Based on real MA permit data from 2025. See methodology →
Estimate the cost of building an accessory dwelling unit in Massachusetts. This calculator uses real permit data from local building departments to project construction costs by town, ADU type, and square footage.
Select your town and project type to see estimated costs, ROI projections, and how your town's ADU bylaws may affect your project. Cost data is derived from building permits issued across Massachusetts in 2024–2025.
MA law allows ADUs up to 900 sf or 50% of principal dwelling, whichever is less.
See the full value picture based on your goal — not just the sticker price.
Milton’s bylaw predates the state ADU law and has not been reconciled with G.L. c. 40A §3. Three provisions — owner-occupancy, family/caregiver restrictions, and the special permit requirement — are preempted by state law. Homeowners should cite G.L. c. 40A §3 if the town tries to enforce them.
See full compliance analysis →Tell us which town to add next. We'll email you when it's live.
| Location | Type | Sqft | Cost | $/sf | Notes |
|---|---|---|---|---|---|
| 1450 Canton AveMilton | detached | 701 | $82K | $117 | Nest Home Plans design |
| 1112 Randolph AveMilton | detached | 900 | $200K | $222 | ADU above 2-car garage |