Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Calculate your Cambridge ADU cost →Building an ADU in Cambridge 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 | Building permit $15 per $1,000 of construction cost for buildings of 3 dwelling units or fewer ($20 per $1,000 otherwise), $50 minimum (ISD Building Fees table): about $3,750 on a $250,000 ADU, $5,250 on $350,000. Plan review included; certificate of occupancy $100; trade permits extra. |
| Impact Fees | None. Cambridge has no residential impact fee; its Incentive Zoning housing contribution ($36.36 per sq ft) applies only to non-residential projects over 30,000 sq ft, and inclusionary housing (11.203) only to larger residential projects. 760 CMR 71.03(3)(b)6 bars any ADU-only impact fee. Water and sewer connection charges from DPW and the Water Department may apply to a detached unit; confirm before design. |
| 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 Cambridge, where we have them — free, no obligation.
Get Cambridge builder quotes →Permit fees in Cambridge are Building permit $15 per $1,000 of construction cost for buildings of 3 dwelling units or fewer ($20 per $1,000 otherwise), $50 minimum (ISD Building Fees table): about $3,750 on a $250,000 ADU, $5,250 on $350,000. Plan review included; certificate of occupancy $100; trade permits extra.. The review timeline is Not published. ISD posts no review target for building permits; the state ADU law makes the unit as-of-right so there is no hearing, but Historical Commission review in a historic or conservation district and a curb-cut vote add weeks., with plan check: Included in the building permit fee per the ISD Building Fees table; there is no separate ADU plan-check queue.. Impact fees are None. Cambridge has no residential impact fee; its Incentive Zoning housing contribution ($36.36 per sq ft) applies only to non-residential projects over 30,000 sq ft, and inclusionary housing (11.203) only to larger residential projects. 760 CMR 71.03(3)(b)6 bars any ADU-only impact fee. Water and sewer connection charges from DPW and the Water Department may apply to a detached unit; confirm before design.. Pre-approved plans: Not available. Cambridge publishes no ADU program page, pre-approved plan catalogue or ADU handbook (CDD and ISD sites checked 2026-09-20)..
| 1-Bedroom Rent | $3,200-$3,600/month |
| 2-Bedroom Rent | $3,800-$4,300/month |
| Annual Rental Income (est.) | $40,800/year |
| Payback Period (est.) | 7.6 years |
| Short-Term Rental | Only as an owner-adjacent short-term rental: the whole unit, inside a building of 4 or fewer units that the operator owns entirely and lives in (Zoning Ordinance 4.60, Ord. |
| Overall ROI Estimate | 9-14% gross rent yield |
A typical ADU in Cambridge generates approximately $40,800 per year in rental income based on average 1-bedroom rents. At an average construction cost of $310,000, the estimated payback period is 7.6 years — before accounting for the resale value the ADU adds, which an appraiser determines from comparable sales in Cambridge 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 Cambridge: $100,000-$250,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $250,000-$480,000.
3. Pre-approved plans — Cambridge 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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