ADU Construction Cost by Size: 400, 700, and 1,200 Square Feet Compared

ADU construction cost in California runs roughly $150 to $600 per square foot, putting a finished 400-square-foot studio near $180,000 and a 1,200-square-foot three-bedroom near $450,000.

The spread is wide because two units of the same size sit on very different lots, with different soil, panel capacity, and city fees.

Size is the first lever, but it is rarely the one that blows a budget.

This guide breaks the number down by unit type, by square footage, and by line item, with every figure traced to a published source.

For a scoped estimate on your own lot, start with a feasibility visit from a team that handles ADU construction cost planning and permitting under one roof.

What You Are Actually Paying For

An accessory dwelling unit is a self-contained second home on a lot that already has a house, with its own kitchen, bathroom, and entrance.

Accessory dwelling units come in three shapes: a converted garage, an attached wing, and a freestanding backyard cottage.

Construction cost covers labor and material costs for the structure itself.

Total ADU cost adds design, engineering, permit costs, utility connections, project management, and contingency, and it is the number that matters.

A builder quoting only the vertical build is quoting 75 to 90 percent of what you will actually pay.

Every ADU project runs through five stages: feasibility and design, construction documents, plan check and permits, site work and shell, then rough-ins and finishes.

Price Ranges by ADU Type and Unit Size

Garage conversions are the cheapest path, detached new construction ADUs are the most expensive, and attached units land between them.

Published 2026 pricing from Los Angeles ADU builders puts garage conversions at $140,000 to $245,000 and detached units at $219,000 to $459,000, while San Diego builder data puts turnkey detached builds at $375 to $600 or more per square foot.

Here is the part that surprises many homeowners: smaller units cost more per square foot, not less.

A kitchen, bathroom, water heater, panel, and sewer connection cost nearly the same in a 400-square-foot studio as in a 1,200-square-foot three-bedroom, so those fixed costs spread across fewer square feet.

A studio layout can pencil at $520 per square foot while a two-bedroom ADU on the same street pencils at $390.

Garage Conversion Numbers, Start to Finish

A garage conversion costs $140,000 to $245,000 or more in Los Angeles, based on published 2026 builder price lists.

The existing structure already has a slab, walls, and a roof, so you skip foundation costs and most of the shell.

What you do not skip is everything that makes a garage habitable.

The real cost drivers are slab condition, ceiling height, insulation and egress windows, a full plumbing run for the new bathroom and kitchen, and bringing the existing garage up to current local building codes.

Older slabs often fail moisture and level requirements, and cutting one open for drain lines adds thousands.

Average per square foot for conversions lands around $250 to $400, so the cost savings are closer to 30 percent than the 60 percent people expect.

What a Detached Backyard Unit Runs

Detached ADUs cost $219,000 to $459,000 in Los Angeles and commonly $300,000 to $450,000 in San Diego for a complete build.

You are paying for a full foundation, shell, and roof, plus a longer trench back to the main house.

Detached units carry the strongest effect on resale value, because appraisers and buyers treat a private, freestanding second home differently than a converted garage.

A separate entrance, no shared walls, and its own yard space make the unit rentable at a higher rate and easier to compare against small homes nearby.

Going two stories halves the foundation and roof area for the same living space, but stairs, a second-floor structural system, and fire separation usually eat the difference.

Expect a two-story unit to cost about the same as a single-story unit of equal square footage, with the payoff being lot coverage rather than dollars.

Running the Math on 400, 600, and 900 Square Feet

Cost per square foot is useful for comparing bids and useless for setting a budget.

The metric hides fixed costs, ignores site conditions, and shifts depending on whether a quote includes soft costs, so two builders can both say $400 per square foot and be $80,000 apart.

Here is the arithmetic at low, mid, and high rates for the vertical build, before soft costs and utilities.

The low column reflects a simple garage conversion with a sound existing slab.

The middle column is the realistic center of the market for a standard detached build with mid-grade finishes.

The high column reflects difficult access, a sloped lot, custom cabinets, and upgraded systems.

Soft Costs, Hard Costs, and the Money People Forget

Soft costs are everything that is not lumber and labor, and they claim a significant portion of the budget.

The share is genuinely contested: one San Diego builder reports plans and permits at about 6 percent of project cost, while a contractor’s published breakdown of a real build shows $56,250 in design, planning, permits, setup, and project management against a roughly $393,000 total, or about 14 percent.

The gap is a definitions problem, since the low figure counts drawings and permit fees only while the higher one folds in project management.

Budget 10 to 20 percent for soft costs, which cover design, structural engineering, Title 24 compliance, plan check and building permits, surveys, inspections, and the contractor’s management fee.

Hard Costs Line by Line

• Site work and foundation costs: excavation, grading, forms, and slab, often $25,000 to $50,000
• Rough carpentry: walls, roof structure, sheathing, and windows
• Mechanical, electrical, and plumbing: HVAC, panel and circuits, supply and drain lines, commonly $30,000 to $60,000 combined
• Insulation, drywall, stucco or siding, roofing, and paint
• Interior finishes: flooring, tile, cabinetry, countertops, light fixtures, and cabinet hardware
• Appliances: range, refrigerator, dishwasher, laundry, and water heater, usually $6,000 to $15,000

Finish level is the lever you control most easily.

Stock cabinets and mid-grade tile versus custom cabinets and stone can swing a small unit by $25,000 without changing a single structural decision.

Structural Engineering Fees

Structural engineering is required whenever you build new, add a second story, cut into an existing structure, or change a load path, which covers nearly every California ADU because seismic rules apply to conversions too.

Published fees vary by scope: one contractor lists $2,915 for engineering on one project, while an ADU builder bundles design and structural engineering at $12,500 for a 550-square-foot model.

That spread reflects bundling, not different rates, so confirm what a quoted fee actually covers.

Get any required seismic bracing, shear wall, or garage retrofit work documented in the construction documents before you sign, because scope discovered mid-build is the most common source of unexpected costs.

Utility Upgrades and Trenching

Utility upgrades run $10,000 to $30,000 on most projects, and San Diego builders report $15,000 to $30,000 or more depending on property specifics.

Common triggers are a panel that cannot carry a second dwelling, an undersized or aging sewer lateral, a water meter sized only for the main house, and gas capacity.

Trenching distance is the sleeper cost, since many builders include the first 75 linear feet of electrical, water, and sewer trenching and bill by the foot beyond that.

A cottage at the back of a deep lot can add $8,000 to $15,000 in trenching alone versus the same unit placed near the house.

Call your electric, water, and sewer providers during design, because meter set fees and connection scheduling routinely add weeks.

Site Conditions and Local Rules That Move the Number

Local regulations influence ADU construction cost as much as finish level does, and two identical plan sets can differ by $20,000 in fees across a city line.

Before design starts, walk the site for how a concrete truck and lumber delivery will reach the build area, how much the lot slopes, and where the existing sewer and panel sit.

Tight side yards that force wheelbarrow work instead of machine access raise labor hours on every trade.

Then check the paper constraints, because protected trees, recorded easements, setbacks, and lot coverage limits can shrink or relocate a footprint after drawings are already paid for.

Permits and City Fees

Permit fees run about $3,000 to $16,000 in Los Angeles and roughly $13 to $28 per square foot in the City of San Diego, which is $5,200 to $11,200 on a 400-square-foot unit.

Sources disagree for a knowable reason: one builder quotes $3,000 to $6,000 for LA city permits, while another reports $5,000 to $15,000 or more, because the higher figure includes school fees, plan check revisions, and connection charges that the lower one treats separately.

Neighboring cities are not comparable either, since published builder data puts Encinitas at $2 to $4 per square foot against San Diego’s $13 to $28.

Ask your building department for a written plan check timeline, because state law requires agencies to act on a complete ADU application within 60 days, but the clock starts only once the application is deemed complete, and most owners see three to six months from submittal to permit.

Confirm your impact fee waiver in writing.

Under California Government Code section 66324, subdivision (c)(1), an ADU with 750 square feet or less of interior livable space is exempt from impact fees charged by local agencies, special districts, and water corporations, and larger units must be charged proportionally to the primary dwelling.

SB 543 renumbered that provision as section 66311.5, so a fee schedule citing the old section may predate the change.

That single rule is why a 740 square foot two-bedroom ADU often costs less to permit than a 780 square foot version of the same design.

Panels, Sewer Laterals, and Meters

An electrical panel upgrade is triggered when the existing property’s service cannot carry the new load, which usually means anything under 200 amps once you add a second kitchen, HVAC, and an EV charger.

A service upgrade generally runs $3,000 to $8,000, more if the utility pulls new wire from the street.

Sewer lateral replacement is triggered by age, undersized pipe, or a failed camera inspection, and typically costs $5,000 to $15,000 depending on trench length.

A separate meter is optional for most attached units and often required for a detached rental, and meter set fees plus a service drop add $2,000 to $10,000.

Paying for It: Loans, Equity, and Cash Flow

Most homeowners fund an ADU with one of these tools, and the right one depends on your equity and your plans for the unit.

• Construction loans: funds released in draws as work is inspected, underwritten against completed value
• HELOC: a revolving line against existing equity, drawn as needed
• Home equity loans: a lump sum at a fixed rate
• Cash-out refinance: replaces the first mortgage, worth it only if current rates beat yours
• Renovation mortgages such as FHA 203(k), which finance purchase plus construction together

A HELOC is the fastest and cheapest to open, with low or no closing costs, interest charged only on what you draw, and no draw inspections slowing the build.

The trade-offs are real, because HELOC rates are usually variable, the line depends on equity you already have rather than finished value, and payments can jump if rates move during a long build.

Construction loans solve the equity problem by underwriting against appraised value after completion, which is how owners with modest equity get built.

To qualify, plan and gather a contractor bid with a line-item cost breakdown, stamped plans, license and insurance, two years of tax returns, and a draw schedule.

Resale Value, Rent, and Room for Family

Resale value improves because you are adding permitted, livable square footage with its own kitchen and bath, which appraisers compare against small homes and condos nearby.

Increased property value from a permitted unit tracks a meaningful share of build cost rather than a fixed percentage, and unpermitted work adds far less because it cannot be counted as living space.

The rental income math is arithmetic worth running yourself.

Take a hypothetical 700-square-foot two-bedroom that costs $320,000 all in and rents for $2,600 a month.

Gross annual rent is $31,200, and after roughly 25 percent for vacancy, maintenance, insurance, and management, you keep about $23,400.

That is a 7.3 percent annual return on build cost, before appreciation and before any resale uplift.

Raise the rent to $3,000, and the return moves to 8.4 percent, which shows the outcome depends on local rent more than on your build cost.

Multigenerational living is the other reason people build, and it shows up in no spreadsheet.

A private unit lets aging parents or adult children live independently on the same lot at a fraction of assisted living costs, and families often convert the unit to a rental years later, which is why flexible layouts and energy efficiency upgrades pay off twice.

Seven Ways to Bring the Budget Down

1. Convert instead of building new, since an existing garage skips the foundation and shell.
2. Price a prefab ADU against site-built. Factory units compress the schedule and stabilize labor rates, but craning and site prep can erase the savings on tight or sloped lots.
3. Use pre-approved plans, because many California cities keep permit-ready plan sets that cut design fees and shorten the permitting process.
4. Stay at or under 750 square feet to keep the impact fee exemption.
5. Place the unit close to the main house, since every trench foot avoided is a direct saving on utility connections.
6. Pick mid-grade finishes, which deliver most of the look for a fraction of the cost.
7. Hold a 10 to 20 percent contingency, which on a $300,000 build is $30,000 to $60,000 reserved for unforeseen costs.

Get three bids from licensed general contractors who work with consistent trade partners, then compare line items rather than bottom lines.

California Snapshot: Los Angeles and San Diego Compared

Both metros sit well above broader averages.

National guides put typical ADU cost at $150 to $300 per square foot, with complex projects in expensive areas exceeding $600. A national cost survey found totals from $112,000 to $285,000, with an average of $181,000.

California coastal metros land in the top third of that distribution because of labor rates, seismic engineering, Title 24 requirements, and fee schedules.

On recent code changes, confirm your city’s fee schedule reflects the renumbering of section 66324 to section 66311.5 before you accept a quoted fee.

Three Sample Budgets

These are planning budgets for standard Southern California lots with reasonable access, built from the builder pricing cited above.

400 sq ft studio, detached: construction $150,000, soft costs and permits $28,000, utilities and trenching $14,000, contingency at 12 percent $23,000, total about $215,000.

600 sq ft one-bedroom, detached: construction $200,000, soft costs and permits $34,000, utilities and trenching $16,000, contingency at 12 percent $30,000, total about $280,000.

900 sq ft two-bedroom, detached: construction $270,000, soft costs and permits $42,000, utilities and trenching $18,000, contingency at 12 percent $40,000, total about $370,000.

Notice what changes between the 400 and the 900.

Construction rises 80 percent while utilities rise only 29 percent, because the trench, the meter, and the connection are close to fixed regardless of unit size.

That is the clearest argument for building the largest unit your lot, your total budget, and the 750 square foot fee threshold will support.

For more on how local rules shape what you can build, see the guide to Los Angeles ADU requirements. If you are weighing a second unit against expanding the house you have, compare it with planning a room addition. You can also see how the build reshapes your backyard and outdoor space.

The California Department of Housing and Community Development publishes a free ADU Handbook covering state law, fees, and approval timelines.

Your Next Three Moves

Start with a site feasibility visit before you pay for a single drawing.

An hour on the lot establishes access, slope, panel capacity, sewer location, setbacks, and tree or easement conflicts, which is what determines whether your budget is realistic.

Next, collect three written bids from licensed general contractors, each with a line-item cost breakdown covering site work, shell, MEP, finishes, and soft costs separately.

Bids that arrive as a single number cannot be compared and usually hide the items that become change orders.

Finally, build your ADU budget with soft costs and contingency included from day one.

Take your expected construction cost, add 15 percent for soft costs, add your utility allowance, then add 10 to 20 percent contingency on the whole thing.

That figure is your real number, and homeowners who start there finish on budget far more often than those who start from a per square foot estimate.

Frequently Asked Questions

How much would it cost to build a 600 square foot ADU?

A 600 square foot ADU costs roughly $240,000 to $310,000 all in across Southern California in 2026. Detached builds at that size price near $400 to $500 per square foot for construction, then add soft costs, permits, and utility connections.

A garage conversion of similar size runs closer to $170,000 to $230,000, because the slab and shell already exist. Site conditions, trenching distance, and finish level move the final cost more than square footage does at this size.

Is it cheaper to build an ADU or buy a prefab?

Prefab ADUs are often cheaper on unit price but not always on total project cost. The factory price excludes foundation, site work, crane delivery, utility connections, permits, and on-site finish work, which together can add $80,000 to $150,000.

Prefab wins on lots with clear crane access, level ground, and simple soil, where the schedule compresses and labor rates stay predictable. Site-built wins on tight, sloped, or hard-to-reach lots, so price both before deciding.

How much value does a 1,000 square foot ADU add to a property?

A 1,000 square foot ADU adds value through permitted living space, rental income, and comparable sales, though the uplift varies by market rather than following a fixed percentage.

Appraisers value permitted units against small homes and condos nearby, so local rents and sale comps drive the number.

Unpermitted conversions add far less because they cannot be counted as legal square footage. Keep permits and final inspection records, since documentation converts construction spending into appraised property value.

Can a homeowner pull permits for an ADU?

Homeowners can pull permits for an ADU in California under an owner-builder permit if they own and occupy the property. Doing so makes you legally responsible for code compliance, worker safety, and any unlicensed labor on site, and it voids some contractor protections.

Most lenders and insurers prefer permits pulled by a licensed general contractor. Owner-builder permits also leave you managing plan check corrections, inspections, and trade scheduling, which is where owners lose more in delays than they save in fees.

What is one drawback of an ADU?

One drawback of an ADU is the permanent loss of yard, driveway, or garage space the unit occupies. A detached cottage typically consumes 400 to 1,200 square feet of usable backyard plus setback clearance, which is why the remaining yard space is usually redesigned in the same project, and a garage conversion removes covered parking and storage.

Other trade-offs include higher property taxes on the added improvement, reduced privacy, and landlord responsibilities if you rent. These are manageable, but weigh them against rental income and resale value before committing.

What is the biggest size ADU you can build?

The biggest ADU you can build in California is generally 1,200 square feet for a detached unit, though local ordinances set the specific limit and some cities cap attached units at 50 percent of the main house.

State law prevents cities from limiting detached ADUs below 800 square feet for a unit with up to two bedrooms. Staying at or under 750 square feet keeps your impact fee exemption, so the largest allowable unit is not always the cheapest to permit.

Talk to a Local ADU Builder

Master in Design has built ADUs, room additions, and full home remodels across the San Fernando Valley since 2012, with design, permits, structural, and construction handled by one in-house team.

Call (855) 741-3167, email bymasterindesign@gmail.com, or visit 6011 Reseda Blvd, Suite 1, Tarzana, CA 91356.