Everything homeowners ask about building an ADU in Quincy — answered with 2026 data.
Yes, ADUs are allowed in One ADU by right in single-family zoning districts (internal, addition, or detached); not allowed in Bus C, Ind A, or Ind B districts, or on a lot subject to a variance in Quincy, MA. The maximum size for a detached ADU is 900 sq ft or 50% of the principal structure's gross floor area (per 760 CMR 71.02), whichever is less; minimum 320 sq ft for a detached ADU. Attached ADUs can be up to 900 sq ft or 50% of the principal structure's gross floor area, whichever is less. Quincy has an ADU-friendliness score of 6/10. Check our full Quincy ADU rules page for complete zoning details.
ADU construction in Quincy typically costs $250,000-$425,000. The cost per square foot averages $350-$550. Garage conversions are more affordable at $150,000-$250,000, while prefab ADUs cost $180,000-$350,000 installed (site work, foundation, and utility connections extra — confirm with manufacturer). Permit fees add Valuation-based building permit fee — Confirm with Quincy Inspectional Services to your budget. See our detailed Quincy ADU cost guide.
Detached ADUs in Quincy can be up to 900 sq ft or 50% of the principal structure's gross floor area (per 760 CMR 71.02), whichever is less; minimum 320 sq ft for a detached ADU. Attached ADUs are limited to 900 sq ft or 50% of the principal structure's gross floor area, whichever is less. Junior ADUs (JADUs) have a cap of No JADU category in Massachusetts law. Maximum height is Detached ADUs limited to one story (city guideline). You are allowed up to 1 ADU per lot; ADU may not be sold or owned separately from the principal dwelling, and no ADU is permitted below the FEMA 100-year base flood elevation on a qualifying lot.
Rear setback: Detached ADUs must meet the principal-structure yard setbacks in QZO Table 4.1 — varies by district; Confirm with Quincy Inspectional Services. Side setback: Per QZO Table 4.1 (district-specific) — Confirm with Quincy Inspectional Services. Front setback: Per QZO Table 4.1 (district-specific) — Confirm with Quincy Inspectional Services. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
1 additional off-street space required only if the ADU is outside a 1/2-mile radius of a transit station (Quincy has 4 Red Line stops); existing off-street parking may not be reduced to create the ADU. 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 allowed — prohibited statewide by the Affordable Homes Act effective Feb 2, 2025. 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 Quincy is typically Standard building-permit review; typically 4-8 weeks — Confirm with Quincy Inspectional Services. Plan check: Site plan (drainage, utilities, parking) + architectural drawings required; flood-zone screening against the FEMA 100-Year Flood Hazard Map for Quincy. Pre-approved plans availability: None — Quincy does not offer a pre-approved ADU plan library.
Average ADU rental income in Quincy: 1-bedroom units rent for $2,200-$2,700/month, 2-bedroom units for $2,700-$3,300/month. Short-term rentals: No — Quincy's ADU guidelines expressly prohibit using an ADU as a short-term rental. Expected ROI: 5-8% annual return.
The minimum lot size for an ADU in Quincy is None stated in the ADU guidelines — underlying district lot standards apply; Confirm with Quincy Inspectional Services. 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 Quincy, a garage conversion typically costs $150,000-$250,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.