ADU rules in Stonington

Detached ADUs are allowed on single-family properties in all nine residential zones (GBR-130, RC-120, RR-80, RA-40, RM-20, RM-15, RH-10, RA-20 and RA-15) and in the DB-5, CS-5, LS-5, GC-60, TC-80, MC-80 and PV-5 districts, but never in the M-1, LI-130, HI-60 or HM districts. The conditions bite: Section 6.3.2.K bars an ADU outright where the property does not meet the district's minimum lot size, a new detached structure needs a Site Plan Application rather than a plain Zoning Permit, and converting an existing detached garage or barn is permitted only if that structure already meets current setbacks. A setback-nonconforming detached structure can be converted only if it qualifies as a historic structure and only by Special Use Permit, with screening and Commission findings. New detached construction may not sit closer to the street than the primary dwelling, and detached ADU structures are capped at 30 feet or the district height limit, whichever is less, and at two stories. Since the March 4, 2026 amendments the permit for new detached construction is a Site Plan Application, not the Special Use Permit that applied before.

An ADU inside the house is allowed by Zoning Permit in the same districts, but Section 6.3.2.B permits an ADU located within a principal structure only in a single-family residential structure of at least 2,000 square feet total gross floor area, exclusive of accessory buildings. That single threshold is the main thing keeping Stonington homeowners out: the Town Planner told the Planning and Zoning Commission at the October 4, 2022 opt-out hearing that reducing it 'is the main item that would enable more homeowners to permit ADU's,' and the March 2026 rewrite left it untouched. Section 6.3.2.F allows a building addition to accommodate an ADU provided all other requirements of the section are met, and Section 6.3.2.K's minimum-lot-size bar applies here too. Secondary access doors must be on the side or rear, and external stairs to upper stories must be at the rear.

Section 6.3.2.D caps an ADU at 1,100 square feet including stairs internal to the unit and excluding garage space, and at no more than two bedrooms. Section 6.3.2.C independently requires the ADU to be clearly subordinate to the primary residence and to contain no more than 50 percent of the total floor area of the primary residential unit, so the binding number is whichever is smaller: at the 2,000 square foot minimum house size an internal ADU tops out near 1,000 square feet, and the 1,100 square foot ceiling only binds once the primary unit passes about 2,200 square feet. A detached ADU structure is separately capped by Section 6.3.2.M.6 at 30 feet or the district's maximum building height, whichever is less, and at two stories including any ground-level parking even where the structure is elevated for flood hazard purposes. The 3-4-2026 amendment reworded Section 6.3.2.C from 'primary residential structure' to 'primary residential unit' to fix confusion about internal ADUs.

Nothing in Stonington's ADU regulation requires the owner to live on the property. Section 6.3.2 lists its conditions exhaustively at items A through M (one ADU per principal residence, the 2,000 square foot house minimum, the size caps, health code certification where there is no public water or sewer, door and stair placement, parking, curb cuts, minimum lot size, code compliance and the detached-structure rules) and none of them is an owner-occupancy or residency condition; the word 'occupancy' appears in the regulations only in relation to certificates of occupancy and unrelated uses. This is a local policy choice rather than a state constraint: CGS 8-2o's list of things municipalities may not require, at subsection (a)(6), does not include owner occupancy, and Stonington opted out of 8-2o(a) to (d) entirely in October 2022, so it could impose one if it chose to.

Section 6.3.2.I requires a minimum of one parking space measuring 9 feet by 18 feet for the ADU, and Section 6.3.2.J bars any additional curb cut to serve it except for a detached ADU on a corner or through lot. That one-space local rule is now in tension with state law: CGS 8-2(d)(9), as rewritten by Nov. Sp. Sess. P.A. 25-1, S. 18 effective July 1, 2026, forbids zoning regulations from requiring any minimum off-street parking for residential development except as provided in CGS 8-3n, and 8-3n permits minimums only for developments of more than sixteen dwelling units or inside one of at most two locally adopted conservation and traffic mitigation districts. A one-unit ADU is neither. Stonington's October 2022 opt-out reached only CGS 8-2o subsections (a) to (d) and does not shield it from 8-2(d)(9), and its regulations are expressly adopted under CGS 8-2 and 8-3, so the preemption applies squarely. The 29th Edition took effect March 4, 2026, before that change, and the town has not yet amended Section 6.3.2.I; confirm current practice with the Planning Department at 860-535-5095 before assuming the space is still required.

The use tables list the ADU as 'ZP/SPA' with a footnote pointing to Section 6.3.2 for the specific permit, and the answer depends on the kind of ADU. An ADU inside the house, or a conversion of an existing detached garage or barn that already meets the district's setbacks, needs only a Zoning Permit, signed off by the Zoning Official on the town's one-page Zoning Permit Application. Construction of a new detached structure containing an ADU requires a Site Plan Application under Section 6.3.2.M.1; that is a compliance review rather than a discretionary hearing, and Sections 15.3.2.A and 17.1.4 treat single-family dwellings and their associated accessory uses in residential districts as reviewed by the Zoning Enforcement Officer, though the regulations never say in so many words who decides a residential-district Site Plan Application for an ADU. Only one path is genuinely discretionary: converting a historic detached accessory structure that does not meet setbacks requires a Special Use Permit with a Commission hearing under Section 6.3.2.M.2.a. The March 4, 2026 amendments cut the permit for new detached ADU structures from a Special Use Permit down to a Site Plan Application and removed Architectural Design Review Board review of ADUs.

Section 6.3.2 says nothing about renting an ADU: there is no lease-term minimum, no primary-residence test, no registration and no cap on how long a tenant may stay. Short-term rental is likewise unregulated townwide, which matters in a town whose Mystic and borough-adjacent neighborhoods carry heavy visitor demand. The Department of Planning & Land Use's own handout states plainly that 'The Town of Stonington does not currently regulate short-term rentals,' and the town's A-to-Z index of Special Acts and Local Ordinances contains no short-term rental ordinance among its 133 entries. A proposed ordinance regulating short-term rentals went to a townwide referendum on March 13, 2023 and was not adopted. Stonington could still act: CGS 8-2o(a)(7)(B) expressly preserves a municipality's power to prohibit or limit ADU short-term rentals, its 2022 opt-out leaves it a free hand, and the town's own zoning consultant flagged in July 2025 that 'There is concern about the use of ADUs for short-term rentals, which is not the intended use of the ADU regulation.' An ADU cannot be sold off separately: by definition it sits on the same lot as, and is subordinate to, the principal single-family dwelling.