Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Calculate your Somerville ADU cost →Building an ADU in Somerville costs between $275,000-$450,000 for new construction, with per-square-foot costs averaging $400-$600. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $150,000-$250,000, while prefab/modular ADUs fall in between at $180,000-$350,000 installed (site work, foundations, and buried utilities extra — confirm with manufacturer).
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | Confirm with Inspectional Services — building permit fees are valuation-based; budget several thousand dollars on a typical $300K+ build |
| Impact Fees | None for a single ADU — Confirm with Inspectional Services |
| Site Prep & Foundation | $10,000 – $35,000 |
| Framing & Structure | $15,000 – $45,000 |
| Plumbing | $8,000 – $20,000 |
| Electrical | $5,000 – $15,000 |
| HVAC | $3,000 – $8,000 |
| Interior Finishes | $15,000 – $40,000 |
Quotes for the same ADU commonly vary 20–40% between builders. Get up to 3 free quotes from ADU builders serving Somerville — no obligation.
Get Somerville builder quotes →Permit fees in Somerville are Confirm with Inspectional Services — building permit fees are valuation-based; budget several thousand dollars on a typical $300K+ build. The review timeline is Building-permit plan review typically 4-8 weeks — Confirm with Inspectional Services (617-625-6600 ext. 5600, isd@somervillema.gov), with plan check: Full architectural drawings plus site plan; sprinkler plan required for most mid-block cottages. Impact fees are None for a single ADU — Confirm with Inspectional Services. Pre-approved plans: None — Somerville does not offer a pre-approved ADU plan library.
| 1-Bedroom Rent | $2,600-$3,000/month |
| 2-Bedroom Rent | $3,300-$3,700/month |
| Annual Rental Income (est.) | $33,600/year |
| Payback Period (est.) | 10.8 years |
| Short-Term Rental | Effectively no — Somerville's STR ordinance (registration required since April 2023) limits short-term rentals to the operator's primary residence, so a standalone ADU generally cannot be an Airbnb |
| Overall ROI Estimate | 6-9% annual return |
A typical ADU in Somerville generates approximately $33,600 per year in rental income based on average 1-bedroom rents. At an average construction cost of $362,500, the estimated payback period is 10.8 years — before accounting for the resale value the ADU adds, which an appraiser determines from comparable sales in Somerville and varies by market (often less than build cost).
1. Garage conversions save 40-60% vs new construction since the shell already exists. Average cost in Somerville: $150,000-$250,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $180,000-$350,000 installed (site work, foundations, and buried utilities extra — confirm with manufacturer).
3. Pre-approved plans — available in Somerville, which can save $3,000-$10,000 in design costs and 2-3 months in review time.
4. State grants — California's CalHFA ADU grant (up to $40K) is currently paused; check calhfa.ca.gov/adu for status and your state for active programs.
5. Get multiple bids — ADU contractor pricing varies 20-40% for the same project. We recommend getting at least 3 quotes.
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