Ask five contractors for an ADU price and you'll get five different numbers. Here's the real math behind the range — and how to tell a legitimate quote from one that's missing scope.
An Accessory Dwelling Unit is one of the highest-leverage investments a homeowner in Greater Los Angeles or Orange County can make — but it's also one of the most inconsistently quoted. Search online and you'll see numbers anywhere from $80,000 to $500,000 for what sounds like the same project. The truth is that ADU cost is genuinely a range, driven by a small number of specific factors — and once you understand them, you can tell in minutes whether a quote you've received is realistic or missing something.
| Typical Range | |
|---|---|
| Starting investment | $150,000 |
| Cost per square foot | $250–$400 |
| Property value added | $150,000–$300,000 |
| Monthly rental income range | $1,500–$2,500 |
A detached ADU with TrioTimber starts at $150,000, and typically runs $250–$400 per square foot depending on plumbing layout, finishes, and site conditions. If your budget is well under that, we'll tell you honestly rather than value-engineer your project into something we're not proud of — and if you're already doing the math on those two numbers, the next section explains exactly how they reconcile.
We believe in radical transparency about pricing, so let's do the math you're probably already doing. If ADU construction runs $250–$400 per square foot, a 400 square foot starter studio at the low end of that range would come out to $100,000 — well under our $150,000 project floor. So which number is real?
Both are — because the relationship between size and price per square foot isn't linear. Every detached ADU, regardless of size, needs the same Fixed Mechanical Core before a single piece of drywall goes up: site excavation, a structural concrete foundation, trenching and running separate utility connections to the primary house, a full kitchen, and a full bathroom. Those systems cost roughly the same to install whether the unit is 400 square feet or 1,200 square feet.
Slab foundations are the baseline. Sloped lots, poor soil, or a site that needs significant grading, retaining work, or utility trenching adds cost before a single wall goes up.
A kitchen and a full bathroom are the single biggest cost drivers in any ADU. A studio layout with an efficient, compact wet wall costs meaningfully less than a spread-out one-bedroom plan with plumbing on opposite sides of the unit.
Whether your existing panel has capacity for a new, separately-metered ADU circuit — or needs a full panel upgrade — is a real cost variable most online calculators don't account for.
Builder-grade finishes versus the kind of custom cabinetry and millwork TrioTimber is known for is the widest swing in the budget — and the one homeowners have the most direct control over.
Setback rules, FAR limits, and plan check timelines vary by city across Greater LA and Orange County. This mostly affects timeline rather than hard construction cost, but city-required upgrades (like fire sprinklers past a certain size) do affect the number.
An ADU is one of the few home improvements that pays for a meaningful share of itself. Across Greater Los Angeles and Orange County, a well-built ADU typically adds $150,000–$300,000 to a property's value, while generating $1,500–$2,500 per month in rental income depending on unit size, finish level, and neighborhood.
That combination — meaningful equity gain plus ongoing rental income — is why ADUs have become one of the most requested projects we build, and why understanding the real cost math matters before you commit to a number that sounds good but doesn't hold up once construction starts.
We'd rather tell you the real number up front, in writing, than win the bid with a low estimate that turns into surprise costs three weeks into construction.
A detached ADU built by TrioTimber starts at $150,000, and typically runs $250–$400 per square foot. Those two numbers can look like they don't match on a small unit — a 400 sq ft studio at $250/sq ft would be $100,000, under the floor — but every detached ADU needs the same fixed core systems (foundation, utility trenching, a full kitchen, a full bathroom) regardless of size. On a 400 sq ft studio that core works out to roughly $375 per square foot to reach the $150,000 floor; on larger one-bedroom+ units, that same fixed cost spreads over more floor area and blends down toward $250 per square foot.
Because every detached ADU needs the same Fixed Mechanical Core — site excavation, a structural concrete foundation, utility trenching to the primary house, a full kitchen, and a full bathroom — whether the unit is 400 or 1,200 square feet. On a compact studio, that core is concentrated into a small footprint, working out to roughly $375 per square foot to reach the $150,000 floor. On a larger unit, you're mostly adding dry living space — framing and finishes for bedrooms — which costs far less per foot, blending the average down toward $250 per square foot.
Across Greater Los Angeles and Orange County, an ADU typically adds $150,000–$300,000 in property value, while generating $1,500–$2,500 per month in rental income.
Be cautious of it. A genuine detached ADU includes a foundation, full kitchen, full bathroom, and its own electrical service — the same core systems as a small house. A number well under $150,000 usually signals a scope gap that surfaces later as change orders, not real savings.
Get a free feasibility review. Tell us about your property and a member of our team will walk you through what's realistic for your site, your city's permit process, and a genuine cost range — in writing.
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