ADU Cost Estimator — guide & how to use
Planning-stage budget range — not a quote, appraisal, or guarantee of financing. Actual costs vary widely by site conditions, contractor, and local labor; only a site visit and contractor quote give a firm price. The tool deliberately outputs a range and a contingency reserve for this reason. Confirm all figures with a licensed contractor, designer, and your local building department before committing funds. All math runs privately in your browser.
What it does
This estimator turns your ADU type, square footage,
finish level, and region into a low–high all-in cost range
with an itemized breakdown — hard construction, foundation, utilities, soft costs (design &
permits), impact fees, and a contingency reserve. The result is always a range,
because construction reliably surprises.
How to use it
- Pick your ADU type — the single biggest cost driver.
- Enter the conditioned square footage (typical 400–1,200 sq ft).
- Choose a finish level and your region.
- For a garage conversion or attached unit, toggle new foundation and
new utilities if you need them (these are auto-included in detached/prefab rates).
- Keep soft costs on and read the all-in range, the $/sq ft, and the breakdown.
How the math works
- Base hard-cost rate per sq ft (national mid-finish midpoint): garage conversion $225, attached $275, detached $350, prefab $325.
- Finish multiplier: Basic ×0.80, Mid-range ×1.00, High-end ×1.35.
- Region multiplier: West Coast ×1.45, National ×1.00, Lower-cost ×0.82.
- Adjusted rate = base × finish × region; hard-cost base = adjusted rate × sq ft.
- Foundation adder (garage/attached only, if checked) = $18 × sq ft × region.
- Utility adder (garage/attached only, if checked) = $12,000 + $28,000 if sewer/septic, × region.
- Detached & prefab base rates are all-in — foundation & utility adders are set to $0 to avoid double-counting.
- Soft costs (if included) = 20% of hard-cost total, with a $12,000 floor; impact fee of $8,000 × region added when sq ft > 750.
- Contingency = (hard + soft + impact) × your % (default 15%).
- Point estimate = subtotal + contingency; range = point × 0.85 (low) to × 1.20 (high) — asymmetric because overruns skew high.
Worked example
A 600 sq ft detached new-build, Mid-range finish,
National region, soft costs included, 15% contingency:
- Adjusted rate = $350 × 1.00 × 1.00 = $350/sq ft.
- Hard-cost base = $350 × 600 = $210,000 (foundation & utilities already inside).
- Soft costs = 20% × $210,000 = $42,000 (above the $12k floor); no impact fee (≤750 sq ft).
- Subtotal = $210,000 + $42,000 = $252,000; contingency = 15% = $37,800.
- Point estimate ≈ $290,000 → range $247,000 – $348,000, or roughly $485/sq ft all-in.
A 600 sq ft garage conversion at the same finish/region (no new foundation or utilities, soft costs included) lands near $158,000 – $223,000 — that gap is the cost of building from scratch versus reusing existing structure. Switch the ADU type to see it in the comparison strip.
Accuracy notes & assumptions
- This is a planning budget, not a bid. Site conditions (grading, fire-rated walls, utility upgrades), contractor, and local labor move the real number.
- Base rates are 2025–2026 U.S. figures weighted toward California/West-Coast-reported data (where most ADU activity is reported); the region multiplier corrects toward national/lower-cost markets but is approximate.
- Detached & prefab rates are all-in and already bundle foundation, utilities, and site work — mixing a prefab's advertised "box only" price ($80–$160/sq ft) with site costs is the #1 source of low-balled prefab estimates.
- Per-sq-ft cost falls as size rises (fixed kitchen/bath/utility costs amortize). A flat per-sq-ft assumption overstates cost for large units and understates for tiny ones; the $12k soft-cost floor partially corrects the small-unit case.
- The >750 sq ft impact fee is a generic placeholder. California waives ADU impact fees under 750 sq ft by state law; other states differ — confirm locally.
- Not included: financing/interest, landscaping beyond the unit, furniture, property-tax reassessment, or solar (required on some new detached units in CA).
Common mistakes this avoids
- Double-counting foundation and utilities on detached/prefab builds — the base rate already includes them.
- Using a prefab's advertised box price as the total and forgetting site prep, foundation, hookups, delivery/crane, and permits — the classic 50% under-estimate.
- Forgetting soft costs (design, structural, Title-24, permits) — 15–25% of the project, surprising homeowners at the lender stage.
- Applying one flat $/sq ft across all sizes.
- Pricing a teardown-and-rebuild as a garage conversion when walls/roof/slab must be replaced.
- Omitting a contingency on a six-figure build, then treating the point estimate as the ceiling.
Frequently asked questions
How much does it cost to build an ADU?
ADUs typically run from about $40,000 for a small garage conversion to $360,000+ for a large, high-finish detached new-build. The biggest driver is type: conversions reuse an existing slab, walls, and roof (cheapest); detached builds construct everything (most expensive). Square footage, finish, region, and new foundation/utilities round it out. This tool turns those inputs into an itemized low–high range.
Why is a detached ADU so much more than a garage conversion?
A garage conversion reuses slab, walls, and roof, so you mostly pay to finish the interior and add a kitchen and bath. A detached build constructs the foundation, framing, roof, and runs new utilities to a free-standing structure — pushing per-sq-ft rates from roughly $150–$400 up to $300–$500+.
Does the estimate already include foundation and utilities for detached/prefab?
Yes. The detached and prefab per-sq-ft rates are all-in and already bundle foundation, utility runs, and site work. To avoid double-counting tens of thousands of dollars, the tool suppresses the separate foundation and utility line items for those types. Those adders only apply to garage conversions and attached additions.
What are soft costs and why are they 15–25%?
Soft costs are everything that is not hands-on construction: plans, structural and Title-24 engineering, permit fees, and (over 750 sq ft) impact fees. They add 15–25% on top of hard costs and are the most-forgotten line item. This tool applies 20% with a real-world $12,000 floor.
Why does prefab look higher than the manufacturer's advertised price?
Prefab marketing often quotes $80–$160/sq ft for the box only — the factory shell delivered to a yard. That leaves out site prep, foundation, utility hookups, delivery and crane, and permits, which can double the real cost. This tool uses an all-in prefab rate.
Is it really free?
Yes — 100% free, and everything runs in your browser. It is a planning-stage budget range, not a quote, appraisal, or guarantee of financing.
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