Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Calculate your Somerville ADU cost →Building an ADU in Somerville costs between $220,000-$400,000 for new construction, with per-square-foot costs averaging $275-$450. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $100,000-$250,000, while prefab/modular ADUs fall in between at $250,000-$480,000.
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | Not published online. ISD charges a plan-review fee at application, credited toward the building permit fee if no plan review is needed; the building permit fee itself is set on the Estimated Cost Affidavit — confirm the current rate with ISD before budgeting. |
| Impact Fees | None. No impact or capacity fee appears in the Zoning Ordinance for a single accessory unit, and the state bars any impact fee or study on a Protected Use ADU that is not required for a single-family house (760 CMR 71.03(3)(b)6). Water/sewer connection charges: confirm with the Engineering Division. |
| 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. We pass your request to up to 5 local builders serving Somerville, where we have them — free, no obligation.
Get Somerville builder quotes →Permit fees in Somerville are Not published online. ISD charges a plan-review fee at application, credited toward the building permit fee if no plan review is needed; the building permit fee itself is set on the Estimated Cost Affidavit — confirm the current rate with ISD before budgeting.. The review timeline is Not published. ISD posts no review-time target; a plan-review fee is paid when you apply and full plan review precedes issuance — ask ISD (617-625-6600 x5600) for the current queue., with plan check: Full architectural drawings and a site plan showing the 3 ft ambulance-cot walkway, buried utility route and the sprinkler design where required (SZO §3.1.12.f); Fire Prevention Bureau sign-off on the sprinkler exemption for corner, driveway or alley lots.. Impact fees are None. No impact or capacity fee appears in the Zoning Ordinance for a single accessory unit, and the state bars any impact fee or study on a Protected Use ADU that is not required for a single-family house (760 CMR 71.03(3)(b)6). Water/sewer connection charges: confirm with the Engineering Division.. Pre-approved plans: None. Somerville offers no pre-approved or pre-reviewed ADU plan library; the only related Council action is a Jan 2026 order to discuss an incentive pilot (item 26-0100)..
| 1-Bedroom Rent | $2,900-$3,150/month |
| 2-Bedroom Rent | $3,450-$3,750/month |
| Annual Rental Income (est.) | $36,300/year |
| Payback Period (est.) | 8.5 years |
| Short-Term Rental | No for a rented ADU. |
| Overall ROI Estimate | 9-14% gross rent yield (estimate) |
A typical ADU in Somerville generates approximately $36,300 per year in rental income based on average 1-bedroom rents. At an average construction cost of $310,000, the estimated payback period is 8.5 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: $100,000-$250,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $250,000-$480,000.
3. Pre-approved plans — available in Somerville, which can save $3,000-$10,000 in design costs and 2-3 months in review time. See which cities offer pre-approved ADU plans.
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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