How Does ADU Construction Work in San Diego, CA?

What Is ADU Construction?

An Accessory Dwelling Unit (ADU) is a self-contained residential unit built on the same lot as a single-family dwelling. In California, ADUs can be detached backyard cottages or garage conversions, offering flexible housing options for families, renters, or multigenerational living while also supporting aging in place (California HCD).

San Diego has become a statewide leader in ADU construction. In 2023, the City issued more than 1,900 building permits for ADUs — the highest total ever recorded (Inside San Diego). This growth reflects strong homeowner demand and local zoning regulations that encourage development. Recent state reforms—including the 60-day permitting process rule and exemptions from certain fees—have made permit approvals faster and the construction site timeline more predictable (Justia Law – Gov Code §65852.2).

Amid rising housing costs and limited supply, ADUs give San Diego homeowners a practical way to generate rental income, provide housing for family members, or add affordable units under the city’s ADU Bonus Program (City of San Diego). By meeting state building standards and passing final inspection, these accessory structures can quickly transform underutilized space into long-term housing solutions.

Smart interior design in ADUs maximizes livable space while meeting California Title 24 energy standards and long-term comfort goals.

Understanding ADU Types (San Diego Homeowners)

San Diego homeowners have several ADU options, each with different benefits, costs, and construction timelines. Choosing the right type depends on budget, site conditions, and local zoning code requirements.

Detached ADUs:

Detached ADUs are standalone units that function as independent living spaces. Under California ADU law, they can be built up to 1,200 square feet, giving families the flexibility to add a full secondary home on their property. These units follow the same permitting process as other residential construction and require utility connections, foundation work, and final inspection before receiving a Certificate of Occupancy (Justia – Gov Code §65852.2).

Garage Conversions:

Garage conversions remain one of the most cost-efficient and time-saving ADU types. In San Diego, they typically take 2–4 months to complete and are ideal for maximizing existing structures without major site work. These projects usually involve interior finishes, plumbing rough-in, and electrical setup, but they avoid the larger scope of work required for detached builds (Groysman Construction).

San Diego’s ADU Bonus Program:

San Diego’s ADU Bonus Program makes the city especially attractive for homeowners pursuing ADUs. This program allows property owners to build additional units if they agree to include deed-restricted affordable housing. In Transit Priority Areas, there is no cap on how many ADUs can be added, making it one of the most flexible programs under current zoning laws (City of San Diego – ADU Toolkit).

Choosing between a detached ADU or garage conversion depends on your site layout, utility connections, and long-term property goals.

How Do ADU Permits and Approvals Work in San Diego?

How long does ADU permitting take in California?

Under California ADU law, cities must review a permit application using a ministerial process with no public hearings or subjective review. By statute, a permitting agency must either approve or deny an ADU within 60 days of receiving complete construction documents. If no action is taken, the application is automatically approved (California Gov. Code § 65852.2). The City of San Diego confirms this in its Permit Services Division guidelines (City of San Diego Information Bulletin 400).

San Diego-Specific Permitting Details

San Diego has streamlined the building process with its ADU Toolkit, helping homeowners prepare submission and review packages that meet local requirements, zoning codes, and design standards (City of San Diego – ADU Toolkit). The city also runs a unique ADU Bonus Program, which allows homeowners to add more units if they agree to provide deed-restricted affordable housing. In Transit Priority Areas, there is no cap on ADUs under this zoning regulation, making it one of the most flexible programs in the state (Terner Center).

Expert Insight

While the permitting process is designed to encourage housing growth, city planners stress that development must still align with community standards. As Heidi Vonblum, San Diego Planning Director, explained: “New homes must be consistent with the scale of existing neighborhoods.” (City of San Diego ADU Reform PDF).

Submit complete, code-compliant plans early—accurate documentation helps speed up the 60-day ADU permit approval process.

What Is the Timeline for Building an ADU?

How long does it take to build an ADU?

The overall construction timeline for an ADU in San Diego typically ranges from 10–18 months from design to final move-in, depending on the scope of work and site conditions. Build times vary by ADU type: a detached ADU generally requires 4–8 months of active construction, while a garage conversion can often be completed in 2–4 months.

Key Phases of ADU Construction

• Feasibility & Design: Initial site assessments, property evaluations, and drafting construction documents with attention to zoning codes and local requirements.
• Permitting & Approvals: A complete permit application must be submitted for review. By law, cities must approve or deny the ADU within 60 days, ensuring faster approvals (California Gov. Code §65852.2).
• Site Prep & Utilities: Includes grading, trenching, and foundation construction, as well as utility connections for water, sewer, and electrical. Detached ADUs often need new hookups, while garage conversions may reuse existing lines.
• Construction & Inspections: Framing, MEP systems (mechanical, electrical, and plumbing rough-in), and interior finishes are completed. City inspectors perform checks at each stage to confirm compliance before final approval and Certificate of Occupancy.

Breaking ground early with clear construction documents and city-approved plans helps prevent costly ADU timeline delays.

How Much Does an ADU Cost in San Diego?

What is the average cost of an ADU?

The construction costs of an ADU in San Diego depend on size, type, and design detail decisions, but several benchmarks provide clarity:

• The median statewide ADU cost is about $150,000 (~$250 per square foot), according to a UC Berkeley Terner Center survey of California homeowners (Terner Center).
• A garage conversion ADU generally costs between $80,000 and $150,000, offering one of the most cost-efficient building processes.
• A detached ADU usually ranges from $150,000 to $300,000+, with higher costs tied to square footage, foundation work, and upgraded interior finishes.

Hidden Costs Homeowners Must Know

In addition to direct construction expenses, homeowners should consider extra cost factors:

• Impact fees are waived for ADUs under 750 sq. ft., which can significantly reduce overall expenses (Better Place Design & Build).
• School fees may apply only for ADUs larger than 500 sq. ft., depending on local district rules.

Financing Options for ADUs

The financial feasibility of a project often depends on available funding:

• Surveys show most homeowners finance ADUs with home equity lines of credit (HELOCs) or cash-out refinancing, both popular among traditional builders and homeowners balancing cost factors.
• California also supports ADU projects through the CalHFA ADU Grant, which offers up to $40,000 in assistance for pre-development needs such as design development, permits, and site preparation (CalHFA ADU Grant).

Understanding labor, material, and permit costs upfront ensures your ADU budget aligns with both your financial feasibility and design goals.

How Do Builders Ensure Quality and Compliance?

Building an ADU in San Diego requires careful adherence to state building standards and local inspection requirements to ensure safety, energy efficiency, and long-term durability. Each project must follow a structured building process that covers design development, construction, and oversight by city officials.

State Building Codes (Title 24)

All ADUs must comply with the California Energy Code (Title 24, Part 6), which establishes requirements for lighting, insulation, and HVAC installation. These standards apply equally to detached units, garage conversions, and other types of site-built ADUs, ensuring every project meets energy efficiency benchmarks (California Energy Commission).

Fire Safety Requirements

Under California law, fire sprinklers are not required in an ADU unless they are already installed in the primary home. This exemption, reinforced by San Diego’s Technical Bulletin RESD-3-4, allows homeowners to reduce construction costs while still meeting state safety requirements (City of San Diego – Technical Bulletin RESD-3-4).

Ministerial and Objective Inspections

The permit application and inspection process is handled ministerially, meaning inspectors rely only on objective standards with no discretionary hearings. This streamlined approach reduces delays and ensures consistency across all ADU projects, whether prefab ADUs or traditional stick-built structures (CALBO – Accessory Dwelling Units).

Quality Assurance Practices

Reputable builders rely on professional engineering teams and licensed contractors to oversee each stage of the project. Quality control includes:

• Foundation construction, framing, plumbing, and electrical setup inspected at multiple phases.
• City oversight to confirm compliance with zoning codes and utilities and services standards.
• A property tax reassessment, typically around 1% of the ADU’s added value, once the project is complete and receives a Certificate of Occupancy.

Hiring licensed builders familiar with local codes ensures smooth inspections and avoids delays during the ADU construction timeline.

Detached ADU vs. Garage Conversion: Which Is Better?

Homeowners in San Diego often weigh the pros and cons of building a detached ADU versus converting an existing garage. Each option has unique benefits, costs, and timelines that can significantly impact a project decision.

Key Takeaway

• A detached ADU is best for homeowners who want more space, design flexibility, and potential rental income.
• A garage conversion ADU is ideal for those seeking a lower-cost, faster build that still provides livable space.

Financing and ROI for Homeowners

Building an ADU in San Diego can provide long-term housing flexibility while also creating a reliable rental income stream. Homeowners considering the financial feasibility of an ADU should understand both projected rental earnings and available funding tools that support the construction timeline.

Median Rental Income

Across California, new ADUs rent for a median of about $2,000 per month, according to UC Berkeley’s Terner Center surveys (Terner Center). San Diego’s strong housing market and limited rental supply make accessory apartments and internal ADUs particularly attractive for homeowners seeking stable returns.

Return on Investment (ROI)

A homeowner who invests around $150,000 to build an ADU and rents it at $2,000/month can typically recover costs in 6–7 years. After this breakeven point, net rental income adds value while the property itself appreciates, especially when the ADU is built with efficient building processes and high-quality interior finishes.

Financing Tools Homeowners Commonly Use

Most San Diego ADUs are financed through home equity, with owners relying on flexible HELOCs or cash-out refinancing to cover cost factors like foundation construction, plumbing, or electrical setup. Some homeowners also turn to construction loans for larger site-built ADUs or prefab ADUs where professional engineering teams oversee the scope of work.

CalHFA ADU Grant

Beyond traditional financing, California offers the CalHFA ADU Grant, which provides up to $40,000 for pre-development costs such as site preparation, permits, and design fees (CalHFA ADU Grant). This program reduces neighborhood disruption during the early building process and is particularly useful for first-time builders seeking end-to-end turnkey service.

When calculating ADU ROI, consider high-efficiency finishes that boost rent value while keeping maintenance costs low.

San Diego Spotlight: Local Programs & Trends

San Diego has emerged as a statewide leader in ADU construction, supported by local requirements and innovative housing programs that build on California’s ADU law. The city has streamlined the building process and created zoning code incentives that encourage both affordability and growth.

Record ADU Growth

In 2023, San Diego issued permits for more than 1,900 ADUs, setting a new record and demonstrating strong homeowner demand for accessory apartments and backyard cottages (Reddit summary of city data). This surge shows how zoning laws and a simplified permitting process are helping meet the region’s housing shortage.

Affordable ADUs through the Bonus Program

The city’s ADU Bonus Program is one of the most innovative in California. It allows homeowners to build additional ADUs if they commit to deed-restricted affordable units. This policy is included in the City zoning code and has already resulted in hundreds of affordable ADUs being approved, making San Diego’s program a model studied by groups like the Casita Coalition (City of San Diego – ADU Toolkit).

2025 Reforms: Refining the Density Bonus

In May 2025, the City of San Diego introduced 24 reforms to its ADU Bonus Program and related zoning regulations. These updates are designed to maintain neighborhood character while ensuring the program continues to support affordable housing growth and compliance with state building standards (City of San Diego Reform Proposal PDF).

Wider Housing Momentum

ADUs are just one piece of San Diego’s broader pro-housing trend. In 2024, the city permitted 8,782 new homes overall, making it one of the most productive years in recent history (Inside San Diego). This reflects a commitment to expanding housing supply through diverse building processes, ranging from site-built ADUs to larger residential projects.

Local programs like San Diego’s ADU Bonus Program reward homeowners who plan efficiently and build within zoning code standards.

Conclusion: Is Building an ADU in San Diego Worth It?

Building an ADU in San Diego is a smart investment for homeowners because it combines streamlined permitting rules, flexible design choices, and strong rental potential. With California’s 60-day ministerial approval law (Gov. Code §65852.2) and San Diego’s innovative ADU Bonus Program (City of San Diego), the permitting process is faster and more predictable than traditional building processes for other housing types. Whether a homeowner chooses a lower-cost garage conversion or a larger detached ADU of up to 1,200 sq. ft., the project can enhance property value while providing new rental income opportunities averaging around $2,000 per month statewide (Terner Center). When paired with financing options like the CalHFA $40,000 ADU Grant (CalHFA), these accessory apartments not only help address local housing needs but also serve as a long-term strategy for financial feasibility, aging in place, and family flexibility.

Maximize your ADU’s value by designing for versatility—multiuse spaces increase both livability and long-term ROI potential.