Validate the property
Check size limits, access, soils and utility capacity with your local planning and building departments. Start with the ADU permit checklist.
Free planning tool · All 50 states + D.C.
ADU Cost Calculator
Plan the full cost of an accessory dwelling unit. Find your city or town, adjust the building and site, and enter local quotes to create a detailed budget with an illustrative low–high range.
· Planning estimate only · Local eligibility must be confirmedView your estimate $273,000–$331,000 ↓
Your quotes & fee amounts Optional · 0/6 entered
Replace any category with your own complete amount. Blank keeps the model; 0 removes an allowance already covered elsewhere. The calculator cannot verify your quote’s scope.
A turnkey quote may include design, sitework or utilities. Enter its total once, then set included categories to 0. Extras override replaces the checkboxes and known extras total. Contingency is still added to the effective subtotal. Amounts stay until cleared, even when you change location or project size.
Rental & financing analysis Optional
Use your own rent and loan assumptions to compare scenarios.
The default rate is an example, not a current lender offer. The loan calculation assumes fixed monthly principal and interest payments.
COMPARE SIZES
How does ADU size change the budget?
Compare three detached ADU scenarios using the same California planning preset. These examples come directly from the calculator, not a survey of contractor prices.
| Size | Planning total | Total per sq ft | Illustrative range | Adjust the example |
|---|---|---|---|---|
| 500 sq ft | $257,085 | $514 | $237,000–$288,000 | Plan 500 sq ft |
| 800 sq ft | $347,547 | $434 | $320,000–$389,000 | Plan 800 sq ft |
| 1,200 sq ft | $464,400 | $387 | $428,000–$520,000 | Plan 1,200 sq ft |
Each example uses one bathroom, standard finishes, one story, typical sitework, moderate utilities, no extras and a 12% contingency reserve. The total includes construction, design, permit allowances, sitework and utilities. Land and financing costs are excluded. The range is an editorial sensitivity band, not a guaranteed price or a statistical confidence interval.
Why is the total per square foot different from the construction rate?
The construction rate covers only part of the budget. Design, utility connections, site preparation and other allowances also contribute to the total. Fixed allowances are spread over fewer square feet in a smaller ADU. Compare quotes with matching inclusions before using a per-square-foot figure.
Have an existing garage? Read the garage conversion cost guide. Comparing building approaches? Explore detached versus attached ADUs and the prefab ADU guide. Confirm permitted size with your local authority before choosing a scenario.
MODEL v2.1
Every calculator assumption is public.
The model starts with a fixed allowance and a construction rate. Your selected cost preset, project choices and quote amounts determine the budget. Location data does not supply local construction prices.
| Approach | Base rate | Fixed allowance |
|---|---|---|
| Detached new-build ADU | $180/sq ft | $40,000 |
| Attached addition | $155/sq ft | $35,000 |
| Garage conversion | $120/sq ft | $25,000 |
| Basement / interior conversion | $110/sq ft | $18,000 |
| Junior ADU / compact conversion | $100/sq ft | $15,000 |
| Above-garage ADU | $190/sq ft | $45,000 |
| Prefab / modular | $150/sq ft | $35,000 |
These are illustrative planning assumptions, not measured market averages. The current methodology explains every factor and the uncertainty range. Older article examples remain labeled as model v1.1.
NEXT STEPS
Turn a scenario into a property-specific budget.
Use these three checks to replace the planning assumptions with a scope your local team can price.
Compare complete scopes
Ask designers and builders to include design, permits, sitework, utilities, delivery and exclusions in written proposals. Read the ADU Cost Guide.
Check income and debt
Use local rental comparables, your actual operating expenses and a lender’s written terms. Review the financing guide.
What does this ADU cost calculator include?
It combines construction and finishes, design and engineering, permit allowances, sitework, utilities, selected extras and a contingency reserve. Optional rental and financing inputs show income and debt-payment scenarios separately.
Which states and cities does the calculator cover?
All 50 states and Washington, D.C., with 48,251 Census places and functioning municipalities from the 2025 Gazetteer. These are geographic choices, not a claim that ADUs are legal or currently under construction in every location. Unincorporated properties can use the state selection.
Are the city presets actual local quotes?
No. The ten cost presets use editorial assumptions, and newly added locations use the U.S. baseline unless mapped to an existing regional preset. You can replace six budget categories with your own quotes or fees. Official activity and policy sources are shown separately from costs.
Can an ADU calculator be 100% accurate?
No. Property conditions, design, utility capacity, local fees, contractor scope and future changes affect the final cost. This calculator provides transparent scenarios, not a guaranteed price. A parcel review, written quotes and confirmed fee schedules make a budget more dependable.
Can I share or save my estimate?
Yes. Copy estimate link creates a URL that restores your selections. Print / save PDF creates a printable budget with the selected city, ADU type, size and other assumptions.