Published: 26 Oct 2026 · Last verified: 26 Oct 2026 · Next review: Jan 2027 Cost figures are re-checked against permit records quarterly. Changes are logged in Corrections.
Cost per square foot is the most quoted ADU number — and the most misleading one. A contractor who says “$400 a foot” sounds cheaper than one who says “$538 a foot.” In this series’ own models, the $400 figure belongs to the more expensive build.
The problem is not that per-foot math is wrong. It is that it hides which costs scale with area and which do not. Once you see that split, the number becomes useful again — as a secondary check, never as the headline.
The paradox in this series’ own numbers
All figures below are this site’s published Los Angeles County models, not new estimates. The per-foot figures are simple division on those published totals.
| Model | Total | Size | Per sq ft |
|---|---|---|---|
| 800 sq ft detached | $320,000 | 800 | $400 |
| 400 sq ft detached | $215,000 | 400 | $538 |
| Garage conversion (typical 2-car, ~450 sq ft living) | $190,000 | ~450 | ~$422 |
The 400 sq ft unit costs $105,000 less in absolute dollars and $137 more per foot — 34% higher. The garage conversion sits between them on both measures, because it reuses a shell but keeps a full kitchen and bath.
Regional project data across Southern California bends the same way: smaller studios often land near $500–$600 per square foot, while 800–1,000 sq ft units commonly land closer to $375–$450 when site conditions are comparable. A 500 sq ft one-bedroom around $300,000 ($600 per foot) and a 1,000 sq ft unit near $425,000 ($425 per foot) are the same curve, not different markets.
Why the curve bends: fixed costs don’t shrink
The full phase comparison explains it. Hold the 400 and 800 models side by side:
| Phase | 400 sq ft | 800 sq ft | What happens when you halve the size |
|---|---|---|---|
| Soft costs (design, permits, plan check) | $28,000 | $32,000 | Falls only $4,000 |
| Site work + foundation | $26,000 | $38,000 | Falls $12,000 |
| Framing + structure | $26,000 | $48,000 | Falls $22,000 |
| Building envelope | $25,000 | $42,000 | Falls $17,000 |
| Systems + interior | $102,000 | $147,000 | Falls $45,000 — still six figures |
| Landscaping & hardscape | $8,000 | $13,000 | Falls $5,000 |
The phases that shrink are the cheaper ones. The phases that barely move are the expensive ones. A kitchen for 400 sq ft is not half the kitchen for 800. A bathroom is a bathroom. The smallest heat pumps on the market are already oversized for many studio ADUs. Permits, drawings, structural calcs, Title 24 documents, a sewer lateral, and an electrical service run do not come in half sizes.
That is why half the size is not half the price — and why the per-foot number rises as the unit shrinks. See the full breakdowns in the 800 sq ft post and the 400 sq ft post, and the soft costs deep dive for the permit and design lines alone.
Where per-foot pricing is real: the fee side
Some government lines genuinely are priced per square foot — and they behave differently from construction per-foot math:
- School district fees are levied per square foot of ADU, up to the 2026 Level 1 state maximum of $5.38 per sq ft. An 800 sq ft ADU in a district charging the maximum owes $4,304. School fees use their own 500 sq ft threshold, not the 750 line — see ADU school fees, explained.
- Impact fees are sometimes per-square-foot rates (Palo Alto’s parks fee runs $18.65 per sq ft before proportionality), sometimes flat schedules, and sometimes dwelling-base fees prorated by the ADU-to-house ratio. The same ADU can owe $0 in one city and five figures in another at the identical size — see the 750 sq ft cliff, explained.
- Permit and plan-check fees are usually valuation-based or flat ADU schedules, not per-foot rates — which is why soft costs fall only $4,000 between the 400 and 800 models.
So a quote that blends construction per-foot with fee per-foot into one rate is mixing two different rulers. Keep them apart: construction per foot for the build, fee schedules for the jurisdiction. Run your address through the fee calculator for the fee side.
How to use per-square-foot without fooling yourself
- Fix size first, then compare per foot. Per-foot comparisons are only valid between units of similar size, scope, and finish level. Never compare a 400 studio to an 800 two-bedroom on per-foot alone.
- Compare totals, then phases. Two bids at the same per-foot rate can hide different inclusions. Put every bid into the same six-phase ledger and find which line doesn’t match.
- Separate the fee side. Ask every bidder which permit, school, and impact fees are included — and check them against your city’s verified schedule before you sign.
- Watch the cliff, not just the curve. At 750 sq ft, city impact fees switch on or off entirely. A 749 sq ft plan and an 800 sq ft plan differ by 51 feet and, in some cities, by $10,000 or more in fees. Per-foot math smooths over that cliff; the fee schedule does not.
- Remember what per foot leaves out. Contingency, loan interest, first-year tax on the ADU only, and vacancy are real year-one costs that no per-foot construction rate includes. See the all-in cost post for that stack.
The bottom line
Per-square-foot cost is a useful secondary metric and a terrible headline. It rewards bigger units even when the bigger unit costs more cash, and it hides the fixed-cost lines that dominate small builds. Use it to sanity-check bids of the same size. Use totals — construction phases plus your city’s verified fees — to decide what to build.
Put your own quote in these six rows
We made a blank version of the six-phase ledger. Drop your contractor’s numbers into it and see which line doesn’t match ours.
Download the worksheet — CSV (import to Google Sheets), no email required
Want the regional benchmark ranges for each phase, updated quarterly?
Methodology and sources
Every figure on this page is drawn from this site’s published, verified series models — no new contractor survey was conducted for this post. Per-foot figures are arithmetic on those published totals.
| Input | Source |
|---|---|
| 800 sq ft model: $320,000 total, six phases | 800 sq ft ADU cost breakdown |
| 400 sq ft model: $215,000 total, six phases | 400 sq ft ADU cost breakdown |
| Garage conversion model: $190,000, ~450 sq ft living | Garage conversion ADU cost |
| Southern California per-foot curve, regional project data | 400 sq ft post, 400 vs 800 section |
| School fee per-foot maximum, 2026 Level 1 | ADU school fees, explained |
| Impact fee per-foot and flat examples | The 750 sq ft cliff, explained |
| Statewide ADU rules, size limits, and fee exemptions | California HCD ADU Handbook |
| Phase-level cost shares for residential construction | NAHB Cost of Construction Survey |
Check this number against your own address
Most California jurisdictions publish permit records online, including the declared construction valuation for each project. That declared valuation is the closest thing to a real local baseline you can get without paying anyone.
- Open EPIC-LA for unincorporated LA County, or the LADBS Permit & Inspection Report for addresses inside the City of Los Angeles.
- Search recent ADU permits within about a mile of your address.
- Compare the declared valuation to the phase totals on this page.
Declared valuations run below contract price — treat them as a floor, not a total. Full detail on Methodology.
If your local numbers diverge meaningfully from ours, we want to know. See Corrections.
Our position
We don’t build ADUs. We don’t sell plans. We don’t sell your information to contractors. No figure on this site has been paid for or reviewed by anyone whose costs we publish. More on About.
Figures are Los Angeles County, 2026, stated as medians of contractor ranges. Costs vary by jurisdiction, site access, and finish level. This is construction cost information — not a quote, and not legal, engineering, or financial advice. Confirm requirements with your local building department and a licensed California contractor.
Frequently asked questions
What is a good per-square-foot price for an ADU in California?
There is no single good number, because per-square-foot cost falls as the unit grows. In this series’ Los Angeles County models, an 800 sq ft detached ADU runs $320,000 ($400 per foot) and a 400 sq ft unit runs $215,000 ($538 per foot). Compare totals for the same size and scope, not per-foot figures across different sizes.
Why is a smaller ADU more expensive per square foot?
Because permits, utility connections, the kitchen, the bathroom, and much of the mechanical package are close to fixed. Halving the floor area mainly cuts lumber, drywall, flooring, and roofing — the cheaper phases. Absolute cost falls; cost per foot rises.
Does cost per square foot include permits and fees?
It depends on who is quoting. This series’ per-foot figures divide the full six-phase total — including soft costs — by floor area. But some fee lines are priced per foot themselves, like school district fees up to the $5.38 per square foot state maximum, while others are flat or prorated, so a single per-foot rate never captures the fee side.
Should I compare contractor bids per square foot?
Only after you fix size, scope, and inclusions. A bid that excludes permits, utility work, or contingency will look cheaper per foot and cost more in total. Put every bid into the same six-phase ledger before comparing any per-foot number.
What is the cheapest per-square-foot ADU size?
In these models, bigger is cheaper per foot up to the point where lot, setbacks, or local rules stop you — but the total keeps rising. The 800 sq ft model costs $105,000 more than the 400 in absolute dollars. Per-foot efficiency is not the same as affordability.