Will It Cost?

The timeline and cost for ADU construction in California depend on the type of unit, design complexity, and city permitting speed. On average, homeowners can expect the entire process to take 10–18 months, including design, permit approval, and construction (Maxable). Simpler projects—like garage conversions—can often finish sooner, while custom detached ADUs may take longer.

In terms of cost, the median construction cost for an ADU in California is around $150,000, or roughly $250 per square foot, according to a statewide survey by the UC Berkeley Terner Center for Housing Innovation (Terner Center). Most ADUs remain far more affordable than traditional new homes, offering a cost-effective way to expand livable space or generate rental income.

Cost & Timeline Benchmarks

Benefits for Homeowners & Investment Potential

Building an Accessory Dwelling Unit (ADU) offers homeowners in California both financial and lifestyle advantages. An ADU can serve as a rental property, guest suite, or multigenerational living space, helping offset mortgage costs or accommodate family needs. Beyond added flexibility, ADUs are a proven way to increase property value and support California’s push for more housing options. The Federal Housing Finance Agency (FHFA) notes that ADUs expand local housing supply while allowing homeowners to build wealth through rental income and appreciation (FHFA).

“ADUs offer many benefits for homeowners — they allow rental income to offset mortgage costs and increase housing stock without the need for new land.” — Federal Housing Finance Agency (FHFA)

Financial Upside At-a-Glance

• Higher resale value: Homes with ADUs sell for 20–35% more than comparable homes without one (NFM Lending).
• Increased property equity: In 2023, the median appraised value of California homes with an ADU reached $1.064 million, compared to $715,000 for homes without ADUs (FHFA).
• Strong rental income: The median rent for new ADUs is around $2,000 per month, providing stable long-term returns for homeowners (Terner Center for Housing Innovation).
• Long-term housing contribution: Only 8% of new ADUs are used as short-term rentals — the majority provide long-term housing, supporting community affordability (Terner Center).

“Actually, it doesn’t really matter what you call it… What you get is more housing.” — Denise Pinkston, President of the Casita Coalition (CalMatters)

ADUs not only enhance home value and rental potential but also offer multi-generational flexibility — allowing families to stay close while maintaining privacy.

Pro Insight: A well-designed ADU kitchen increases rental appeal. Prioritize durable materials and energy-efficient fixtures for higher ROI.

How to Get Started (Homeowner Checklist)

Starting your ADU project in California begins with understanding your property’s potential and assembling the right team. The process includes evaluating your lot, designing your unit, and submitting plans for city approval before construction begins. San Diego homeowners benefit from statewide streamlining laws, which require cities to review ADU permits within 60 days (CalMatters). Working with an ADU-experienced designer and builder helps ensure compliance with local zoning, energy, and safety codes (California HCD).

Building an ADU isn’t just about construction — it’s about planning smartly from the start to save time, money, and stress.

Homeowner Action Plan

Here’s a simple checklist to guide your ADU journey from concept to completion:

• Talk to your local planning department (San Diego): Confirm zoning allowances, lot size limits, and parking rules (City of San Diego ADU Toolkit).
• Hire an ADU designer or architect: Choose professionals experienced with California’s ADU laws to create compliant, cost-effective plans.
• Review your budget & financing options: Plan for design, permits, construction, and contingency costs — typical ADUs cost around $150,000–$200,000 statewide (Terner Center).
• Submit permit applications: Work with your design team to file for city approvals; state law mandates a 60-day turnaround on complete ADU applications (CalMatters).
• Select a contractor and begin construction: Choose a licensed ADU builder with local experience for smooth project delivery and code compliance.

Schedule a Free Consultation → Better Place Design & Build ADU Services

Expert Tip: Before building, verify setbacks and utility connections—early planning prevents costly redesigns and approval delays.

Why Choose Better Place Design & Build for ADU Construction in San Diego

Better Place Design & Build specializes in turnkey ADU design and construction across San Diego County, handling everything from planning and permits to final build. Our team’s local expertise ensures compliance with San Diego’s ADU regulations and the Bonus ADU Program, helping homeowners complete projects efficiently and stress-free. We focus on quality craftsmanship, energy-efficient design, and fast permit approval, delivering ADUs that add both comfort and value. Start your ADU project with a free consultation and property review.

Expert Advice: Partner with an experienced local builder who understands San Diego’s ADU Bonus Program to maximize incentives and avoid costly compliance issues.

Conclusion

Building an Accessory Dwelling Unit (ADU) is one of the most practical and valuable investments a California homeowner can make. With clear state laws, faster permit processes, and flexible design options, ADUs offer a smart way to expand living space, earn rental income, and increase property value. Whether you’re planning a detached backyard home, a garage conversion, or an attached addition, today’s ADU programs make it easier than ever to bring your project to life.

For San Diego homeowners, understanding local incentives and working with an experienced design–build team ensures a smooth process from concept to completion — turning your property into a long-term asset for your family and community.

Final Advice: Partner with an experienced ADU builder who manages permits, design, and construction under one roof for a stress-free process.