Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Calculate your Boston ADU cost →Building an ADU in Boston costs between $75,000-$350,000 for new construction, with per-square-foot costs averaging $150-$400. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $150,000-$275,000, while prefab/modular ADUs fall in between at $180,000-$320,000.
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | Long-form building permit $50 plus $10 per $1,000 of estimated construction cost (ISD fee schedule dated May 15, 2023): about $950 on a $90,000 interior ADU, $3,050 on a $300,000 detached ADU; ZBA appeal $150 for a 1-3 family home. |
| Impact Fees | None: ISD's fee schedule lists no impact or capacity fee. Trade permits are small (electrical $20 plus $1 per fixture, plumbing $20 plus $5 per fixture, gas $20 plus $5 per appliance). Ask BWSC about connection charges if a detached ADU needs its own water or sewer service. |
| 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 Boston, where we have them — free, no obligation.
Get Boston builder quotes →Permit fees in Boston are Long-form building permit $50 plus $10 per $1,000 of estimated construction cost (ISD fee schedule dated May 15, 2023): about $950 on a $90,000 interior ADU, $3,050 on a $300,000 detached ADU; ZBA appeal $150 for a 1-3 family home.. The review timeline is ISD plan review takes about 5 weeks for a complete application (city ADU FAQ); a ZBA variance adds a hearing cycle. City guidance: design up to 3 months, permitting up to 3 months, construction 6-12 months., with plan check: About 5 weeks for a complete long-form application (city ADU FAQ); longer if plans are revised, documents are missing or zoning relief is needed.. Impact fees are None: ISD's fee schedule lists no impact or capacity fee. Trade permits are small (electrical $20 plus $1 per fixture, plumbing $20 plus $5 per fixture, gas $20 plus $5 per appliance). Ask BWSC about connection charges if a detached ADU needs its own water or sewer service.. Pre-approved plans: Not available. The city's ADU Guidebook (November 2024) illustrates designs for common Boston house types but contains no pre-approved plans; every project needs its own architect-stamped drawings..
| 1-Bedroom Rent | $2,500-$2,900/month |
| 2-Bedroom Rent | $3,000-$3,500/month |
| Annual Rental Income (est.) | $32,400/year |
| Payback Period (est.) | 6.6 years |
| Short-Term Rental | Only through Boston's short-term rental registry, which requires owner-occupancy: a second unit in an owner-occupied 2-3 family home can be registered as 'owner-adjacent' ($200/yr). |
| Overall ROI Estimate | 7-12% gross annual yield (estimate) |
A typical ADU in Boston generates approximately $32,400 per year in rental income based on average 1-bedroom rents. At an average construction cost of $212,500, the estimated payback period is 6.6 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: $150,000-$275,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $180,000-$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). 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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