Cambridge is moderately progressive on ADU regulations, allowing accessory units in residential zones with reasonable size limits. The city's focus on housing affordability and proximity to Boston make it attractive for ADU development.
Zoning & Size Requirements
| Allowed Zones | Residential zones R-1 through R-4 |
| Max Size (Detached) | 950 sq ft |
| Max Size (Attached) | 850 sq ft |
| Max Size (JADU) | Not allowed |
| Max Height | 35 feet |
| Max Units | 1 ADU per lot |
Setback Requirements
| Rear Setback | 15 feet |
| Side Setback | 3 feet |
| Front Setback | 15 feet |
| Parking Required | Optional |
| Owner Occupancy | Not required |
| Min Lot Size | 4,500 sq ft |
Permit Process & Fees
| Approval Process | Building permit with design review |
| Review Timeline | 60-90 days |
| Permit Fees | $3,500-$5,500 |
| Impact Fees | $1,500-$2,500 |
| Plan Check | 3-4 weeks |
| Pre-Approved Plans | Not available |
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Construction Costs [1][2]
| Average Build Cost | $210,000-$420,000 |
| Cost per Sq Ft | $220-$440 |
| Garage Conversion | $105,000-$230,000 |
| Prefab ADU (Installed) | $160,000-$340,000 |
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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 [3][4]
| Avg Rent (1 Bedroom) | $1,900-$2,500/mo |
| Avg Rent (2 Bedroom) | $2,300-$3,400/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 Cambridge
- Cambridge's university presence (MIT, Harvard) creates strong rental demand
- Design review adds time but ensures quality neighborhood integration
- Premium rents despite lower ROI make Cambridge attractive for long-term hold
- Walkable neighborhoods command premium rental rates
Sources & References
All data on this page is sourced from official government records and verified market data. View our full methodology.
- 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
- Accessory Dwelling Units — Mass.gov Government Data — Accessed 2026-02-24
- Massachusetts Affordable Homes Act — ADU Provisions Government Data — Accessed 2026-02-24
Frequently Asked Questions
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