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
| Allowed Zones | Select residential zones |
| Max Size (Detached) | 900 sq ft |
| Max Size (Attached) | 800 sq ft |
| Max Size (JADU) | Not allowed |
| Max Height | 35 feet |
| Max Units | 1 ADU per lot |
Setback Requirements
| 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
| 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
| 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 (installed) |
Finance Your ADU Project
Most ADU projects are funded through HELOCs, construction loans, or cash-out refinancing. Compare rates from top lenders in minutes.
Compare ADU Financing Options →Rental Income Potential
| 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 |
Explore Prefab ADU Options
Prefab ADUs can save 20-40% vs. traditional construction and cut build time in half. Compare top prefab manufacturers.
Compare Prefab ADUs →Recent Law Changes
What's Changed Recently
- 2024: Expanded ADU allowance in residential zones
- 2023: Streamlined zoning variance process
- 2024: Added provisions for basement ADUs
- 2023: Created ADU design guidelines
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
Frequently Asked Questions
Can I build an ADU in Boston?
Yes. ADUs are allowed in Select residential zones in Boston. The maximum size for a detached ADU is 900 sq ft. No owner-occupancy is required.
How much does an ADU cost in Boston?
ADU construction in Boston typically costs $200,000-$400,000. Garage conversions run $100,000-$220,000, and prefab ADUs cost $150,000-$320,000 (installed).
How much can I rent an ADU for in Boston?
Average ADU rents in Boston are $1,800-$2,400/mo for a 1-bedroom and $2,200-$3,200/mo for a 2-bedroom unit.
Do I need parking for an ADU in Boston?
1 space per unit. Check with Boston planning department for the latest Massachusetts ADU parking rules, as many states have relaxed requirements in recent years.
How long does it take to get an ADU permit in Boston?
The permit review timeline in Boston is 90-150 days. Fees range from $4,500-$7,500.
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