ADU 101
Let’s start from the beginning
What is an ADU?
ADUs are self-contained homes located on the same lot as a primary residence. They include kitchens, bathrooms, and sleeping areas, and can be attached, detached, or conversions of existing space (like a garage or basement).
ADU Types
There are many types of ADUs, and their names may differ slightly based on where you are.
ADUs do not always fit neatly in one category – like an attached ADU made from converted space, for instance.
Detached
A standalone structure, such as a small cottage or backyard home. These typically provide the most privacy and design flexibility but are often the most expensive to build.
Attached
An addition to the main house or conversion of a connected space like a garage. These are often less costly than detached units and provide easier utility connections.
Internal conversion
Adaptations of existing interior space such as basements, attics, or underutilized rooms. These are usually the most cost-effective and quickest to construct but may involve trade-offs in privacy or storage.
Not an ADU
RVs and tiny homes on wheels typically do not qualify as ADUs in Maine. Contact your town’s planning department for confirmation.
FAQs
Can an ADU be used as a short-term/vacation rental?
Every town may apply its own ADU rental policies, so it’s essential to check with your local Planning Department or Code Enforcement Office to determine whether short-term rentals are allowed, if there’s a required minimum rental period, or if owner-occupancy is mandated.
When is my ADU ready for move-in?
Municipalities require both final inspections and a Certificate of Occupancy. Once the inspection is complete and the certificate issued, your ADU is ready for move-in—just be sure utilities are connected, an address is assigned, and all final preparations are in place.
See our blog post about renting your unit for more details about renting out the unit to long-term tenants. If moving friends or family into the unit, it’s still highly important to have a written agreement. Consult a lawyer or find an agreement template by searching online for phrases like “family rental agreement.”
What’s the difference between a site-built and a prefabricated or manufactured ADU?
There are two types of construction to consider and both have pros and cons, so think about the level of customization you want and how the ADU will work on your property.
Site-built ADUs
Traditionally constructed and built from scratch on your property (AKA “stick-built”), which often slows for a lot of customization and smaller changes throughout construction.
Typical benefits:
- More options for customization
- Can be easier to get financing and permits
- Can be easier to place on unique properties (steep slopes, rocky terrain, small spaces)
- Can typically be paid for phase-by-phase
Prefab, modular, and panelized ADUs
Built in a factory, then shipped to your site and assembled or placed on a foundation. Some companies provide “turnkey” service with support through the entire process (permitting, construction, etc.). If not, you’ll need to hire other professionals to help.
Prefab ADUs are categorized based on how they are built and inspected.
- Partially-fabricated (panelized, framing kits) Parts of the ADU are built off site at a factory, and construction follows a traditional site-built pattern. Local inspectors check work at different stages and a local general contractor and their crew are responsible for putting all the pieces together and passing inspections.
- Prefabricated (modular, manufactured, factory-built) The ADU is built and inspected for building code off site at a factory, then delivered and put in place on your property by a crane or tow. Local inspectors only check work like foundation and utility hook-ups. These types are all built similarly but are called different things based on what building code they meet (state, federal, etc.).
It’s important to note that prefab ADUs may cost the same as site-built ADUs – transportation, crane, foundation, and sales tax can end up being 20-40% of the total cost. When you get in touch with prefab companies, be sure to ask what is included in the price. Some companies do offer a lower price point.
Typical benefits:
- Lower design and engineering costs
- More clarity on what you’ll get in the end
- Faster construction with less on-site disruption
- Quality inspections from the start
- Fewer permits and inspections
How much does it cost to build an ADU?
General ADU cost estimates in Maine:
- Interior Conversion ADU: $30,000 – $100,000
- Garage Conversion ADU: $60,000 – $150,000
- Attached New ADU: $100,000 – $300,000
- Detached New ADU: $200,000 – $500,000
This cost varies widely based on your design and the requirements of your site and local municipality. See Budgeting for more detailed information on costs and financing for your ADU.
How long does it take to build an ADU?
Building an ADU is an investment of time as well as money. Most projects take 12-24 months to complete, with interior conversions being the fastest.
- Planning (learning the rules, setting goals, budgeting): 1-3 months
- Design: 1-6 months
- Permitting: 1-6 months
- Construction: 6-12 months
What do I do first?
This site walks you through each part of the ADU process, from learning the rules and gathering initial inspiration through construction.
We recommend starting with two things:
- Use our ADU Wizard by putting in your address and seeing what’s possible.
- Read our ADU Rules page and then walk through the pages of this site one-by-one for a detailed guide on what to do next.
Why Build an ADU?
Building an ADU (Accessory Dwelling Unit) is an investment in your property, your family, and your community—and it’s especially relevant here in Maine.
ADUs can help address the growing demand for affordable, flexible housing across the state. Whether in coastal towns, rural villages, or urban centers, ADUs provide much-needed housing options for local workers, aging Mainers, and small families.
Aging in Place
An ADU can provide a comfortable, accessible home for aging parents or yourself as you age—close to family, but with privacy and independence.
Keeping loved ones close
Create a safe and stable space for adult children, aging relatives, or family members with disabilities, right on your property.
Workforce Housing
ADUs offer local workers—like teachers, healthcare aides, and seasonal employees—an affordable place to live near their jobs.
Rental income
Earn additional income to support your retirement, property taxes, or unexpected expenses—whether renting the ADU or your main home.
Property value
A well-built ADU can increase the resale value of your property, especially as demand for flexible housing rises across Maine.
Caretaker Housing
Provides a space for a live-in caregiver or home health aide—especially important for elderly or disabled residents.
Downsizing without moving
Retirees can live in the ADU and rent out their main home, staying on their property but reducing space and maintenance needs.
Environmental Impact
Smaller homes typically require fewer resources and encourage walkability or shorter commutes for local workers, reducing carbon footprints.
Peace of Mind
Whether you're traveling or aging in place, an ADU brings an extra set of hands or eyes to the property.
Supporting Maine's Housing Goals
Help meet your town’s housing needs by adding gentle density that fits within the character of your neighborhood.
ADUs are not just about adding a building—they’re about creating options, supporting family, and strengthening communities across Maine.
Ready to ADU?
Navigate project phases using this section on each page.
ADU Rules →
Take the first step
Looking for more background info?
Check out these resources for help.
ADU Glossary
