The complete guide to accessory dwelling units
An ADU is a small, self-contained home on the same lot as your main house — a backyard cottage, in-law suite, garage apartment, or finished basement unit. Here's what they are, the forms they take, and how to know if one is right for your property.
What exactly is an ADU?
An accessory dwelling unit is a fully independent home that shares a lot with a primary residence. It has its own kitchen, bathroom, sleeping area, and private entrance — everything a household needs to live independently.
You can't sell an ADU separately from the main house, but you can use it however your town allows — housing family, generating rental income, or creating flexible space that grows with your needs. People also call them in-law suites, backyard cottages, carriage houses, or granny flats — see the ADU dictionary for plain-English definitions of these and other terms.
Four ways an ADU can take shape
Which one fits depends on your lot, your budget, and what your town allows.
Detached
A freestanding cottage in the backyard, fully separate from the main house. The most private option — and the one most tenants and family members prefer.
Attached
A new unit connected to the main house, often sharing a wall or roofline. Can be more cost-effective by reusing existing structure and utilities.
Garage or above-garage
Convert an existing garage, or build a carriage-house unit above one — a smart way to add a private space without giving up yard.
Basement or internal
A legal apartment carved from existing space inside the home, with its own entrance. Subject to egress, ceiling height, and code review.
One backyard, room for what matters most
Family & care
Keep aging parents, adult children, or a caregiver close — with privacy on both sides.
Rental income
Turn unused yard space into a steady stream that helps offset your mortgage.
Added value
Livable square footage buyers actively seek — a long-term investment in your equity.
Flexibility
A home office today, guest space next year, a downsizing option down the road.
A few things that shape what's possible
Every property is different. These are the factors that most often decide whether — and what — you can build.
Get a free feasibility review →Can you build one in Connecticut?
Connecticut set a statewide baseline in 2021, but roughly two-thirds of towns opted out and now apply their own rules. Most still allow ADUs — the answer starts with your address.