Everything homeowners ask about building an ADU in Cambridge — answered with 2026 data.
Yes, ADUs are allowed in All residential districts - Cambridge eliminated single-family-only zoning in Feb 2025 (Residence C-1 citywide); one ADU is by-right under the state law in Cambridge, MA. The maximum size for a detached ADU is 900 sq ft or 50% of primary dwelling, whichever is less (MA state by-right cap). Attached ADUs can be up to 900 sq ft or 50% of primary dwelling, whichever is less (MA state by-right cap). Cambridge has an ADU-friendliness score of 8/10. Check our full Cambridge ADU rules page for complete zoning details.
ADU construction in Cambridge typically costs $210,000-$420,000. The cost per square foot averages $220-$440. Garage conversions are more affordable at $105,000-$230,000, while prefab ADUs cost $160,000-$340,000. Permit fees add $3,500-$5,500 to your budget. See our detailed Cambridge ADU cost guide.
Detached ADUs in Cambridge can be up to 900 sq ft or 50% of primary dwelling, whichever is less (MA state by-right cap). Attached ADUs are limited to 900 sq ft or 50% of primary dwelling, whichever is less (MA state by-right cap). Junior ADUs (JADUs) have a cap of N/A (Massachusetts has no JADU category). Maximum height is No ADU-specific cap - an ADU may match the principal-dwelling/accessory standards (base residential height is now 4 stories citywide). You are allowed up to 1 ADU per lot on a qualifying lot.
Rear setback: 5 ft base (ADU setbacks may not be stricter than principal/accessory standards under 760 CMR 71.03). Side setback: 5 ft base. Front setback: 10 ft base. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
None - Cambridge abolished all parking minimums citywide in Oct 2022 (state law would otherwise allow 1 space beyond 0.5 mi of transit). Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
Not required (prohibited under MA Affordable Homes Act / 760 CMR 71). Note that owner-occupancy rules vary by state law — California and several other states have eliminated this requirement for standard ADUs.
The ADU permit review timeline in Cambridge is typically 60-90 days. Plan check: 3-4 weeks. Pre-approved plans availability: Not available.
Average ADU rental income in Cambridge: 1-bedroom units rent for $1,900-$2,500/month, 2-bedroom units for $2,300-$3,400/month. Short-term rentals: Not permitted. Expected ROI: 5-7% annual return.
The minimum lot size for an ADU in Cambridge is None - state law prohibits a minimum lot size for a by-right ADU (760 CMR 71.03). This applies to detached ADUs — garage conversions and JADUs may have different or no lot size requirements.
Garage conversions are one of the most popular and affordable ADU options. In Cambridge, a garage conversion typically costs $105,000-$230,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.