What Is an ADU? A Straightforward Answer for Connecticut Homeowners

An Accessory Dwelling Unit (ADU) is a fully self-contained, legally separate living space built on the same property as a single-family home. It has its own entrance, kitchen, bathroom, and bedroom — a complete home at smaller scale. You might also hear it called an in-law suite, granny flat, mother-in-law apartment, backyard cottage, or secondary dwelling. Same thing, different names. In Connecticut, the legal term is ADU.

There are three main types Lagace Construction builds:

• Attached Addition: A new structure connected directly to your main home — sharing a wall but with a completely separate entrance and interior. Common when you have side yard space but limited rear yard.
• Detached Cottage: A standalone structure built in your backyard, side yard, or elsewhere on your lot. Often called a “backyard cottage” or “carriage house.” Offers the most privacy for occupants and owners alike.
• Basement Conversion: Converting an existing unfinished basement into a legal apartment. Usually the most cost-effective ADU type since the foundation and structure already exist. Requires egress windows, separate entrance, and meeting current code for habitable space.
• Garage Conversion / Garage with Living Above: Converting an attached or detached garage, or building a new structure with a garage on the ground floor and living quarters above. Very popular in CT towns with tight lot coverage rules.

Who Builds ADUs in Connecticut — and Why?

In our 26+ years working in eastern Connecticut, we’ve built ADUs for a wide range of homeowners:

• Aging parents moving onto the property — Stay close without living on top of each other. Parents get independence; adult children get peace of mind.
• Adult children returning home — College grads or young families who want to save money but still need their own space.
• Rental income — A legal, permitted ADU can generate $1,200–$2,000+/month in rent in eastern CT. Long-term rental or short-term (Airbnb-style) depending on town regulations.
• Multigenerational living — CT families are increasingly choosing to keep multiple generations on the same land. It makes financial and practical sense.
• Home office or studio — Not legally required to be rented. Some homeowners build ADUs as dedicated workspaces or art studios that could become housing later.

Connecticut ADU Law — What Changed in 2026

Connecticut passed significant ADU legislation that took effect in 2023 and has continued to evolve through 2026. Under current state law, most single-family residential zones in Connecticut must allow at least one ADU per lot as-of-right — meaning towns cannot flat-out ban them. Local zoning can still regulate setbacks, lot coverage, maximum size, and design standards, but they cannot prohibit ADUs entirely in single-family zones.

This means the question for most CT homeowners is no longer if they can build an ADU — it’s how and what size. Town rules still vary significantly. Some towns in Tolland and Windham counties have adopted friendly ADU ordinances; others still have restrictive setback and lot coverage requirements that limit your options. That’s why our first step is always a free on-site feasibility review — we look at your specific lot and your specific town’s rules before you spend a dollar on design.

ADU Cost in Connecticut — 2026 Ranges

One of the first questions we get: “How much does an ADU cost in Connecticut?” The honest answer is — it depends heavily on the type, size, site conditions, and finishes you choose. Here are realistic 2026 ranges based on what we’re actually building:

ADU Type	Typical Size	Cost Range (2026)
Basement Conversion	400–700 sq ft	$80,000–$150,000
Attached Addition	400–800 sq ft	$150,000–$280,000
Detached Cottage	400–700 sq ft	$200,000–$380,000
Garage with Living Above	500–900 sq ft	$220,000–$420,000

What Drives ADU Cost in Connecticut?

These ranges are wide for a reason. Here’s what pushes costs up or brings them down:

• Foundation type: Slab-on-grade is cheapest. Full basement with frost wall adds cost. Pier or helical piles for difficult terrain add more. A basement conversion avoids foundation cost entirely — the biggest cost advantage of that type.
• Utilities and connections: Every ADU needs its own electrical panel and service. Running a new water line and drain from the main house is straightforward; running it 150 feet to a detached structure costs more. In rural Windham and Tolland County, septic capacity is often the biggest variable — you may need a new system or an expansion, which can add $15,000–$40,000 to the project.
• Egress and code upgrades: Basement conversions require code-compliant egress windows (and sometimes window well excavation), smoke/CO detection, ceiling heights, and stair width. These are non-negotiable and add real cost to conversions.
• Kitchen and bath finishes: A functional but modest kitchen package runs $12,000–$22,000. A higher-end kitchen with solid stone counters and custom cabinetry can be $40,000+. Same with bathrooms.
• Permits and engineering: We handle all permit applications. Permit fees in CT towns vary from a few hundred dollars to several thousand. Structural engineering or stamped drawings are sometimes required by towns — especially for detached structures or anything with complex framing.
• Lot conditions: Steep grades, rocky ledge, wetland buffers, and difficult access all add cost. We’ll identify these at the free site visit before you commit to anything.

Our estimates are detailed and written — not ballpark guesses over the phone. We want you to know exactly what you’re getting before you sign anything.

Our ADU Build Process — From First Call to Certificate of Occupancy

We’ve refined this process over 26 years of building in Connecticut. No surprises, no gaps. Here’s how a Lagace ADU project unfolds:

1. Free On-Site Feasibility Visit
   We come to your property — not your kitchen table, your actual lot. We walk the land, look at the existing structure, and talk about what you’re trying to accomplish. We review preliminary zoning setbacks and lot coverage on-site. This visit is free and carries zero obligation.
2. Town Zoning & Lot Review
   After the site visit, we pull your town’s ADU regulations, overlay them against your specific lot dimensions and lot coverage, and confirm exactly what’s buildable. If your town’s rules limit size or placement, we tell you — along with what options you still have. Some towns have ADU-friendly overlays; others require variances for certain configurations. You’ll know before spending a dollar on design.
3. Design Consultation
   We work with you on layout and functionality. What’s the unit for? Who’s living there? Do you need one bedroom or two? Dedicated laundry? Separate entry from the street? We bring floor plan concepts based on your site and needs. Architectural drawings are coordinated at this stage.
4. Permit Application — We Handle It
   We prepare and submit all permit applications: building permit, zoning compliance, electrical, plumbing, and any required structural drawings. We interface directly with the building department and zoning official so you don’t have to navigate town hall. Permit timelines vary by town from 4 weeks to 12+ weeks.
5. Construction with Weekly Updates
   Once permits are approved, construction begins. You get weekly written progress updates — not vague check-ins, actual milestone reports with photos. Our skilled local crew handles framing and electrical directly; we use only trusted licensed trade partners for HVAC and plumbing. You’ll always know who’s on your property and why.
6. Final Inspection & Certificate of Occupancy
   We schedule and manage all inspections — framing, electrical, plumbing, insulation, and final. When the building department signs off, you receive a Certificate of Occupancy. Your ADU is now a legal, permitted dwelling. Ready to rent, ready to occupy, and adds documented value to your property.

ADU Builder Serving Eastern Connecticut — Our Service Towns

We’re based in Andover, CT — the heart of eastern Connecticut. Our crew knows these roads, these town halls, these building departments, and these soil conditions. We don’t subcontract work out to crews from Hartford or New Haven and then disappear. When you hire Lagace, you get a team that’s been building in these specific towns for over 25 years.

Tolland County

Andover, Bolton, Columbia, Coventry, Ellington, Hebron, Mansfield, Somers, Stafford, Tolland, Vernon, Willington

Windham County

Ashford, Brooklyn, Canterbury, Chaplin, Eastford, Hampton, Killingly, Lebanon, Plainfield, Pomfret, Putnam, Scotland, Sterling, Thompson, Windham, Woodstock

New London County

Colchester, Salem, East Haddam

Hartford County

Avon, Canton, East Hartford, Farmington, Glastonbury, Manchester, Marlborough, Simsbury, West Hartford

Not sure if we cover your town? Contact us — if you’re in or near eastern CT, there’s a good chance we do.

Why Homeowners Choose Lagace Construction for Their ADU

• 26+ Years Building in Connecticut: We started in 1998. We’ve seen every soil condition, every town hall attitude toward ADUs, and every curveball a CT build can throw. That experience protects you.
• CT Licensed and Fully Insured: New Home Construction License NHC #NHC-0016939. Electrical License #E1-00191661. Fully insured with general liability and workers’ comp. Verify any contractor before you sign — it takes 30 seconds on the CT DEEP licensing portal.
• Skilled Local Crew — No Framing or Electrical Subs: Our core crew handles framing and electrical directly. We don’t farm these critical scopes to whoever’s available. You get consistent quality, accountable supervision, and people who know the job from day one to day last.
• We Pull All Permits and Manage Every Inspection: You should never have to walk into a town building department on our behalf. We handle permit applications, submissions, follow-up, and all required inspections. If a town wants a revised drawing, we deal with it.
• ★ 4.9 Google Rating: Nearly 5 stars across our reviews. Read them — they’re from real eastern CT homeowners who hired us for additions, ADUs, and renovations. We’re proud of our reputation and we intend to keep it.
• Written Estimates — No Surprises: Our estimates are detailed and in writing. Scope of work is clearly defined. Change orders require written approval. We don’t believe in verbal agreements and neither should you.

In-Law Suite vs. ADU — What’s the Difference?

Short answer: in Connecticut, not much. The terms describe the same type of space. Here’s the distinction worth understanding:

“In-law suite” and “mother-in-law apartment” are common terms homeowners use — often for a space that’s designed with a family member in mind and may or may not be a fully separate legal unit. Historically, some in-law suites in Connecticut were built without permits as interior room conversions — they had a kitchenette and a private entrance but weren’t legally classified as separate dwellings and weren’t on the tax record as such.

“ADU” (Accessory Dwelling Unit) is the legal term used in Connecticut zoning and building codes. An ADU is a formally permitted, legally separate dwelling unit. It has a legal certificate of occupancy, it’s on record with the town, and it legally may be rented to non-family members.

Why does this distinction matter to you?

• Rental legality: An unpermitted in-law suite cannot legally be rented. A permitted ADU can. If you want rental income, you need a real ADU with a CO.
• Home sale: A permitted ADU adds provable value to your property — appraisers can count rental income potential. An unpermitted space adds risk and can create complications at closing.
• Insurance: Unpermitted spaces may not be covered under your homeowner’s policy. A permitted ADU can be properly added to your coverage.
• Safety: CT building codes exist for a reason. Egress windows, smoke detectors, electrical panels, CO detectors — these aren’t bureaucratic boxes to check. They’re the difference between a safe home and a liability.

When homeowners call us asking about an “in-law suite,” we’re going to talk to them about a proper ADU — because that’s what protects them legally, financially, and physically. We’ve done plenty of work legalizing previously unpermitted in-law spaces too — bringing them up to current code and getting them properly permitted. If you have a space like that, let’s talk.

ADU Builder Connecticut — Frequently Asked Questions