Building an ADU in Connecticut: A Straight-Talk Guide for Homeowners

Daniel Peragallo
Jun 10
9 min read

Thinking about adding a backyard cottage, in-law suite, or rental unit to your property? Here's what you need to know in CT.

What is an ADU?
Costs
What Towns in CT Allow ADUs
ADU Build Process
FAQs
Further Resources

What Is an ADU?

An example of a Tiny House interior, a trending approach to building ADUs in Connecticut.

An ADU (Accessory Dwelling Unit) is a small, self-contained living space on the same property as a single-family home. Think: a backyard cottage, an above-garage apartment, or a finished basement with its own entrance. It has its own kitchen, bathroom, and living space. It’s not a duplex, and you can’t legally sell it separately — but it’s a fully livable space.

Some people build them for aging parents, others for returning kids, long-term rentals, or just some extra breathing room. An ADU can also include accessibility features from the start, which is a great option for those interested in "Aging In Place" remodels.

How Much Does It Cost to Build an ADU in CT?

It depends on a ton of details, but as a general starting point, building ADUs in Connecticut typically costs anywhere between $80,000 and $250,000+, depending on what you're building, where you're building it, and how much of the work is custom vs. off-the-shelf.

What Impacts ADU Costs

Type: Detached units are most expensive. Conversions (garage or basement) usually cost less.
Size: Smaller doesn’t always mean cheaper — compact layouts still need plumbing, electrical, HVAC, etc.
Site Work: Slopes, ledge, tree removal, or long utility runs all drive up cost.
Utilities: Septic or sewer work or installing new bathrooms heavily increases costs.
Permitting & Plans: Some towns need just admin approval. Others require full hearings and engineered drawings.
Finishes: Builder-grade vs. custom kitchens, tile, and trim can swing the price tens of thousands.

Industry Average ADU Costs

$40K–$180K

Basement Conversions

$50K–$200K

Garage Conversions:

$80K–$300K

Attached ADUs

$120K–$350K+

Detached ADUs

(This does not represent Peragallo Construction's pricing.)

Main Types of ADUs

There are a few main ADU types allowed in most Connecticut towns:

Detached ADU: A backyard cottage or standalone unit.
Attached ADU: An addition to your existing home.
Garage Conversions: Turn your garage into living space.
Basement/In-Law Suite: Fully separate living space below grade.
Second Story Additions: Building over a garage or existing structure is sometimes allowed.
Converted Barns or Pool Houses: Possible, depending on code.

Some towns will limit square footage, height, or even the number of bedrooms. Always double-check.

Are ADUs Legal in Connecticut?

Short answer: Yes, but it depends on your town.

Public Act 21-29 (House Bill 6107), technically allows ADUs statewide. Towns can still opt out of this law — and many have. Before you go drawing up plans, call your town's building or planning department and ask if ADUs are permitted.

Some towns allow them by right.
Some require a special permit.
Some have opted out altogether.
Some allow ADUs even on nonconforming lots.

Start here: CT General Assembly - Public Act 21-29

Which Towns in Connecticut Allow ADUs?

Unfortunately most lists of which towns have adopted the ADU bylaws are outdated, even ones shared online. The best way to confirm if a given town allows ADUs, without a risk of outdated information, is contacting the town, or the county zoning department using these links

Hartford County	Hartford Planning and Zoning Department
Fairfield County	Fairfield Town Plan & Zoning Department
Litchfield County	Town of Litchfield Land Use Department
Middlesex County	The Middletown Building Department
New Haven County	New Haven Zoning and Regulations Department
New London County	New London Planning, Zoning & Wetlands Division
Tolland County	Tolland Planning and Zoning Commission
Windham County	Windham Planning and Zoning Commission

Connecticut Towns: ADU Allowance Summary

Town	ADUs Allowed	Detached ADUs	Notes
Guilford	✅ Yes	✅ Yes	Up to 1,200 sq ft, rental allowed
Madison	✅ Yes*	❌ No	Opted out of state law, attached only
Old Saybrook	✅ Yes	✅ Yes	More flexible; rental to non-family allowed
Milford	✅ Yes*	❌ No	Attached only, opted out
Westbrook	✅ Yes	✅ Yes	State law applies
Durham	✅ Yes*	❌ No	Local ordinance only, requires renewal
Haddam	✅ Yes	✅ Yes	Local rules align with state guidelines
Berlin	✅ Yes*	❌ No	Restricted to senior or affordable housing use
North Haven	✅ Yes*	❌ No	Family use preferred
Clinton	✅ Yes*	❌ No	Coastal constraints apply
South Windsor	✅ Yes*	✅ Yes	Detached allowed with special permit
New Milford	✅ Yes*	✅ Yes	Very ADU-friendly
Bethel	✅ Yes*	✅ Yes	Admin approval only
Ellington	✅ Yes*	✅ Yes	Capped at 800 sq ft; 1 bed limit
Wethersfield	✅ Yes*	❌ No	Utilities must be shared
East Lyme	✅ Yes*	❌ No	Family use only in most cases

What's the Process to Build an ADU?

Here’s what the typical Connecticut ADU build looks like:

Feasibility Check: Calling your town, checking zoning, and making sure your project can even happen. If you already have a contractor, they usually do this for you.
Assemble a Team: Find your contractor, and if you're hiring a separate designer, architect or design-build firm, make sure they are going to work well alongside your choice of GC.
Develop Plans: Your designer or GC will create floorplans, elevations, structural drawings, etc. These are usually approved alongside permits.
Get Permits: Submit to your local building department.
- If a special permit or zoning variance is needed, expect added time (and paperwork).
- Most contractors pass along the fee for permits to the homeowner.
- How long getting a permit takes depends heavily on your town.
Start Construction: Demolition, foundation + framing, utilities, etc. Most companies will ask you before completion if there are any issues, and fix them on a "punch list" of final tasks.
Final Inspections: Certificate Not all jobs require a final inspection, but most big remodels will. After that inspection, the job's done, and if you have a post-build payment in your contract, you'll likely make your last payment now. This is when most warranties will begin.

2024 + 2025 Connecticut ADU Law Updates

As of 2024, Connecticut has not completely overhauled ADU legislation at the state level, but towns continue to update local zoning in response to the flexibility granted by Public Act 21-29. Here are some key themes and changes:

More towns are revisiting previous opt-outs: Several municipalities are reevaluating whether to rejoin the state’s default ADU framework as demand increases.
Design requirements are getting stricter in some areas, especially in historic zones and coastal districts, with guidelines for materials, setbacks, and height limits.
Rental regulations: Some towns have introduced clearer language restricting short-term rentals like Airbnb, while others now allow ADUs for non-family tenants.
Permit streamlining in progressive towns: A few municipalities now offer simplified, administrative approvals rather than lengthy hearings or variances.
Utility hookups clarified: Towns are better defining when shared utilities are allowed vs. requiring independent water, sewer, or septic systems.
Size flexibility: While many towns still cap ADUs at 800–1,000 sq ft, some have raised limits to 1,200 sq ft to reflect modern usability needs.

These updates show a slow but growing shift toward more practical and accessible ADU integration across Connecticut, especially in towns trying to tackle housing shortages or attract multi-generational households.

Average Timelines

Anywhere from 4 to 10+ months, depending on your contractor’s schedule and how quickly your town moves. Jobs with structural changes, or lots of plumbing, HVAC, and electrical work, tend to take the longest: Especially when the work is due to a surprise pipe or wire found after demo.

Inspections are required at key phases: footings, framing, plumbing, electrical, insulation, and final sign-off. Each inspection step can add days or weeks if not scheduled ahead of time, or if your town is overbooked. Likewise towns with understaffed clerks offices will likely take longer to issue permits.

❓ FAQ: Common ADU Questions in CT

Design, Size & Structure

Are tiny houses considered ADUs in Connecticut?

What’s the smallest size an ADU can be in Connecticut?

Can I build an ADU over a garage or existing addition?

Can I turn a shed or pool house into an ADU?

Can I convert an existing barn, garage, or structure into an ADU?

Occupancy / Ownership

Can I live in my ADU and rent out the main house?

Can I rent out my ADU long-term in Connecticut?

Can I use my ADU as a short-term rental like Airbnb?

Can I sell an ADU separately from my house?

Does an ADU need its own address or mailbox?

If I move or sell my house, does my ADU need demolished?

Maintenance

What kind of maintenance does an ADU require?

An ADU requires regular maintenance just like any home — including roof checks, plumbing, HVAC servicing, and appliance upkeep.

Does an ADU improve resale value in Connecticut?

Utilities, Parking & Systems

Do I need separate utility hookups for an ADU?

Do ADUs need a separate HVAC or heating system?

Can an ADU share a driveway, or does it need its own parking?

Permits

Do I need a permit to build an ADU in Connecticut?

Do I need to notify neighbors before building an ADU?

What inspections are required to build an ADU?

Do ADUs in Connecticut need a full kitchen?