Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Calculate your Boston ADU cost →Building an ADU in Boston costs between $200,000-$400,000 for new construction, with per-square-foot costs averaging $220-$440. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $100,000-$220,000, while prefab/modular ADUs fall in between at $150,000-$320,000.
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | $4,500-$7,500 |
| Impact Fees | $2,000-$3,500 |
| 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 Boston — no obligation.
Get Boston builder quotes →Permit fees in Boston are $4,500-$7,500. The review timeline is 90-150 days, with plan check: 4-6 weeks. Impact fees are $2,000-$3,500. Pre-approved plans: Not available.
| 1-Bedroom Rent | $1,800-$2,400/month |
| 2-Bedroom Rent | $2,200-$3,200/month |
| Annual Rental Income (est.) | $25,200/year |
| Payback Period (est.) | 11.9 years |
| Short-Term Rental | Not permitted |
| Overall ROI Estimate | 5-7% annual return |
A typical ADU in Boston generates approximately $25,200 per year in rental income based on average 1-bedroom rents. At an average construction cost of $300,000, the estimated payback period is 11.9 years — before accounting for the resale value the ADU adds, which an appraiser determines from comparable sales in Boston 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 Boston: $100,000-$220,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $150,000-$320,000.
3. Pre-approved plans — Boston does not offer a pre-approved/standard ADU plan program yet, so budget for custom architectural plans (typically $3,000-$15,000).
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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