A California ADU typically runs $200,000–$400,000 all-in in 2026, with garage conversions at the low end ($120K–$220K) and detached new construction at the high end ($300K–$550K per square foot). Cost varies by type, size, city, and lot conditions — and the wide bid spread you see from contractors (the same project quoted $150K and $350K by different builders) is not random; it reflects which builders include the hidden cost categories up front and which absorb them as change orders later. This guide gives you the field-tracked numbers, the categories contractors omit, and the red flags to catch before you sign. Sometimes the right answer is not to build, and we say that clearly before any money moves.
Every cost band you see online is a starting point. Your actual cost is determined by the interaction of four variables.
The type of ADU sets the base cost band before lot conditions and finishes come in.
Size is less linear than most homeowners expect. The fixed costs — design, permit, kitchen and bath rough-in, utility connections — are nearly identical at 400, 600, or 800 sqft. The marginal cost per added square foot is mostly framing, finishes, and a slightly larger HVAC system.
That's why 600 sqft has the best per-sqft efficiency in California, and why most homeowners default to it.
State law preempts local size rules up to 800 sqft for ADUs (Gov Code §65852.2). Above 800 sqft, local rules apply and the project gets harder to permit.
Cost breakdown — 600 sqft ADU →Three city-level factors move the cost band:
The biggest single source of cost variance is what's already on (and under) your lot.
The lot-specific answer is what the $199 Feasibility & Risk Assessment is built to produce.
$199 Feasibility & Risk Assessment →| ADU Type | Per-sqft (typical) | Typical all-in (CA) | Premium cases |
|---|---|---|---|
| JADU (Junior ADU) | $200–$350/sqft | $80K–$160K | up to $200K with full kitchen |
| Garage conversion | $250–$450/sqft | $120K–$220K | up to $280K with structural retrofit |
| Attached ADU | $300–$500/sqft | $200K–$320K | up to $400K with premium finishes |
| Backyard detached | $300–$550/sqft | $240K–$380K | $400K+ on hillside or HPOZ |
| Prefab / modular | $200–$400/sqft delivered | $180K–$320K (factory $80K–$150K + site $100K–$170K) | up to $370K with full site work |
| Size | Per-sqft (typical detached) | Typical all-in (detached) |
|---|---|---|
| 400 sqft | $350–$600/sqft | $140K–$240K |
| 500 sqft | $320–$570/sqft | $160K–$285K |
| 600 sqft | $300–$550/sqft | $180K–$330K |
| 700 sqft | $290–$520/sqft | $200K–$365K |
| 800 sqft | $280–$500/sqft | $225K–$400K |
| 1,000 sqft | $270–$480/sqft | $270K–$480K |
| 1,200 sqft (state ceiling) | $260–$460/sqft | $310K–$550K |
600 sqft is the most-searched and most-built footprint in California. Garage conversion and prefab run 15–30% below the detached bands above. → 600 sqft full cost breakdown · ADU sizes overview
| Metro | Per-sqft (new detached) | Typical 600 sqft all-in |
|---|---|---|
| Los Angeles | $300–$550/sqft | $200K–$340K |
| San Diego | $280–$500/sqft | $180K–$300K |
| San Francisco / Bay Area | $400–$700/sqft | $300K–$450K |
| San Jose / Silicon Valley | $380–$620/sqft | $250K–$380K |
| Orange County | $310–$540/sqft | $230K–$370K |
The single biggest source of homeowner anger in the ADU process is the gap between the signed bid and the final invoice. Field tracking via the InspectPilot project database (11M California construction inspection records, since 2013) consistently surfaces six categories that drive most of that gap.
Required when the new ADU adds plumbing fixtures and the existing lateral is undersized, broken, or built in clay or cast iron. Common across LA, OC, and SF where pre-1960 stock dominates. Higher in cases needing a street cut.
Slopes above ~15%, hillside ordinance areas, expansive soils, or poor soil class drive geotech reports ($2.5K–$5K), grading permits ($1.5K–$4K), engineered foundation upgrades ($20K–$60K vs. slab), and slope stability analysis ($3K–$8K).
Garage conversions on pre-1970 stock frequently require framing reinforcement, foundation work, and code upgrades. Most initial bids assume the existing structure is sound. Field experience says otherwise about a third of the time.
ADUs adding bathrooms or laundry often trigger a water-meter upsize. Cost varies sharply by city; LADWP, San Diego Water, and SFPUC have different fee structures.
Older homes with 100A or 125A panels need an upgrade to 200A to handle the ADU load. Cheaper if bundled into the ADU electrical scope; more expensive if it comes up mid-project.
Permits, design, soils report, financing fees, builder's risk insurance, and contingency. Often presented as “you'll figure this out later” rather than a line item in the contractor bid. They're not optional.
Run the Cost Calculator — it includes the hidden categories above as toggles, so the number you see is closer to the all-in cost than what you'll get from a contractor's first quote.
Run the Cost Calculator →A clean bid for a California ADU has four characteristics. If any are missing, the bid is incomplete and the final number will be higher than the headline.
The 2026 California floor for legitimate new detached ADU construction is roughly $250/sqft. Anything quoted below $200/sqft is either missing major scope (utilities, permit fees, finishes) or assumes change orders will close the gap. This is the most reliable single red flag.
A clean bid carries a contingency of 5–10% of construction cost. If there's no contingency line, you are the contingency.
“Cabinet allowance $5,000” with no spec sheet means “we'll use whatever's cheapest, and if you want something specific you'll pay the difference as a change order.” A clean bid carries finish specs by brand and product, not a dollar placeholder.
A bid that reads “$245,000 for a 600 sqft ADU, turnkey” tells you nothing about what's included. A clean bid breaks down design, permit, site work, construction, utilities, finishes, and soft costs as separate line items.
Lump-sum bids are not comparable. Itemized bids let you see which builder is pricing the hidden categories and which is omitting them.
Bids that fall above or below the published California range need a written reason. “We're cheap and high-quality” is rarely a reason. “We're cheap because we cut corners” is.
The general cost band is a starting point. The lot-specific band is the answer. $199 written report — the one upfront payment before the managed build, which costs you nothing extra.
ADUscale is paid by the contractor out of his existing customer-acquisition budget, so the homeowner pays the same price as going direct. The verified-bid review and change-order discipline are built in, at no extra cost to you.
ADUscale is a California build-side ADU partner: we help homeowners secure one of the state's top contractors, expand that contractor's capacity to take the project, and protect the budget with inspection-gated milestone payments — at the same price as going direct. Cost data on this page is calibrated from industry cost-benchmark data, California HCD ADU resources, LADBS permit records, CSLB complaint and bond data, and the InspectPilot inspection database (filtered to California ADU projects, 11M records since 2013). Permit-volume context anchored to the CA YIMBY ADU Reform Retrospective and UC Berkeley Terner Center ADU research. ADUscale is paid by the contractor — the homeowner pays the same price as going direct.
Last updated: June 2026.
The answer is the lot-specific band, calibrated against your city, your site conditions, the hidden categories, and the bid spread you're looking at. The free Reality Check gives you the first read in two minutes. The $199 Feasibility & Risk Assessment gives you the written report that catches the hidden categories before any contract is signed. And sometimes, after the analysis, the right answer is not to build on this lot, with this budget, in this market — we say so clearly when it applies, before any money moves.
