How Much Does an ADU Cost in Massachusetts? (2026)
Short answer: most new ADUs in Massachusetts cost about $275 to $450 per square foot to build in 2026. A 600 sq ft backyard ADU usually lands between $165,000 and $270,000. A garage or basement conversion often costs less because part of the structure already exists.
Typical ADU cost by size (new construction)
| ADU size | Typical total |
|---|---|
| 400 sq ft | $110,000 – $180,000 |
| 600 sq ft | $165,000 – $270,000 |
| 800 sq ft | $220,000 – $360,000 |
| 900 sq ft (state by-right max) | $248,000 – $405,000 |
These are typical Massachusetts market ranges for 2026, not a quote. Your price depends on your lot, size, finishes and utilities. FJ Global gives a free written estimate.
What drives the price
- Detached vs. attached: a detached ADU needs its own foundation, roof and utility runs. An attached ADU or addition shares walls and is often less per square foot.
- Conversions: turning a garage or basement into an ADU is usually the lowest-cost option, if the structure is sound.
- Site work: excavation, drainage and utility trenching can add $20,000 to $80,000.
- Utilities: electrical service, water and sewer or septic upgrades can add $5,000 to $30,000.
- Design and permits: drawings, engineering and permit fees typically run $10,000 to $40,000.
- Finishes: kitchen, bath and flooring choices move the total more than anything else inside.
How to budget for your ADU
- Decide the goal: rental income, aging parents, or space for adult kids.
- Pick the type: detached, attached, garage or basement conversion.
- Set aside a 10–15% contingency for surprises.
- Get a site visit and written estimate before you finalize plans.
FAQ
How much does a 600 sq ft ADU cost in Massachusetts?
Typically $165,000 to $270,000 for new construction in 2026, before unusual site work.
Is a garage conversion cheaper than a new ADU?
Usually yes, because the foundation, walls and roof already exist. The garage still needs insulation, utilities and code upgrades.
Do I need a permit to build an ADU in Massachusetts?
Yes. ADUs need a building permit from your town. Since February 2, 2025, most single-family zones must allow one ADU by right.
Do you build ADUs in New Hampshire too?
Yes. FJ Global builds ADUs in Lowell, MA and southern NH, including Nashua, Salem and Hudson.
Related services and guides
Planning an ADU? Get a free estimate from a licensed MA contractor with $1M insurance. Call (781) 527-9445 or fill out the form.
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