How Much Does an ADU Cost? Averages and What Drives Price

An ADU, or accessory dwelling unit, is a small, self-contained home on the same lot as a main house. It can be a backyard cottage, a converted garage, or an attached addition with its own kitchen, bathroom, and entrance. How much does an ADU cost depends mostly on type and size, but the national average is about $180,000, within a full range of roughly $40,000 to $360,000. Most guides put new construction at $150 to $300 per square foot, and small units often run higher.

That range is too wide to budget from, so this article narrows it. You will see why the number matters before you hire anyone, how to build your own estimate from square footage, what each ADU type costs relative to the others, which factors push the price up or down, and where you can save.

We finish the walls and ceilings of ADUs across the Sacramento Valley, so we see the last stretch of the budget on real jobs. That view shapes the advice below, especially the parts about sequencing and interior finishing.

Why your ADU budget needs a real number first

You need a real ADU cost number early because the price swings by six figures on choices you make before construction starts, and the money usually comes from your own savings or a loan against your home. A rough guess can leave you short with half a building standing.

What the national average hides

The average of about $180,000 sits inside a $40,000 to $360,000 range , and the gap between those ends is the real story. A garage conversion with plumbing nearby sits at the low end. A large detached unit with new utility trenches and high-end finishes sits at the top. Your project lands somewhere specific, and the average says little about where.

Size matters less than you would expect. The expensive parts of an ADU are fixed costs : foundation or structural work, utility connections, the kitchen, and the bathroom. A 400 square foot unit needs nearly all of them, which is why small ADUs cost more per square foot than large ones. Adding another 100 square feet mostly adds framing, drywall, and flooring.

The drawbacks worth budgeting for

The biggest drawback of an ADU is the high up-front price for a small amount of space. Financing is also less straightforward than buying a home, so owners commonly combine savings with a home equity line of credit or a renovation loan.

Time is the other cost. Design, permitting, and construction often stretch across many months, and payback from rent alone commonly takes many years. Housing a relative or adding long-term property value are sound reasons to build. Just match the budget to the reason.

How to estimate what your own ADU will cost

To estimate your ADU cost, multiply the square footage by a per-square-foot rate, then add design, permits, site work, and a contingency. The method takes about ten minutes and gives you a range you can test against real bids.

The estimate in six steps

1. Pick the type and size. A one-bedroom ADU usually falls between 500 and 700 square feet.
2. Choose a rate. Use $150 for a lean build, $250 for a mid-range build, and $350 for higher finishes or costly sites.
3. Multiply the size by the rate to get the construction cost.
4. Add 15 to 30 percent for design, permits, fees, utility connections, and site prep.
5. Add 10 percent contingency on the new total for surprises behind walls and under soil.
6. Compare the result with at least three itemized written bids.

Some guides quote all-in rates and others quote construction only, so ask every bidder what their number includes.

Construction cost by size

This table applies the three rates to the sizes people search for most. It shows construction cost only, before the soft costs in step 4.

Small units usually land at the higher rate and larger ones at the lower rate, because fixed costs spread over more area. Many cities and counties cap ADU size, often between 800 and 1,200 square feet, so confirm what your lot allows before pricing the bottom rows.

A worked example

Take a 600 square foot detached ADU at the mid rate of $250. Construction comes to $150,000 . Add 20 percent for design, permits, and site work, which is $30,000, for a subtotal of $180,000. A 10 percent contingency adds $18,000. Your planning budget is $198,000 .

ADU cost by type, from cheapest to most expensive

Converting space you already own is the cheapest way to get an ADU, and a garage conversion is the usual starting point. Attached additions come next, and detached new builds cost the most because they need everything from scratch.

What each type costs relative to the others

A garage conversion at the low end of the $40,000 to $360,000 range avoids a new foundation, roof, and framing. The catch is that you need a garage with enough usable square footage, and it has to meet habitability rules for light, ventilation, and exits.

Is prefab cheaper than building on site?

Prefab is not automatically cheaper. The factory price covers the unit, but site prep, foundation, delivery, crane time, and utility hookups are still your cost, and they can add tens of thousands of dollars. The advantage is speed and price certainty, because a factory quote has fewer change orders than a site-built job.

Compare an all-in prefab quote against a general contractor bid, never the sticker price against the bid. If a unit costs less once both include foundation and hookups, prefab wins. If it does not, build on site and keep the design flexible.

What drives an ADU's cost up or down

The factors that move an ADU price most are site conditions, utility connections, design complexity, finish level, and local fees. Size matters, but these five decide where inside the range you land.

Site and utilities

A flat, accessible lot with sewer, water, and power close to the planned unit keeps costs down. A sloped lot, a long utility trench, or a main electrical panel that needs an upgrade can add thousands. Properties on septic may need a larger tank or an added drain field. Ask your contractor to walk the lot and price these items separately.

Foundation work is the other big site variable. Slab-on-grade is the cheapest option, and poor soil or drainage problems push you toward more engineering. These costs are unavoidable once you build new, which is one more reason conversions stay cheaper.

Design, finishes, and local fees

Complex rooflines, many corners, large window walls, and custom cabinets all cost more. Putting the kitchen and bathroom back to back on a shared plumbing wall costs less than separating them. Finish level is the fastest lever you control, because flooring, cabinets, counters, and fixtures can swing the total by tens of thousands.

Fees depend on where you build. In California, state law bars impact fees on ADUs under 750 square feet, which helps smaller Sacramento Valley projects. Permit fees, plan review time, and local design rules still vary by city and county, so call your planning department before you design.

How to lower your ADU cost and plan the interior finish

You lower ADU costs by keeping the design simple, reusing existing structure and utilities, and collecting itemized bids. Then you plan the interior finish early, because walls and ceilings touch every room and fall late in the schedule.

Practical ways to save

• Convert before you build. A garage or existing space avoids the most expensive structural work.
• Keep the footprint simple. A rectangle costs less to frame and roof than a shape with many corners.
• Stack the wet rooms. Share a plumbing wall between kitchen and bathroom.
• Spend on what lasts. Put money into windows, insulation, and flooring, and save on easily replaced fixtures.
• Hire out the technical trades. Structure, electrical, and plumbing belong to licensed pros, while painting and landscaping are fair DIY work.
• Verify licenses and get three bids. Compare scope line by line, not just totals.

Budgeting drywall and ceilings

Drywall is a smaller line item than the structure, but it sets the quality of every room. The work covers hanging, taping, mudding, sanding, and texture, and the price follows wall and ceiling area , ceiling height, corner count, and your choice of smooth or textured finish. It happens after framing, rough-in, and insulation inspections pass, so a failed inspection delays it.

Our ADU drywall service delivers smooth, ready-to-paint surfaces, and we give free in-home estimates around Sacramento and nearby cities. With over 30 years in business, we also don't ask for payment until the job is complete. Ask us for a scope, then check that any bid names the texture, number of coats, and cleanup.

Plan your ADU budget with real numbers

The answer to how much an ADU costs is a range, and your job is to narrow it. Start with the type, multiply by a realistic per-square-foot rate, add 15 to 30 percent for soft costs, and hold back 10 percent for surprises. Then replace estimates with three itemized bids.

When the framing is up and inspections are passed, the interior finish is next. If you are planning that stage, our crew can quote drywall installation for your ADU with a free in-home estimate.