Zoning & Size Limits
| Requirement | Cambridge, MA | Somerville, MA |
|---|
| Allowed Zones | All residential districts - Cambridge eliminated single-fami | By right in Neighborhood Residence (NR) district as the 'Bac |
| Max Size (Detached) | 900 sq ft or 50% of primary dwelling, whichever is less (MA state by-right cap) | Local backyard cottage: 576 sq ft max floor plate, max massing 24 ft wide x 32 ft deep; state protected-use ADU: up to 900 sq ft or 50% of principal dwelling GFA, whichever is smaller (reasonable local dimensional standards still apply) |
| Max Size (Attached) | 900 sq ft or 50% of primary dwelling, whichever is less (MA state by-right cap) | 900 sq ft or 50% of principal dwelling gross floor area, whichever is smaller (state cap); internal conversions (basement/attic) follow the same cap |
| Max Height | No ADU-specific cap - an ADU may match the princip | Backyard cottage: 1.5 stories max, ground story he |
| Parking Required | None - Cambridge abolished all parking minimums citywide in | None required for the ADU — state law bars parking requireme |
| Owner Occupancy | Not required (prohibited under MA Affordable Homes Act / 760 | Not allowed — prohibited statewide by the Affordable Homes A |
Setback Requirements
| Setback | Cambridge, MA | Somerville, MA |
|---|
| Rear Setback | 5 ft base (ADU setbacks may not be stricter than principal/accessory standards under 760 CMR 71.03) | 3 ft minimum (3 or 20 ft where lot abuts an alley); 10 ft minimum separation from the principal building |
| Side Setback | 5 ft base | 3 ft minimum |
| Front Setback | 10 ft base | Backyard cottages sit behind the principal buildin |
Permits & Timeline
| Detail | Cambridge, MA | Somerville, MA |
|---|
| Permit Timeline | 60-90 days | Building-permit plan review typically 4-8 weeks — Confirm with Inspectional Services (617-625-6600 ext. 5600, isd@somervillema.gov) |
| Permit Fees | $3,500-$5,500 | Confirm with Inspectional Services — building permit fees are valuation-based; budget several thousand dollars on a typical $300K+ build |
| Impact Fees | $1,500-$2,500 | None for a single ADU — Confirm with Inspectional Services |
| Pre-Approved Plans | Not available | None — Somerville does not offer a pre-approved AD |
Construction Costs
| Cost | Cambridge, MA | Somerville, MA |
|---|
| Avg Build Cost | $210,000-$420,000 | $275,000-$450,000 |
| Cost Per Sq Ft | $220-$440 | $400-$600 |
| Garage Conversion | $105,000-$230,000 | $150,000-$250,000 |
| Prefab ADU | $160,000-$340,000 | $180,000-$350,000 installed (site work, foundations, and buried utilities extra — confirm with manufacturer) |
Rental Income Potential
| Rental | Cambridge, MA | Somerville, MA |
|---|
| 1BR Avg Rent | $1,900-$2,500/mo | $2,600-$3,000/mo |
| 2BR Avg Rent | $2,300-$3,400/mo | $3,300-$3,700/mo |
| Short-Term Rentals | Not permitted | Effectively no — Somerville's STR ordinance (registration re |
| Est. ROI | 5-7% annual return | 6-9% annual return |
Which City Is Better for ADUs?
Cambridge scores 8/10 while Somerville scores 7/10 on our ADU-friendliness index.
Cambridge edges ahead overall. Both cities offer viable ADU development opportunities — the best choice depends on your specific property, budget, and goals.
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