Cost by Build Path
Four primary ADU paths in MA:
| Path | Cost | Timeline |
|---|---|---|
| Interior basement conversion | $95K-$165K | 3-5 months |
| Garage conversion | $140K-$220K | 4-6 months |
| Second-story addition | $185K-$280K | 5-8 months |
| Detached new build | $240K-$310K+ | 6-12 months |
Frequently Asked Questions
Does Mass Save apply to ADUs?
Yes — Mass Save treats ADUs as separate dwelling units. Each ADU captures its own full rebate stack: $10K-$16K heat pump, $750-$2,250 HPWH, $4K panel upgrade, 75-100% insulation. Total ADU rebate stack: $11K-$17K typical.
What's the most cost-effective ADU build path?
Interior basement conversion typically lowest cost at $95K-$165K. Already-built foundation + roof + walls — minimum new construction. Best for homes with 7'+ basement ceiling height + walkout potential.
References & Sources
- MGL c. 40A §3A — full text. https://malegislature.gov/Laws/GeneralLaws/PartI/TitleVII/Chapter40A/Section3A
- Mass Save residential rebate program. https://www.masssave.com/saving/residential-rebates
