What Is an ADU Cost Breakdown?

How Much Does an ADU Cost in California?

Building an ADU in California typically costs $150,000–$300,000+, depending on size, ADU type, finish level, and site conditions. On a per-square-foot basis, most backyard cottages and granny flats fall within $250–$600+ per sq.ft., with higher-cost markets—like Los Angeles and Southern California—pushing toward the upper range. According to statewide research, the median construction cost of a California ADU is $150,000, or $250 per sq.ft. (Terner Center). ADUs also add significant long-term value, with homes containing ADUs selling for approximately 35% more than comparable properties without them (Better Place Design & Build). These numbers reflect real building costs that include construction labor, structural engineering, building plans, and typical utilities connections required for detached new construction.

Statewide ADU pricing swings most with finish level, layout complexity, and site prep—so anchoring your budget to a realistic per-sq-ft range helps avoid underestimating total build cost.

How Much Does an ADU Cost in San Diego?

ADUs in San Diego generally cost more than the statewide average due to higher labor rates, material prices, and local permitting fees. Most homeowners spend $300,000+ for a detached ADU, $200,000–$300,000 for an attached ADU, and $150,000–$200,000 for a garage conversion, especially when accounting for zoning regulations and utilities like electrical systems or plumbing fixtures. Local builders report that construction costs in San Diego average $375–$600 per sq.ft. for a turnkey ADU, reflecting real building expenses such as a concrete foundation, HVAC systems, and project management by general contractors. Detached units typically start around $300,000, while garage conversions remain the most affordable option at $150,000–$200,000 (Better Place Design & Build). These ranges also apply whether you’re building a smaller 500 sq. ft. ADU or a larger ground-up design through a local design-build company.

In San Diego, costs climb fastest when utilities need upgrades or runs are long, so a site evaluation early in planning is one of the best ways to protect your budget.

ADU Hard Costs: Materials & Labor Breakdown

What Counts as Hard Costs?

Hard costs are the physical construction expenses required to build an ADU, including labor and materials. These costs cover everything involved in creating the structure itself—from the building foundation and concrete work to the finished interiors. According to industry analysis, the vertical build (labor + materials) makes up roughly 80–85% of the total ADU cost, whether you’re constructing a smaller 500 sq ft ADU or a larger ground-up design.

Labor-heavy phases like insulation, drywall, and finishes are where budgets swing—lock your finish package early to avoid upgrade-driven overruns.

Labor Breakdown

Labor costs include all skilled trades needed to construct the unit. This typically covers:

• Foundation work (slab, land leveling, excavation)
• Framing & structural carpentry (including metal framing where required)
• Roofing & exterior siding
• Electrical wiring & panel upgrades
• Plumbing installation
• Insulation & drywall
• Interior finishes (paint, trim, installation of fixtures)

These trades make up the largest portion of ADU spending due to California’s higher labor rates and the need for licensed professionals such as a structural engineer and certified general contractors who ensure compliance with local building regulations.

Material Costs

Material costs include all building components that go into the ADU structure and interior, such as:

• Lumber, framing materials, or panelized modular components
• Concrete and rebar for foundation pouring
• Roofing materials
• Drywall, insulation, and sheathing
• Windows and exterior doors
• Cabinets, tile, countertops, and flooring (including options like Kitchen Cabinets, Luxury Vinyl Plank, or Quartz Solid-Surface)
• Plumbing and electrical fixtures
• Heating and cooling equipment such as a Mini Split HVAC System

Choosing premium or energy-efficient features can quickly increase the overall price—upgraded materials and finishes can add up to 30% to the total project cost.

ADU Hard Costs Table

ADU Soft Costs: Permits, Design & Fees

How much do ADU permits cost in San Diego?

ADU permits in San Diego typically cost $15–$28 per sq.ft., which includes plan check fees, school fees, utility fees, and other permitting fees required by the local building department. Thanks to California state law, ADUs under 750 sq ft. pay zero development impact fees, creating meaningful savings for homeowners (California HCD). These permit costs vary by local city and often differ based on ADU size, zoning entitlements, and additional requirements such as utility upgrades or Boundary & Survey work.

Required Soft Costs

Soft costs include all professional, administrative, and regulatory expenses associated with planning and approving an ADU project. These typically include:

• Architectural plans and overall Design and Architecture
• Engineering and structural calculations
• Title 24 energy compliance reports (often completed with support from an energy consultant)
• Permit application and plan check fees
• Utility connection or upgrade fees

Industry experts note that soft costs can account for roughly 15% of the total ADU budget (RenoFi). These expenses are essential in Building Construction and must be factored into early cost estimation—especially for larger projects like a 1,200 sq ft ADU or ground-up builds requiring more detailed architectural cost and regulatory review.

ADU Soft Costs Table

ADU Cost by Type

Detached ADU Cost in San Diego

Detached ADUs are the most expensive ADU type because they require a new foundation, full utility connections, and stand-alone structural systems. In San Diego, most detached units cost $300,000–$400,000+, depending on size, finish level, and site conditions. Local builders report that construction pricing averages $375–$600 per sq.ft., which drives detached ADU costs to the top of the range. These higher expenses reflect additional permitting fees, utility trenching, and site work often coordinated with local government requirements before a Certificate of Occupancy is issued.

Attached ADU Cost

Attached ADUs typically cost $200,000–$300,000, making them more affordable than detached units. Because they share at least one structural wall with the primary home, attached ADUs reduce framing, exterior siding, and utility installation costs. Homeowners often choose attached ADUs when lot size is limited or when overall project efficiency is a priority, especially when exploring financing options like a home equity loan or construction loans to manage the budget effectively.

Garage Conversion Cost

Garage conversions are the fastest and most affordable ADU option in San Diego, typically ranging $150,000–$200,000. Converting an existing structure lowers costs because the slab foundation and exterior walls can often be reused, although structural reinforcement and utility upgrades are still required. According to regional cost data, garage conversions consistently fall at the bottom of the ADU price spectrum and generally require fewer furnishings and appliances since layouts remain compact, with minimal post-construction cleanup or landscaping costs.

Detached units typically cost the most because they require a new foundation and full utility runs—attach or convert when the site allows to lower your total budget.

What Factors Increase or Decrease ADU Cost?

ADU pricing varies significantly depending on the home, lot conditions, design decisions, and construction requirements. Builders across California—including Better Place Design & Build, GreatBuildz, and Maxable—consistently highlight four major cost drivers: size and layout, site conditions, utility capacity, and finish level. Understanding these factors helps homeowners budget realistically and choose the most cost-effective ADU type for their property.

Size & Layout

Size is the single biggest cost driver for ADUs. Larger units cost more overall, but the price per square foot decreases as the ADU gets bigger. Maxable says, “The first 400 square feet are the most expensive because they must include a kitchen and a bathroom, plus there are lots of fixed costs like permits and design fees. The larger your unit is the less your cost per square foot will be.” Layouts with multiple bedrooms, complex rooflines, or additional plumbing walls (such as adding a second bathroom) also increase cost.

Existing Site Conditions

The condition of the lot can significantly impact construction cost. Challenging or complex sites often require additional engineering and labor, including:

• Sloped or uneven lots needing grading or retaining walls
• Poor soil conditions requiring deeper footings or reinforced foundations
• Limited access for equipment or material delivery
• Drainage improvements or stormwater management upgrades

Builders emphasize that site work is one of the most unpredictable components of an ADU budget.

Utility Upgrades

If the existing home’s utilities cannot support an additional dwelling, upgrades may be required. Common cost-drivers include:

• Sewer lateral replacement or extension
• Main electrical panel upgrades (common for older homes)
• Water line upsizing to support added fixtures

Utility work can add tens of thousands of dollars depending on distance to existing connections and the age of the home.

Finish Level

Interior and exterior finish choices can shift an ADU budget substantially. Standard finish packages help control cost, while premium materials or custom upgrades increase spending. According to industry insights, upgraded materials and finishes can add up to 30% to the total project cost.

Finish choices that most affect the budget include:

• Custom cabinetry vs. standard stock cabinets
• High-end tile or flooring
• Premium windows and doors
• Architectural lighting or smart-home upgrades

ADU Permits, Timelines & Approval Process

How long does it take to permit an ADU in San Diego?

Most ADU permits in San Diego take 2–4 months to process, depending on project complexity, permits and fees required, city review volume, and any needed corrections. San Diego also offers expedited review options, which can shorten timelines for homeowners using standard plans or streamlined submittals. The permitting phase includes plan check reviews, zoning compliance checks, and Title 24 energy verification before final approval—similar to what a homeowner would experience in other California markets like the City of Los Angeles or other Cali ADU jurisdictions.

Key local permit requirements

San Diego follows California’s statewide ADU laws but also applies several city-specific requirements that homeowners must meet before approval. These local regulations include:

• Fire setbacks based on access routes and fire-code clearances
• Maximum height limits depending on zoning and ADU type
• Owner occupancy rules (state law removed owner-occupancy requirements for newly built ADUs through 2025)
• Parking exemptions, including no new parking required for ADUs within ½ mile of public transit

Meeting these requirements—and addressing Permits and Impact Fees early—helps streamline the process and reduces delays. Proper preparation also ensures smoother ADU ownership by avoiding redesigns that may arise during review by the local government or building department.

Before submitting plans, confirm setbacks, utility tie-in paths, and access for construction—clean site planning helps prevent permit corrections that add months.

ADU Cost-Saving Strategies for California Homeowners

Building an ADU in California doesn’t have to exceed your budget—especially when you take advantage of state incentives, streamlined design options, and smart planning practices. The most effective cost-saving strategies focus on minimizing soft costs, reducing site complexity, and avoiding unnecessary upgrades. These approaches help homeowners control expenses without sacrificing functionality or long-term value.

Build Under 750 sq.ft. (Avoid Impact Fees)

California law waives all development impact fees for ADUs 750 sq.ft. or smaller, saving homeowners thousands in upfront permitting costs. This exemption applies statewide and is confirmed by the California Department of Housing and Community Development (Source: California HCD).

Use Standardized or Pre-Approved Designs

Standardized ADU plans—offered by many California cities and builders—significantly reduce architectural and engineering fees. These plans also shorten plan-check timelines, lowering soft costs by reducing revisions and reducing the number of billable hours required for professional services.

Set Aside a 10% Contingency Budget

A 10% contingency fund helps homeowners avoid financial surprises, especially during excavation, utility upgrades, or structural reinforcement. Experts consistently recommend carrying a buffer to handle unforeseen costs that arise once construction begins.

Simplify the Layout

Simple, efficient floor plans reduce both material and labor costs. Limiting structural complexity—such as avoiding extra plumbing walls, vaulted ceilings, or custom rooflines—keeps the project within budget while still delivering a functional living space.

Avoid Basement ADUs

Basement ADUs require excavation, moisture mitigation, structural reinforcement, and often extensive utility work, making them one of the most expensive ADU types. Avoiding basement conversions ensures significantly lower labor and material expenses while reducing construction risks.

Keeping the structure simple (clean rooflines, fewer plumbing walls) is one of the fastest ways to cut hard costs without shrinking livable square footage.

Prefab vs. Traditional ADU Costs in California

Prefab ADUs are often marketed as a faster and cheaper alternative to traditional, site-built ADUs—but in California, especially in higher-cost regions like San Diego, the price difference is usually much smaller than homeowners expect. While factory construction can reduce labor time, the total project cost still depends on foundation work, utilities, delivery, crane placement, and site preparation, which are required regardless of whether the ADU is prefab or stick-built.

Prefab ADUs Often End Up Within 10–15% of Site-Built Costs

Industry analyses show that prefab ADUs typically fall within 10–15% of the total cost of a site-built ADU, largely because homeowners still pay for major on-site expenses. These include utility trenching, foundation work, grading, crane setup, and transportation—costs that are not included in most advertised prefab pricing.
Although this insight appears frequently across California ADU builders, it is reinforced by builders who compare real-world pricing between factory-built and site-built ADUs.

Advertised Prefab Prices Exclude Major Site Costs

Prefab pricing online often lists the unit only, leaving out many required expenses:

• Foundation construction
• Cranes and rigging to lift the module into place
• Transportation and delivery fees
• Site prep and grading
• Utility connections (sewer, water, electrical)
• Permits, engineering, and Title 24 reports

These items can add hundreds of hours of labor and significant cost to the project. Because prefab companies frequently exclude these items from their list price, homeowners may find that the real total cost is much closer to that of a traditional build.

Prefab ADUs Are Rarely Cheaper in San Diego

Due to San Diego’s labor rates, complex permitting environment, and the region’s higher cost of site preparation, prefab ADUs are rarely cheaper than traditional ADUs. Local builders note that once all on-site work is factored in—especially transportation logistics and crane placement—prefab units often cost nearly the same as site-built ADUs.

Homeowners typically choose prefab ADUs for predictability and speed, not necessarily savings.

When comparing prefab to site-built, always separate the factory unit price from real on-site costs like foundation, utilities, and crane placement—those are what narrow the savings gap.

Local ADU Rules, Zoning & Regulations in San Diego

San Diego follows California’s statewide ADU laws while applying additional local zoning standards that guide where and how ADUs can be built. These rules influence size, placement, parking, height, and utility requirements. Understanding these regulations helps homeowners avoid redesigns and delays during the permitting process. Most ADU regulations in San Diego align with California’s ADU Handbook provided by the California Department of Housing and Community Development (HCD) (Source: California HCD).

Before choosing a design, confirm setbacks, height limits, and parking exemptions for your exact lot—matching the plan to zoning upfront prevents permit delays later.

Lot Size

San Diego does not impose minimum lot size requirements for ADUs, consistent with California state law. This means nearly all single-family and multifamily lots are eligible for at least one ADU, provided they meet setback and building-code requirements.

Setbacks

California requires ADUs to follow reduced, ADU-specific setbacks:

• 4-foot side and rear setbacks for detached ADUs
• Front setbacks depend on zoning but often use flexible ADU allowances

These reduced setbacks make ADUs feasible even on small or irregular lots (California HCD).

Height Limits

Height allowances vary depending on the ADU type:

• Single-story detached ADUs typically allowed up to 16 feet
• Attached ADUs may follow the height of the primary home
• Some multifamily zones allow taller ADUs depending on overlay zones

San Diego aligns these rules with the state’s mandate to ensure ADUs are not restricted by overly strict height standards.

Parking Rules

Parking requirements for ADUs are significantly reduced under California law:

• No parking required if the ADU is within ½ mile of public transit, part of an existing structure, or located in a historic district
• Detached ADUs not meeting exemptions may require one parking space

These rules expand ADU eligibility and simplify development (California HCD).

Utility Requirements

Utility connections depend on ADU type:

• Garage conversions often reuse existing utilities
• Detached ADUs typically require new or upgraded water, sewer, and electrical connections
• Some lots need panel upgrades or sewer lateral replacements, which can add cost

These requirements must be evaluated early to avoid unexpected expenses.

Fire Sprinklers

Fire sprinklers are not required for ADUs unless the primary residence already has sprinklers, in accordance with California state building code (California HCD).

SB 9 Differences

While ADUs allow homeowners to add units on a single parcel, SB 9 enables the lot to be split and developed with duplexes. Key differences:

• SB 9 = lot split + duplex potential
• ADU laws = adding units without splitting the lot
• SB 9 projects follow different zoning and utility requirements

Homeowners should evaluate whether ADU development or SB 9 offers better long-term value based on lot geometry and goals.

San Diego ADU Local Compliance Checklist

ADU Cost Comparison Table

The table below provides a quick, high-clarity comparison of ADU costs in San Diego based on verified regional pricing. These numbers reflect real construction averages shared by leading San Diego ADU builders – particularly the $375–$600 per sq.ft. construction range and the $150k–$200k garage conversion range.

San Diego ADU Cost Comparison

Conclusion

San Diego homeowners can expect to spend $150,000–$400,000+ on an ADU, depending on whether they choose a garage conversion, attached unit, or detached structure. Most of the budget – 80–90% goes toward hard construction costs, including materials and labor, while the remaining 10–15% covers soft costs like design, engineering, and permitting. Local data shows that San Diego permit and impact fees typically range from $15–$28 per sq.ft., though homeowners building ADUs 750 sq.ft. or smaller pay $0 in impact fees under California law. Overall, San Diego remains one of the most ADU-friendly regions in the state, offering homeowners strong long-term value, rental income potential, and significant property appreciation benefits.

Expect most of your spend to land in the vertical build; pairing realistic hard-cost ranges with a 10% contingency is the safest way to finish on budget.