Building an ADU in Connecticut

What state law lets you build

Connecticut's ADU law took effect on January 1, 2022, through Public Act 21-29. It is now C.G.S. 8-2o. Under it, town zoning rules must allow "at least one accessory dwelling unit" as of right "on each lot that contains a single-family dwelling." Your town also cannot require the ADU to be an affordable (deed-restricted) unit.

"As of right" means the town approves the ADU by checking it against its zoning rules. No public hearing, variance, special permit or other discretionary vote is needed, other than a finding that your site plan meets the zoning rules.

State law defines an ADU as a separate home on the same lot as a larger main home. It has its own cooking facilities and meets the building code, fire code and health and safety rules. The ADU can be inside the main home, attached to it, or detached on the same lot.

Public Act 26-7 took effect on October 1, 2026. It renamed "accessory apartment" to "accessory dwelling unit" throughout the statutes. It also extended the ADU rules to towns that zone under a special act rather than the general zoning statute, and extended the utility-fee limits to investor-owned water companies (see below). [1][2][3][4]

Did your town opt out?

The 2021 law let each town opt out of the ADU rules. Opting out took a two-thirds vote of the zoning commission after a public hearing, then a two-thirds vote of the town's legislative body (or the board of selectmen in a town-meeting town). The window closed on January 1, 2023. The statute says that "on and after January 1, 2023, no municipality may opt out."

The opt-out covers subsections (a) to (d) of the law, which hold the as-of-right rule, the size, setback, parking and design limits, the 65-day decision and the utility-fee limits. In a town that opted out, its own zoning rules decide whether and how ADUs are allowed, except for the transit-district rule below.

We did not find an official state list of which towns opted out. Ask your town's planning and zoning office whether it opted out and what its ADU rules say. A 2026 bill to repeal the opt-out, HB 5507, was tabled in the House on April 15, 2026, and the opt-out wording is still in the statute as Public Act 26-7 rewrote it.

If your town did not opt out and never updated its rules, the law says any noncompliant local rule became void. The town must then decide ADU applications under the state standards and "may not use or impose additional standards" beyond them. [1][3][5]

The transit-district exception

Since January 1, 2026, you can build an ADU as of right even in a town that opted out, if two things are true. Your property must be inside a "transit-oriented district," and you must have owned property in that district for at least three years. The ADU still has to meet any structural or architectural requirements in the town's zoning rules.

Under C.G.S. 8-13hh, a transit-oriented district is a group of parcels that a town designates itself, with zoning "designed to encourage increased density of development, including mixed-use development." Ask your planning and zoning office whether your lot is in one. [1][6][7]

What your town can and can't require

In a town that is covered, zoning rules for ADUs must:

• allow an ADU that is attached, inside the house, or detached on the same lot;
• set any maximum size at no less than 30% of the main home's net floor area or 1,000 square feet, whichever is less (towns may allow bigger);
• require setbacks, lot size and frontage no stricter than for the main home;
• not require height, landscaping or design standards stricter than those for single-family homes in town.

For example, for a 2,000 sq ft house, 30% is 600 sq ft, so your town's size cap must allow at least 600 sq ft. For a 4,000 sq ft house, 30% is 1,200 sq ft, so the cap must allow at least 1,000 sq ft.

Your town also cannot require:

• a passageway between the ADU and the main home;
• an exterior door, unless the building or fire code requires one;
• more than one parking space, or a fee in lieu of parking;
• that the people living there be related to you by family, marriage or employment;
• a minimum age for the people living in the ADU;
• separate utility billing, or periodic permit renewals.

A town cannot make you fix a nonconforming use, structure or lot before it approves the ADU. It cannot require fire sprinklers in the ADU unless the main home needs them or the fire code requires them. Building code rules, and rules for lots on a well or septic system, still apply, but approval "shall not be unreasonably withheld." [1][2]

How fast your town must decide

The as-of-right process must produce a decision within 65 days after the zoning commission receives your application. You can agree to one or more extensions of up to another 65 days, or withdraw the application.

Building code requirements still apply to the ADU. See our ADU permit process guide for the usual steps. [1][2]

Water, sewer and utility fees

A town, special district, or sewer or water authority cannot treat an ADU as a new residential use when it calculates connection fees or capacity charges, unless the ADU was built along with a new single-family home on the same lot. It also cannot require a new or separate utility connection to the ADU, or charge a related connection fee or capacity charge.

Since October 1, 2026, these limits also apply to investor-owned water companies. [2][3]

Garage conversions

State law does not mention garages by name. It does allow detached ADUs on the same lot as the main home, and it bars a town from making approval depend on correcting a nonconforming use, structure or lot. The building and fire codes still apply. Read our garage conversion guide for the general steps. [1][2]

Renting it out

Nothing in the law limits a town's power "to prohibit or limit the use of accessory dwelling units for short-term rentals or vacation stays." Check your town's rules before listing an ADU on a nightly-rental site. [1][2]

If your town's rules don't match the state law

In a covered town, state law voids local rules that don't comply, and the town must judge your application by the state standards. You can read the current statute on the Connecticut General Assembly website. The changes from Public Act 26-7 apply from October 1, 2026.

Start with your town's planning and zoning office. Ask whether the town opted out, which accessory dwelling unit rules apply to your lot, and whether your lot is in a transit-oriented district. The legislature's research office has a plain-English summary of Public Act 26-7. [1][2][3]

Connecticut laws to know

• Public Act 21-29, Section 6 (2021)

  Created C.G.S. 8-2o, effective January 1, 2022: towns must allow at least one accessory apartment as of right on each lot with a single-family home, with a one-time opt-out. [1][4]

• C.G.S. Section 8-2o

  The ADU rules themselves: size floor, setbacks, one parking space at most, a 65-day decision, utility-fee limits, the opt-out process and the transit-district exception. [1]

• November Special Session Public Act 25-1, Section 13 (2025)

  Added subsection (g), effective January 1, 2026: an owner of at least three years in a transit-oriented district may build an ADU as of right even if the town opted out. [1][6]

• Public Act 26-7 (2026)

  Effective October 1, 2026: renames accessory apartments to accessory dwelling units, extends the rules to towns that zone under a special act, and bars water companies from separate-connection and connection-fee charges. [2][3]

Questions people ask