Connecticut ADU permit requirements
Last reviewed: June 20, 2026
Quick answer
Permitting an accessory dwelling unit (ADU) in Connecticut means pulling a building permit from the local authority that issues it. Connecticut has a statewide ADU law that sets baseline standards local agencies must follow. ArchPermit tracks 14 source-cited requirements (12 required, the rest conditional) for this jurisdiction. These requirements were last reviewed June 20, 2026; confirm the exact forms and fees with the local permitting authority.
Connecticut has a statewide ADU law. This baseline reflects that law plus the universal new-dwelling deliverables. Your local city or county still administers the permit and may add its own forms and fees, so confirm the exact checklist with them.
Connecticut's Public Act 21-29 (2021) allows one ADU as of right on single-family lots, BUT towns could opt out by a two-thirds vote before Jan 1, 2023, and many did. Confirm the specific town's opt-out status; in opt-out towns the local rules govern.
- Adopted building code compliancerequired
The ADU must comply with Connecticut's adopted residential building code: 2022 Connecticut State Building Code, Residential (2021 IRC). Confirm the in-force edition with the local AHJ.
source · reviewed 2026-06-20 - Building cross-sectionsrequired
Cross-sections through the structure showing ceiling heights, wall/roof assemblies, insulation, and fire/sound-rated assemblies where required.
source · reviewed 2026-06-20 - Confirm the town's opt-out status FIRSTrequiredverify locally
The state as-of-right rule applies only if the town did NOT opt out by Jan 1, 2023; opt-out towns set their own ADU rules. Verify before relying on the state default.
source · reviewed 2026-06-20 - Egress & life-safetyrequired
Emergency escape and rescue openings (egress windows), smoke and carbon-monoxide alarms, and required exits per the adopted residential code.
source · reviewed 2026-06-20 - Energy code compliancerequired
Energy-code compliance documentation for the conditioned space. Connecticut: 2022 Connecticut State Building Code, Energy (2021 IECC).
source · reviewed 2026-06-20 - Exterior elevationsrequired
Elevations of each affected building face showing height, exterior materials, and openings; existing vs. proposed where exterior changes occur.
source · reviewed 2026-06-20 - Floor plansrequired
Dimensioned floor plans of the ADU (and affected areas of the primary dwelling) showing rooms, uses, door/window locations, egress, and fixture/equipment locations.
source · reviewed 2026-06-20 - Foundation planrequired
Foundation plan with footing/stem-wall sizes and details (or a cited site-specific design), per the adopted residential code.
source · reviewed 2026-06-20 - Framing / structural plansrequired
Floor, wall, and roof framing showing member sizes, spans, spacing, connections, and lateral bracing; structural calculations where the framing is non-conventional.
source · reviewed 2026-06-20 - Local zoning / setback compliancerequiredverify locally
Compliance with the local jurisdiction's zoning: permitted use, setbacks, height, lot coverage, and any ADU-specific standards. These are set locally. Confirm with the permitting agency.
source · reviewed 2026-06-20 - Mechanical / plumbing / electricalrequired
HVAC, plumbing fixtures and lines, and electrical (panels, circuits, outlets, lighting, smoke/CO alarms) for the new unit.
source · reviewed 2026-06-20 - Site / plot plan (to scale)required
Scaled site plan showing property lines, setbacks, existing structures, the proposed ADU footprint with dimensions to property lines, driveways/parking, and utility locations.
source · reviewed 2026-06-20 - ADU size cap (≤1,000 sf or 30%)conditionalverify locally
In non-opt-out towns, an ADU may be limited to ≤1,000 sq ft or 30% of the principal dwelling, whichever is less.
source · reviewed 2026-06-20 - Parking limited to one spaceconditionalverify locally
A municipality may not require more than one parking space for an ADU under the state default.
source · reviewed 2026-06-20
Connecticut ADU permit questions
- Do I need a permit to build an ADU in Connecticut?
- Yes. An accessory dwelling unit (ADU) in Connecticut requires a building permit, and Connecticut has a statewide ADU law that sets baseline standards local agencies must follow. ArchPermit tracks 14 cited requirements for this jurisdiction (12 required, the rest conditional). Always confirm the exact forms, fees, and code edition with the local permitting authority that issues the permit.
- What does an ADU permit package require in Connecticut?
- Core required items include: Adopted building code compliance; Building cross-sections; Confirm the town's opt-out status FIRST; Egress & life-safety; Energy code compliance; Exterior elevations, plus 6 more. Each requirement links to its authoritative source, and nothing is permit-ready until an independent licensed professional reviews and seals it.