Everything homeowners ask about building an ADU in Quincy — answered with 2026 data.
Yes, ADUs are allowed in One ADU by right (no special permit or variance) on any lot in a district where single-family homes are allowed, i.e. Residence A and B (760 CMR 71.02-71.03; ISD Guidelines Dec 11, 2025) — within the dwelling, as an addition, or detached. Not allowed in Bus C, Ind A or Ind B districts, or on a lot subject to a variance. QZO Chapter 375 has no ADU section; the state law and the ISD guidelines govern. in Quincy, MA. The maximum size for a detached ADU is 900 sq ft or 50% of the principal dwelling's gross floor area, whichever is less (760 CMR 71.02, applied by the ISD guideline). Minimum 320 sq ft for a detached ADU — a local ISD figure, not a state or 780 CMR rule. Example: a 1,500 sq ft house allows 750 sq ft; 1,800 sq ft or larger allows the full 900. Attached ADUs can be up to Same cap as detached: 900 sq ft or 50% of the principal dwelling'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 $220,000-$400,000. The cost per square foot averages $275-$450. Garage conversions are more affordable at $100,000-$250,000, while prefab ADUs cost $250,000-$480,000. Permit fees add Valuation-based building permit fee under Municipal Code Ch. 150 Art. III (Inspectional Services fees); no ADU-specific fee is published. Confirm the current figure with ISD (617-376-1450) before budgeting. 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 dwelling's gross floor area, whichever is less. Attached ADUs are limited to Same cap as detached: 900 sq ft or 50% of the principal dwelling's gross floor area, whichever is less. Junior ADUs (JADUs) have a cap of No JADU category in Massachusetts law — an internal unit is simply an ADU under the same 900 sq ft / 50% cap. Maximum height is Detached ADU limited to one story (ISD guideline). District caps in QZO Table 4.1: Residence A 2.5 stories, Residence B 3 stories; a garage as an accessory building is capped at 16 ft (§375-3.2.3.1). An internal or attached ADU follows the principal building's limit. You are allowed up to 1 ADU per lot on a qualifying lot.
Rear setback: Detached ADU must meet the principal-building yards in QZO Table 4.1 (ISD guideline): Residence A 20 ft rear; Residence B (1-3 units) 25 ft rear; Residence C half the building height, never less than 25 ft. Note: a garage or shed may sit 6 ft from the rear line (§375-3.2.3.1) and 760 CMR 71.03(3)(b)2 bars ADU dimensional rules stricter than those for an accessory structure — ask ISD in writing which figure it will apply. Side setback: Per QZO Table 4.1 (ISD guideline): Residence A 13 ft; Residence B (1-3 units) 13 ft; Residence C half the building height, minimum 25 ft. Accessory buildings (garages, sheds) are otherwise allowed 6 ft from a side line (§375-3.2.3.1); see the 760 CMR 71.03(3)(b)2 note under rear setbacks. Front setback: Per QZO Table 4.1: 25 ft in Residence A and B. In Residence A the building must also match the average front setback of houses within 300 ft on the same side of the street, never less than 25 ft or more than 55 ft (Table 4.1 note 1). Residence C: half the building height, minimum 25 ft. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
None if the ADU is within 0.5 mile of a Transit Station (760 CMR 71.02 counts subway, commuter rail, ferry and MBTA bus stops — Quincy has four Red Line stations); otherwise 1 additional off-street space (ISD guideline; state max). Existing off-street parking may not be removed to create the ADU and must comply with QZO §375-5.1: in Residence A/B, spaces 3 ft from side and rear lines, no enclosed parking in the front yard, driveway max 20 ft wide. 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 — 760 CMR 71.03(2)(a) bars any owner-occupancy condition on a Protected Use ADU and Quincy's guidelines impose none. The ADU may not be sold or owned separately from the house (ISD guideline: 'ADUs shall not have separate ownership'). 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 Not published — Quincy ISD does not post building-permit review times; 760 CMR 71.03(1) requires an as-of-right review with no hearing or discretionary approval. Confirm the current queue with Inspectional Services. Plan check: ISD reviews the site plan (drainage, utilities, parking) and architectural drawings against 780 CMR and the guidelines, screens the parcel against the FEMA 100-year Flood Hazard Map (no ADU below base flood elevation) and the QZO §8.1 Floodplain Overlay District; the Conservation Commission reviews construction in flood plain areas and near wetlands. Pre-approved plans availability: None — Quincy publishes no pre-approved or pre-reviewed ADU plan set; ISD's 'Helpful Forms and Checklists' list only the ADU guidelines and the Table of Dimensional Requirements.
Average ADU rental income in Quincy: 1-bedroom units rent for $2,250-$2,650/month, 2-bedroom units for $2,650-$3,150/month. Short-term rentals: No — the ISD ADU guidelines bar using an ADU as a short-term rental (permitted by 760 CMR 71.03(3)(c)), and Municipal Code §196-14 (Order 2020-125, adopted Mar 15, 2021) limits short-term rentals (under 31 days) to the o. Expected ROI: 7-14% gross rent yield (estimate).
The minimum lot size for an ADU in Quincy is None — 760 CMR 71.03(3)(b)2 bars a minimum lot size for a Protected Use ADU, and 71.02 protects a principal dwelling 'regardless of whether it, or the Lot it is situated on, conforms to Zoning'. District minimums (Residence A 7,650 sq ft, Residence B 6,750 sq ft, Table 4.1) do not block an ADU on an existing undersized lot. 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 $100,000-$250,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.
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