Zoning & Size Limits
| Requirement | Somerville, MA | Newton, MA |
|---|
| Allowed Zones | By right in Neighborhood Residence (NR) district as the 'Bac | All zones allowing single-family use (by right per MGL c.40A |
| Max Size (Detached) | 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) | 1,000 sq ft or 50% of the main building by right; 1,200 sq ft in an existing garage or if the owner occupies the site; up to 1,500 sq ft by special permit |
| Max Size (Attached) | 900 sq ft or 50% of principal dwelling gross floor area, whichever is smaller (state cap); internal conversions (basement/attic) follow the same cap | Internal/attached: up to 1,000 sq ft (1,200 sq ft for conversions within existing homes), max 50% of the main building's floor area |
| Max Height | Backyard cottage: 1.5 stories max, ground story he | Detached: 2.5 stories with standard new-house setb |
| Parking Required | None required for the ADU — state law bars parking requireme | No additional parking required (state law caps at 1 space an |
| Owner Occupancy | Not allowed — prohibited statewide by the Affordable Homes A | Not required for by-right ADUs (prohibited by state law); re |
Setback Requirements
| Setback | Somerville, MA | Newton, MA |
|---|
| Rear Setback | 3 ft minimum (3 or 20 ft where lot abuts an alley); 10 ft minimum separation from the principal building | 5 ft for a 1.5-story/22-ft detached ADU; standard principal-dwelling setbacks for taller units |
| Side Setback | 3 ft minimum | 5 ft for a 1.5-story/22-ft detached ADU; standard principal-dwelling setbacks for taller units |
| Front Setback | Backyard cottages sit behind the principal buildin | Per underlying zone — Confirm with Newton Planning |
Permits & Timeline
| Detail | Somerville, MA | Newton, MA |
|---|
| Permit Timeline | Building-permit plan review typically 4-8 weeks — Confirm with Inspectional Services (617-625-6600 ext. 5600, isd@somervillema.gov) | Building permit typically 4-8 weeks; special-permit route adds roughly 3-6 months — Confirm with Newton Planning Department |
| Permit Fees | Confirm with Inspectional Services — building permit fees are valuation-based; budget several thousand dollars on a typical $300K+ build | Building permit fees based on construction value — Confirm with Newton Inspectional Services |
| Impact Fees | None for a single ADU — Confirm with Inspectional Services | No ADU-specific impact fees identified — Confirm with Newton |
| Pre-Approved Plans | None — Somerville does not offer a pre-approved AD | Not available |
Construction Costs
| Cost | Somerville, MA | Newton, MA |
|---|
| Avg Build Cost | $275,000-$450,000 | $250,000-$450,000 |
| Cost Per Sq Ft | $400-$600 | $250-$450 |
| Garage Conversion | $150,000-$250,000 | $120,000-$220,000 |
| Prefab ADU | $180,000-$350,000 installed (site work, foundations, and buried utilities extra — confirm with manufacturer) | $150,000-$320,000 |
Rental Income Potential
| Rental | Somerville, MA | Newton, MA |
|---|
| 1BR Avg Rent | $2,600-$3,000/mo | $2,500-$3,300/mo |
| 2BR Avg Rent | $3,300-$3,700/mo | $3,100-$4,200/mo |
| Short-Term Rentals | Effectively no — Somerville's STR ordinance (registration re | No — minimum 30-day rental/occupancy term for ADUs |
| Est. ROI | 6-9% annual return | 4-6% annual return |
Which City Is Better for ADUs?
Somerville scores 7/10 while Newton scores 6/10 on our ADU-friendliness index.
Somerville 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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