Boston is moderately ADU-friendly, with increasing allowances for accessory units in residential neighborhoods. The city's housing shortage has prompted more liberal policies, though strict zoning and historic district rules can complicate projects.
Zoning & Size Requirements [1][2]
| Allowed Zones | Select residential zones |
| Max Size (Detached) | 900 sq ft or 50% of living area (whichever is less) |
| Max Size (Attached) | 900 sq ft or 50% of living area (whichever is less) |
| Max Size (JADU) | Not allowed |
| Max Height | 35 feet |
| Max Units | 1 ADU per lot |
Setback Requirements [1][2]
| Rear Setback | 20 feet |
| Side Setback | 5 feet |
| Front Setback | 20 feet |
| Parking Required | 1 space per unit |
| Owner Occupancy | May be required in some zones |
| Min Lot Size | 5,000 sq ft |
Permit Process & Fees [3]
| Approval Process | Building permit + zoning variance |
| Review Timeline | 90-150 days |
| Permit Fees | $4,500-$7,500 |
| Impact Fees | $2,000-$3,500 |
| Plan Check | 4-6 weeks |
| Pre-Approved Plans | Not available |
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Construction Costs [4][5]
| Average Build Cost | $200,000-$400,000 |
| Cost per Sq Ft | $220-$440 |
| Garage Conversion | $100,000-$220,000 |
| Prefab ADU (Installed) | $150,000-$320,000 |
Finance Your ADU Project
Most ADU projects are funded through HELOCs, construction loans, or cash-out refinancing. Compare rates from top lenders.
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Rental Income Potential [6][7]
| Avg Rent (1 Bedroom) | $1,800-$2,400/mo |
| Avg Rent (2 Bedroom) | $2,200-$3,200/mo |
| Short-Term Rental | Not permitted |
| Est. Annual ROI | 5-7% annual return |
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Recent Law Changes
What's Changed Recently
- Affordable Homes Act §8 (Feb 2025): Statewide ADU by right in ALL single-family zones
- 352 MA jurisdictions now required to allow ADUs without special permits
- Boston ADU pilot program expanded citywide with pre-approved designs (2026)
- Cambridge fast-track ADU permits: 30-day approval for compliant applications
Tips for Building an ADU in Boston
- Boston's high property values and strong rental market support premium rents
- Historic district restrictions can significantly complicate projects
- Zoning variance requirement adds time and complexity
- Strong tenant base ensures reliable long-term income despite lower ROI
Sources & References
All data on this page is sourced from official government records and verified market data. View our full methodology.
- Accessory Dwelling Units (ADUs) in Boston — City of Boston Planning Department — Accessed 2026-02-25
- ADU Zoning — Boston Planning & Development Agency Planning Department — Accessed 2026-02-25
- Permitting Your ADU | Boston.gov Planning Department — Accessed 2026-02-24
- HomeAdvisor — ADU Construction Cost Data Market Data — Accessed 2026-02-24
- Angi — Local ADU Contractor Costs Market Data — Accessed 2026-02-24
- Zillow Rental Manager — Market Rent Data Market Data — Accessed 2026-02-24
- HUD Fair Market Rents Government Data — Accessed 2026-02-24
- ADU FAQ | Boston.gov Planning Department — Accessed 2026-02-24
- Research your ADU Options | Boston.gov Planning Department — Accessed 2026-02-24
- Massachusetts Affordable Homes Act — ADU Provisions Government Data — Accessed 2026-02-24
Frequently Asked Questions
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