Building an Accessory Dwelling Unit (ADU) in San Diego County has become an increasingly popular way to add value to your property, create additional living space for family members, or generate rental income. With California's progressive ADU laws and San Diego's housing demand, now is an excellent time to consider adding an ADU to your property.
This comprehensive guide will walk you through everything you need to know about ADU construction in San Diego County—from understanding what an ADU is, to navigating permits and regulations, to planning your budget and timeline.
In This Guide:
What is an ADU?
An Accessory Dwelling Unit (ADU) is a secondary housing unit on a single-family residential lot. ADUs are also commonly called:
- Granny flats – when used for elderly family members
- In-law units – for extended family living
- Backyard cottages – detached units in the yard
- Casitas – especially common in Southern California
- Guest houses – for visitors or rental income
ADUs are complete, independent living spaces with their own kitchen, bathroom, and sleeping areas. They can be attached to the main house, detached in the backyard, or converted from existing structures like garages.
Types of ADUs Allowed in San Diego County
1. Detached ADU
A separate, standalone structure in your backyard or side yard. This is the most popular option in San Diego County as it provides maximum privacy and can be designed to complement your property's aesthetics.
Best for: Maximizing rental value, multi-generational living, or creating a separate workspace.
2. Attached ADU
An addition to your existing home that shares at least one wall with the primary residence. Often more cost-effective than detached units since utilities and foundation work may be shared.
Best for: Keeping family close while maintaining independence, or when lot size is limited.
3. Garage Conversion ADU
Converting an existing garage into living space. This is typically the most affordable ADU option since the structure already exists.
Best for: Budget-conscious homeowners or those with limited yard space.
4. Junior ADU (JADU)
A smaller unit (up to 500 sq ft) created within the existing primary residence. JADUs must share at least one bathroom with the main home and have an efficiency kitchen.
Best for: Creating a small rental unit or accommodating a family member with minimal construction.
San Diego County ADU Regulations & Requirements
California has some of the most progressive ADU laws in the country, and San Diego County follows state guidelines while adding some local requirements.
Key Regulations (2025):
- Lot eligibility: Most single-family residential lots are eligible for at least one ADU
- Size limits: Up to 1,200 sq ft or matching the primary dwelling size (whichever is less)
- Height limits: Generally 16 feet for detached ADUs (up to 25 feet in some cases)
- Setbacks: Typically 4 feet from side and rear property lines
- Parking: No additional parking required in most cases
- Owner occupancy: Not required as of 2025
Important Note: While these are general guidelines, specific requirements can vary by municipality within San Diego County. Cities like San Diego, Carlsbad, Oceanside, and El Cajon may have additional local ordinances. Always verify current regulations with your local planning department before beginning your project.
ADU Cost Breakdown for San Diego County
The cost of building an ADU in San Diego varies significantly based on size, complexity, materials, and type. Here's what you can expect:
Average Cost Ranges (2025)
| ADU Type | Size Range | Cost Range | Per Sq Ft |
|---|---|---|---|
| Garage Conversion | 300-600 sq ft | $60,000 - $150,000 | $200-$250/sq ft |
| Attached ADU | 400-800 sq ft | $120,000 - $280,000 | $300-$350/sq ft |
| Detached ADU | 600-1,200 sq ft | $180,000 - $420,000 | $300-$400/sq ft |
What's Included in the Cost?
- Design & Planning: Architectural drawings, engineering (3-8% of total)
- Permits & Fees: Building permits, impact fees, inspections ($5,000-$25,000)
- Site Preparation: Grading, utilities, foundation work
- Construction: Framing, electrical, plumbing, HVAC, finishes
- Materials: Quality of finishes significantly impacts cost
- Labor: Skilled trades and project management
Permits & Application Process
Navigating the permit process is one of the most critical—and sometimes frustrating—parts of ADU construction. Here's what you need to know:
Required Documents:
- Site plan showing property boundaries and ADU location
- Architectural drawings (floor plans, elevations)
- Structural calculations and engineering stamps
- Title 24 energy compliance calculations
- Utility connection plans
- Fire and life safety plans
Application Timeline:
Design Considerations for San Diego Climate
San Diego's Mediterranean climate offers unique opportunities and challenges for ADU design:
Climate-Optimized Features:
- Natural ventilation: Cross-breeze design reduces AC needs
- Solar orientation: Position windows to maximize winter sun, minimize summer heat
- Outdoor living: Patios and covered porches extend usable space
- Drought-tolerant landscaping: Reduce water usage and maintenance
- Energy-efficient windows: Double-pane, low-E glass for coastal areas
- Insulation: Proper insulation maintains comfort year-round
Maximizing Space in Small ADUs:
- Open floor plans to create flow and visual space
- High ceilings (9-10 feet) make rooms feel larger
- Built-in storage to minimize clutter
- Multi-functional furniture and spaces
- Large windows to bring in natural light
- Light color palettes to enhance spaciousness
ADU Construction Timeline
From initial concept to move-in ready, here's what to expect:
Phase 1: Planning & Design (2-3 months)
- Initial consultation and site evaluation
- Preliminary design concepts
- Final architectural plans
- Engineering and structural plans
Phase 2: Permits (2-4 months)
- Permit application submission
- Plan review and revisions
- Permit approval and issuance
Phase 3: Construction (4-8 months)
- Site preparation and foundation (2-4 weeks)
- Framing and rough-ins (4-6 weeks)
- Exterior finishes (3-4 weeks)
- Interior finishes (4-8 weeks)
- Final inspections and certificate of occupancy
Total Timeline: 8-15 months from start to finish
Note: Garage conversions typically take 3-6 months less since no foundation work is required.
ROI & Property Value Impact
Building an ADU is a significant investment, but the return can be substantial:
Rental Income Potential:
San Diego rental market rates for ADUs (2025):
- Studio/1-bedroom (400-600 sq ft): $1,800-$2,500/month
- 1-bedroom (600-800 sq ft): $2,200-$3,000/month
- 2-bedroom (800-1,200 sq ft): $2,800-$3,800/month
Property Value Increase:
Studies show that ADUs can increase property value by 20-30% or more, often recovering 80-100% of construction costs. In San Diego's competitive real estate market, homes with ADUs sell faster and command premium prices.
Calculate Your Potential ROI:
Example: 600 sq ft detached ADU
- Construction cost: $210,000
- Monthly rental income: $2,400
- Annual income: $28,800
- Simple payback period: ~7.3 years
- Plus increased property value: $150,000-$200,000
Other Financial Benefits:
- Potential property tax benefits (varies by situation)
- Reduced living costs if housing family members
- Increased borrowing capacity (home equity)
- Flexibility for future life changes
Why Choose a Licensed Contractor for Your ADU
While the DIY route or unlicensed builders might seem tempting to save money, working with a licensed contractor is essential for ADU construction:
Benefits of Working with Licensed Contractors:
- Legal compliance: Licensed contractors understand building codes, zoning laws, and permit requirements
- Proper insurance: Protection for you and the workers on your property
- Quality assurance: Licensed contractors stake their reputation and license on quality work
- Permit expertise: Experienced contractors know how to navigate the permitting process efficiently
- Warranty protection: Licensed work typically includes warranties and recourse if issues arise
- Resale value: Permitted work with licensed contractors protects your investment
What to Look for in an ADU Contractor:
- Valid California contractor's license (verify at CSLB.ca.gov)
- Bonded and insured
- Experience specifically with ADU construction
- Knowledge of local San Diego regulations
- References and portfolio of completed ADUs
- Clear, detailed written estimates
- Good communication throughout the project
Ready to Build Your ADU?
Building an ADU in San Diego County is an excellent investment that can provide rental income, accommodate family members, or increase your property value. While the process involves careful planning, permits, and construction, the result is a valuable asset that enhances your property for years to come.
At Moore Quality Builders, we specialize in ADU construction throughout San Diego County. With over 40 years of experience and a California contractor's license, we guide you through every step—from initial design to final inspection.
Start Your ADU Project Today
Contact Moore Quality Builders for a free consultation. We'll assess your property, discuss your goals, and provide a detailed estimate for your ADU project.