Tool · 01 · cost calculator
How much does an ADU
cost in your area?
A defensible low–mid–high estimate in under 90 seconds. Calibrated to 2025–2026 builder data by state, ADU type, and finish level — so the numbers reflect what people are actually paying.
How the ADU cost calculator works
We start with a baseline $/sq ft for each ADU type — detached, attached, garage conversion, JADU, and prefab — based on the median 2025 build costs reported by ADU-specialist builders. We then adjust for the state's construction cost index, your specific metro (when known), finish level, site complexity, and foundation type.
The result is a range, not a point estimate. The low number assumes everything goes smoothly; the high number assumes the normal mix of surprises real projects encounter. Most homeowners should plan to the mid-to-high end of the range to avoid the budget panic that hits around month four of construction.
Common ways homeowners overspend
- Mid-build finish upgrades. The cabinet decision you make at week 12 cascades — bigger cabinets pull bigger counters, more electrical, taller walls.
- Underestimating utilities. Sewer extensions and panel upgrades can add $10K–$40K and are often missing from early estimates.
- Permitting changes mid-design. Each redraw costs design fees and pushes the construction-loan clock.
- Skipping contingency. 10% is the floor. On older homes or complex sites, 15% is realistic.
When you're ready for real numbers, run our ROI calculator to see whether the project pencils out, then compare loan options. Or jump straight to the full ADU process guide.
ADU cost FAQ
- How accurate is the ADU cost calculator?
- It's a planning estimate, not a quote. Our coefficients are calibrated to 2025–2026 U.S. builder data and adjusted by state and metro. Real quotes from a contractor walking your lot will land within roughly 15% of the midpoint we show — though site conditions, finishes, and code requirements can move that either direction.
- What's included in the estimate?
- Design and engineering, permits and impact fees, site prep, foundation, framing and shell, MEP rough-in, interior finishes, utility hookups, and a 10% contingency. It does not include landscaping, furniture, or the homeowner's own time.
- Why is California so much more expensive?
- Labor, materials, permitting timelines, seismic engineering, and impact fees all run higher in California than the national average. Within California, costs vary by another 20–25% between metro areas — Bay Area and Malibu run highest, inland markets run lower.
- Is a garage conversion really that much cheaper?
- Yes — typically 30–40% cheaper than a comparable detached ADU. You're reusing the slab, walls, and roof. The savings shrink if you need to extensively reframe, raise the ceiling, or run new utilities a long distance.