ADU rules in Greenwich

Greenwich allows one detached ADU (the regulations call it an 'External ADU', an ADU in a separate new or pre-existing accessory building) per lot as an accessory to a single-family dwelling, in all seven single-family residential zones: RA-4, RA-2, RA-1, R-20, R-12, R-7 and R-6. Only one ADU is permitted per property, External or Internal, not both. Two conditions bite: the ADU must sit within lawful setbacks on a lot that conforms with or exceeds the zone's minimum lot size, and in R-6 it is allowed only where the existing structure is a single-family dwelling. An ADU on a nonconforming lot is still possible but requires a Special Permit from the Planning and Zoning Commission under Sec. 6-99(a)(3)(E), and in R-6 a lot under 7,500 sq ft (or one needing cross easements for access, circulation or drainage) goes to Special Permit under Sec. 6-98(a)(4)(A). This is Greenwich's own rule, not the state floor: the town opted out of CGS 8-2o before the January 1, 2023 deadline.

An 'Internal ADU', one contained within the existing or proposed interior of the Primary Unit, is allowed on the same terms as a detached one, in RA-4, RA-2, RA-1, R-20, R-12, R-7 and R-6. Sec. 6-99(a)(2) permits only one ADU per property, so a homeowner chooses Internal or External, not both. A basement, attic or wing conversion counts as Internal; attached ground-floor garage space may be converted only if the required off-street parking is still provided on the property (Sec. 6-99(a)(3)(I)). Design rules apply: the ADU must avoid any architectural characteristic that makes the lot appear to contain a two-family dwelling, and for an Internal ADU the front doors of the Primary Unit and the ADU should not have equal prominence, especially if both are visible from the street.

Sec. 6-99(a)(3)(D) sets the caps by zone for detached units and a single percentage cap for internal ones. The Planning and Zoning Commission may authorize a larger ADU on a Special Permit application under Sec. 6-99(a)(3)(E), on an express finding that the ADU's scale, design, size or location keeps it compatible with its zone and, if External, subordinate in appearance to the Primary Unit; a July 2026 application for a 2,238 sq ft barn conversion at 295 Taconic Road in RA-4 is a live example of that path. Two limits sit outside Sec. 6-99 and are easy to miss. A detached ADU is an accessory building, so Sec. 6-144(a) caps it at 25 feet in height and requires 5 feet of separation from another structure. And the ADU's floor area counts toward the lot's overall Floor Area Ratio, which Sec. 6-5(a)(23) computes across 'all buildings on a lot (including accessory structures having walls and a roof)'; Sec. 6-99(a)(3)(K) requires FAR calculations for both the ADU and the Primary Unit with the application, so on a tight lot the FAR ceiling, not the square-foot cap, is often the real constraint.

Sec. 6-99(a)(3)(A) requires the owner of record to reside in either the ADU or the Primary Unit. The requirement is both at application and ongoing: an initial affidavit is filed with the application and an annual affidavit acknowledging continued compliance is required every year thereafter, on the town's ANNUAL AFFIDAVIT for ACCESSORY APARTMENTS form (2026 edition posted on the Land Use forms page). The same paragraph adds that the owner 'shall retain title to both structures at all times', which forecloses selling the ADU separately. Enforcement has teeth: Sec. 6-99(a)(5) makes any ADU approval automatically revocable if the owner fails to file a required affidavit on time, or if the Zoning Enforcement Officer finds the property does not in fact comply with the occupancy standards. Nothing in state law disturbs this: CGS 8-2o(a)(6) lists the conditions a town may not impose and owner occupancy is not among them, and Greenwich opted out of 8-2o entirely in 2022.

Sec. 6-99(a)(3)(F) requires at least one off-street parking space on an appropriate solid surface dedicated to the ADU; no additional driveway or curb cut may be created for the primary purpose of serving it (Sec. 6-99(a)(3)(H)), and attached ground-floor garage space or an existing detached garage may be converted to an ADU only if the required off-street parking is still provided on the property (Sec. 6-99(a)(3)(I)). In R-6 that space may be tandem, by the cross-reference in Sec. 6-99(a)(3)(M) to Sec. 6-155(1)(d). State law now constrains enforcement of the one-space rule: effective July 1, 2026, CGS 8-2(d)(9), as rewritten by Nov. Sp. Sess. P.A. 25-1, S. 18, bars zoning regulations from requiring 'a minimum number of off-street motor vehicle parking spaces for any residential development except as provided in section 8-3n', and CGS 8-3n(a) bars rejecting a residential application solely on parking grounds unless the commission finds a specific adverse impact on public health and safety that approval conditions cannot mitigate. Greenwich's opt-out does not reach that: CGS 8-2o(f) covers only 8-2o(a) to (d), and the town's separate 2022 parking opt-out ran through CGS 8-2p, which P.A. 25-1, S. 53 repealed effective January 1, 2026. Greenwich responded under CGS 8-3n(d) by adopting two conservation and traffic mitigation districts (the statutory maximum) on July 21, 2026, effective July 24, 2026, in which minimum parking may still be required for residential developments of fewer than sixteen units. Treat the one dedicated space as squarely enforceable inside those two districts and as constrained by 8-2(d)(9) and 8-3n elsewhere in town, and confirm with Planning and Zoning which regime covers a given address (chapter 124 as amended: https://www.cga.ct.gov/2026/sup/chap_124.htm).

The normal path is administrative. Sec. 6-99(a)(4)(A) requires approval from the Planning and Zoning Office (the Town Planner or designee) before any conversion or construction, or any building permit or certificate of occupancy; the applicant files the ADU application form, the owner-occupancy affidavit, scaled drawings with FAR calculations, and a Health Department septic endorsement or DPW Sewer Division endorsement. No public hearing is involved. The PLPZ 2026 00190 amendment approved 6/23/2026 (effective 7/1/2026) made this explicit by listing 'Creation of an ADU subject to Sec. 6-99' under Administrative Site Plan Approval in Sec. 6-13(c). A discretionary hearing before the Commission is triggered only in defined cases: an ADU larger than the size cap or on a nonconforming lot needs a Special Permit under Sec. 6-99(a)(3)(E) and Sec. 6-17; an R-6 lot under 7,500 sq ft or one requiring cross easements needs a Special Permit under Sec. 6-98(a)(4)(A); and any proposal inconsistent with the standards of Sec. 6-99 is referred to the Commission for formal site plan review.

Renting the ADU long term is the point of the regulation: Sec. 6-95(a)(4) permits 'the rental and use for residential purposes of dwelling units in accessory buildings' that comply with Sec. 6-99. Short-term rental is banned outright for ADUs: Sec. 6-93(a)(3)(e) states that 'Short-term Residential Rental of accessory units (Section 6-99) ... is prohibited', and Sec. 6-5(a)(43.4) defines a Short-term Residential Rental as a rental of part or all of a residential dwelling for fewer than thirty consecutive nights. That ban was adopted 1/27/2022 and is zoning-based; Greenwich has no short-term-rental license or registration ordinance, and its Municipal Code has no STR chapter. Because the owner must live on the property and 'shall retain title to both structures at all times' (Sec. 6-99(a)(3)(A)), the ADU cannot be conveyed or subdivided away. The current regulation imposes no tenant income, age or family restrictions; the pre-2023 Affordable and Elderly Accessory Apartment regime that did impose them was repealed by the 7/14/2023 rewrite, though units approved under it still file the matching branch of the town's annual affidavit.