Everything homeowners ask about building an ADU in Medford — answered with 2026 data.
Yes, ADUs are allowed in SF-1, SF-2, GR, APT-1 and APT-2 — every Medford district in which a detached single-family dwelling is a permitted use (Medford Zoning Ordinance Table A, row A.1; districts listed at § 94-2.1). That is wider than most homeowners assume, because 760 CMR 71.02 defines a "Single-family Residential Zoning District" as "[a]ny Zoning District where Single-family Residential Dwellings are a permitted or an allowable use, including any Zoning District where Single-family Residential Dwellings are allowed as-of-right or by Special Permit" — so the state's by-right ADU protection is not confined to SF-1 and SF-2. Detached single-family dwellings are not permitted in ROS, APT-3, C-1, C-2, I, O, O-2 or MUZ, so no protected-use ADU arises there. Confirm your parcel's district with Medford Planning, Development & Sustainability at (781) 393-2480 or the Building/Zoning Department at (781) 393-2509 in Medford, MA. The maximum size for a detached ADU is The smaller of 900 sq ft or half the gross floor area of the principal dwelling. On this point Medford and the Commonwealth agree: "The accessory dwelling unit shall not be larger in floor area than one-half the floor area of the principal dwelling, or 900 square feet, whichever is smaller" (Medford Zoning Ordinance § 94-8.2.3(4)), and M.G.L. c. 40A § 1A defines an ADU as one that "is not larger in gross floor area than 1/2 the gross floor area of the principal dwelling or 900 square feet, whichever is smaller." Note which way that cuts: if your house is under 1,800 sq ft of gross floor area, the half-GFA rule binds long before the 900 does — a 1,400 sq ft house buys a 700 sq ft ADU. Gross floor area is measured under 760 CMR 71.02 as the sum of all stories of compliant ceiling height including basements and lofts, "but excluding crawl spaces, garage parking areas, attics, enclosed porches and similar spaces," so an unfinished attic or an attached garage does not enlarge your allowance. Medford's own text lets the Community Development Board make an exception "in the event that the detached accessory structure has some preservation value, as with a carriage house or a barn" — that is a discretionary route above 900 sq ft, not a right. Attached ADUs can be up to The same limit — the smaller of 900 sq ft or half the principal dwelling's gross floor area. Medford has an ADU-friendliness score of 7/10. Check our full Medford ADU rules page for complete zoning details.
ADU construction in Medford typically costs $230,000-$400,000. The cost per square foot averages $300-$500. Garage conversions are more affordable at $115,000-$230,000, while prefab ADUs cost $165,000-$330,000. Permit fees add From the city's posted Schedule of Building Fees (effective July 28, 2009 and still the version linked from the Building Department page): $15.00 per $1,000.00 of estimated construction cost, plus a $35.00 application fee; a Plan Review Fee for 1-4 family dwelling units of $100.00; and a certificate of occupancy for an existing residential building at $125.00. On a $250,000 ADU that is roughly $3,750 + $35 + $100, about $3,885, before separate plumbing and gas, electrical and sheet metal/mechanical permits, which are billed on their own schedules. Note the sting in the tail: work started without a building permit is charged a QUADRUPLE FEE to your budget. See our detailed Medford ADU cost guide.
Detached ADUs in Medford can be up to The smaller of 900 sq ft or half the gross floor area of the principal dwelling. On this point Medford and the Commonwealth agree: "The accessory dwelling unit shall not be larger in floor area than one-half the floor area of the principal dwelling, or 900 square feet, whichever is smaller". Attached ADUs are limited to The same limit — the smaller of 900 sq ft or half the principal dwelling's gross floor area. Junior ADUs (JADUs) have a cap of Not a category in Massachusetts — there is no junior ADU designation in M.G.L. c. 40A, in 760 CMR 71.00 or in the Medford Zoning Ordinance. An ADU carved out of the existing house is simply an ADU, subject to the same 900 sq ft / half-GFA cap. Maximum height is 35 feet and 2½ stories for the principal dwelling in every Medford residential district (Medford Zoning Ordinance Table B, row 1). Medford separately limits structures accessory to one- and two-family dwellings to 15 feet (§ 94-4.3.1(1)), which read literally would squeeze a detached ADU into a single storey. It should not apply: 760 CMR 71.03(3)(b)(2) makes unreasonable any dimensional standard — expressly including "bulk and height, and number of stories" — "that are more restrictive than is required for the Principal Dwelling, or a Single-family Residential Dwelling or accessory structure in the Zoning District in which the Protected Use ADU is located, whichever results in more permissive regulation." The more permissive of the two is 35 feet. Take the regulation with you and confirm with the Building/Zoning Department at (781) 393-2509 before you draw a second storey. You are allowed up to 1. Medford allows one: "Not more than one accessory dwelling unit may be established on a lot" on a qualifying lot.
Rear setback: 3 feet for a detached ADU, or 1 foot if it is built with fire-resistance-rated wall assemblies. Medford requires a 15-foot rear yard for a detached single-family dwelling (Table B, row 1) but only that "[a]ny accessory structures must be set back a minimum of three feet from rear and side lot lines, or a one foot separation from lot lines if constructed of fire-resistance wall assemblies, as defined by the State Building Code" (§ 94-4.3.2(3)). Under 760 CMR 71.03(3)(b)(2) the more permissive of the two standards controls, which is the 3-foot accessory-structure rule. One local constraint that does bite and is not a setback: a building accessory to a single- or two-family detached dwelling "shall not be less than ten feet from that detached dwelling unless part of and attached to that dwelling" (§ 94-4.1.2(3)(a)), so plan for a 10-foot gap between the house and the cottage. Side setback: 3 feet from the side lot line for a detached ADU (Medford Zoning Ordinance § 94-4.3.2(3)), or 1 foot with fire-resistance-rated wall assemblies — again more permissive than the 7½-foot side yard Table B requires of the house, and so the standard that governs under 760 CMR 71.03(3)(b)(2). Read it together with § 94-4.3.2(1): "In residential districts, no accessory structure shall be located within the required front or side yard or any court but may be built in the rear yard." The practical result is that a detached ADU belongs in the rear yard, and inside that rear yard it needs 3 feet of clearance to the side and rear lines. Front setback: 15 feet for the principal dwelling in all residential districts (Medford Zoning Ordinance Table B, row 1), and a detached ADU may not sit in the required front yard at all — § 94-4.3.2(1) bars accessory structures from the required front or side yard. An ADU inside or attached to the house takes the house's own 15-foot front yard. Medford also bars off-street parking within the front yard in the SF-1, SF-2 and GR districts (§ 94-6.1.4(3)). Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
Probably none, and never more than one space. Medford's by-law asks for "no more than one (1) additional parking space, when necessary and feasible" (§ 94-8.2.3(7)). State law caps that and, over most of Medford, cancels it: a municipality may not require "[a]ny additional on-street or off-street parking space for a Protected Use ADU if any portion of its Lot is located within a 0.5 mile radius of a Transit Station" (760 CMR 71.03(2)(b)(2)), and may not require more than one space outside that radius. "Transit Station" is defined at 760 CMR 71.02 to include a "Bus Station," which is in turn "[a] location serving as a point of embarkation for any bus operated by a Transit Authority" — on its face, any MBTA bus stop. Medford also has the Orange Line at Wellington, the Green Line Extension at Medford/Tufts and Ball Square, and Lowell Line commuter rail at West Medford. Between them there is very little of the city that sits more than half a mile from something that qualifies. Confirm how the Building Department is applying the half-mile radius at (781) 393-2509 before you give up a parking space, and note that Medford's general requirement of 2 spaces per dwelling unit (parking code A, § 94-6.1.3) does not reach a protected-use 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 required — and this is the single provision where Medford's published rules are most clearly wrong. Section 94-8.2.3(2) still says "[a]n affidavit shall be provided stating that one of the two dwelling units shall be occupied by the owner of the property," and § 94-8.2.3(8) makes the permit expire after five years unless the owner re-files that affidavit. Both are superseded. M.G.L. c. 40A § 3 provides that the use of land or structures for an ADU "shall not require owner occupancy of either the accessory dwelling unit or the principal dwelling," and 760 CMR 71.03(2)(a) lists an owner-occupancy requirement first among the Prohibited Regulations. 760 CMR 71.03(4) goes further and bars a municipality from enforcing such a condition even where it was imposed before January 31, 2025. The related ban on holding the ADU "in separate ownership from the principal structure/dwelling unit" in the same subsection travels with the owner-occupancy scheme — ask the Building Commissioner in writing how the city is applying it. 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 Medford is typically Confirm with the Medford Building Department at (781) 393-2509 — the city publishes no ADU-specific review timeline. What you can count on is that there is no hearing to wait for: because one ADU is a protected use under M.G.L. c. 40A § 3, there is no Community Development Board agenda, no abutter notice and no 20-day appeal period on a special permit, so the clock is ordinary building-permit review rather than a discretionary land-use calendar. Plan check: Standard building plan review; the state building code requires three sets of plans, and Medford charges a $100.00 Plan Review Fee for 1-4 family dwelling units. Two Massachusetts-specific items to check early. First, Title 5: if the property is on a septic system rather than sewer, 310 CMR 15.000 applies and is expressly preserved by M.G.L. c. 40A § 3, and MassDEP issued Title 5 guidance for ADUs updated September 11, 2025 — the local Board of Health, not the Building Department, is the authority. Second, utilities: 760 CMR 71.03(3)(b)(3) says a municipality "may not require a separate utility connection, such as water, sewer, electric," though the utility itself, a state or regional board, or a court still can. Every ADU must also be assigned an address to the MassGIS standard and reported to MassGIS and EOHLC (760 CMR 71.03(8)). Pre-approved plans availability: None from the City of Medford — no city plan gallery appears on the Building Department or Planning pages as of September 1, 2026. The Commonwealth fills the gap: EOHLC's Accessory Dwelling Units page hosts a free, searchable library of downloadable ADU designs alongside financing and technical-assistance programmes. Start there before you commission a custom set.
Average ADU rental income in Medford: 1-bedroom units rent for $2,100-$2,600/month, 2-bedroom units for $2,600-$3,200/month. Short-term rentals: No — 30-day minimum. "Where the accessory dwelling unit or the principal dwelling is occupied as a rental unit, the minimum occupancy or rental term shall be 30 days" (Medford Zoning Ordinance § 94-8.2.3(6)). This one survives the state law rather than falling to it: M.G.L. c. 40A § 3 expressly allows an ADU to "be subject to restrictions and prohibitions on short-term rental, as defined in section 1 of chapter 64G," and 760 CMR 71.03(3)(c) repeats the point. Long-term letting is unrestricted and, with owner occupancy gone, you may rent both units. Expected ROI: 5-8% annual return.
The minimum lot size for an ADU in Medford is None may be imposed on the ADU itself: 760 CMR 71.03(3)(b)(2) provides that "a Municipality may not require a minimum Lot size for a Protected Use ADU." The underlying district minimums for the lot are 7,000 sq ft for a detached single-family dwelling in SF-1 and 5,000 sq ft in all other districts, with 35 feet of frontage, 50 feet of width and 55 feet of depth (Medford Zoning Ordinance Table B, row 1). Maximum lot coverage is 40 percent for a detached single-family dwelling. And if your lot or your house is already nonconforming, that is not a bar: 760 CMR 71.03(3)(b)(9) says a municipality "may not prohibit the development of a Protected Use ADU in an existing structure or Principal Dwelling, or Lot due to nonconformance". 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 Medford, a garage conversion typically costs $115,000-$230,000. 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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