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
| 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 (installed) |
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| 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 |
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: Streamlined ADU design review process
- 2023: Increased max attached ADU size from 750 to 850 sq ft
- 2024: Reduced parking requirements for ADUs
- 2023: Created new ADU provisions in transitional zones
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
Frequently Asked Questions
Can I build an ADU in Cambridge?
Yes. ADUs are allowed in Residential zones R-1 through R-4 in Cambridge. The maximum size for a detached ADU is 950 sq ft. No owner-occupancy is required.
How much does an ADU cost in Cambridge?
ADU construction in Cambridge typically costs $210,000-$420,000. Garage conversions run $105,000-$230,000, and prefab ADUs cost $160,000-$340,000 (installed).
How much can I rent an ADU for in Cambridge?
Average ADU rents in Cambridge are $1,900-$2,500/mo for a 1-bedroom and $2,300-$3,400/mo for a 2-bedroom unit.
Do I need parking for an ADU in Cambridge?
Optional. Check with Cambridge 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 Cambridge?
The permit review timeline in Cambridge is 60-90 days. Fees range from $3,500-$5,500.
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