ADU Rules
ADU Wizard
The Wizard tool is a starting point to imagine and explore possible options on your property. For the most accurate and up-to-date information, particularly for properties in a shoreline zone, check in with your town’s planning and/or code enforcement staff.
Overview
- Learn what you can build on your property
- Understand local rules
- Meet with local staff to discuss your project
Before you start dreaming up your ADU, you need to know what you’re allowed to build. We will help you navigate the state and local laws that apply to your property.
Project Timeline
Learning the rules is part of the Planning phase, which typically takes 1-3 months. Most ADU projects take 12-18 months to complete, but some extend to 24 months or more.
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.
What do I need to know about becoming a landlord?
Renting an ADU comes with many responsibilities, including understanding local and state housing laws, executing a lease, finding and managing a tenant, and maintaining a rental property. It’s important to understand the laws as they may affect things like future rent increases, changing use over time, evicting tenants, and moving family into the unit. See our blog post about renting your unit for more details.
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.”
Do I need to tell my neighbors?
You’re not required to tell your neighbors about your ADU, but it’s always a good idea to communicate with them early in the process. Your project will run more smoothly if they are kept informed, and they may have great ideas for your project!
If you live in a Neighborhood or Homeowners Association, talk with your representative or board early in the process. HOAs or subdivision deeds can explicitly restrict ADUs, rentals, multi-dwelling units, and similar uses.
What if I have an unpermitted ADU on my property already?
An unpermitted ADU can make it difficult to sell or refinance your property. If the ADU is being built, the City or County can stop construction and increase permitting and inspection fees. Existing unpermitted ADUs can go through the permitting process, and local staff will work with you to determine what would be involved to bring your ADU up to code.
If you already have an ADU on your property that was built without permits, you may be wondering how Maine’s new housing law (LD 2003) affects your situation. Here’s what you need to know:
Under LD 2003, the Unpermitted Units aren’t “Grandfather”, therefore LD 2003 does not automatically legalize unpermitted ADUs. Homeowners with existing, unpermitted units must still go through the local permitting process to bring their ADU into compliance. You may be able to retroactively permit the unit if it meets current zoning, building, life safety, and utility requirements.
What You Can Do If You Have an Unpermitted ADU:
- Contact your local Code Enforcement Officer or Planning Department
- Ask what the process is to legalize an existing ADU.
- They may require inspections, as-built drawings, or upgrades to meet current code.
- Prepare for inspections and possible upgrades
- You may need to upgrade electrical, plumbing, egress, or fire safety components.
- In some cases, structural work may be needed if the unit doesn’t meet building code.
- Check zoning compliance
- LD 2003 may make it easier to legalize your unit if it now meets the new standards (e.g., no lot size minimums, reduced parking, etc.).
- However, if the unit violates setback, size, or health/safety rules, it may not be permitted even under LD 2003
Be Aware of Possible Penalties
- Some municipalities may charge double permit fees or issue notices of violation if an unpermitted unit is discovered.
- However, many towns are willing to work with property owners to bring units into compliance—especially in support of affordable housing goals.
Note: Learning the rules about ADUs includes quite a few topics. Find the answers to these questions in the content below:
- Am I allowed to build an ADU? How many?
- Can I put an ADU in my garage?
- How large can my ADU be?
- Do I need to live in the main house or ADU?
- Can I rent my ADU as an AirBNB or other short-term tenal?
- Will I need to add parking?
- What if my property is in an Historic District or other special zone?
- I have a septic system or well – can I still build an ADU?
Learning the Rules, Step-by-Step
Keep in mind Understanding how local and state rules apply to your project can feel like a lot at first. Remember: Local staff are here to help. Contact Planning staff for your location to ask all your ADU questions. You may also want to check out our Glossary to get familiar with key terms.
Step 1
Use the ADU Wizard to See What’s Allowed on Your Lot
Insert your address above to be redirected to our ADU Wizard tool, which uses artificial intelligence to provide zoning information about your property. Note: This is just a first step and you will want to contact your code enforcement officer or the planning department in your municipality before making any decisions.
Using the ADU Wizard, local assessor websites, and public GIS portals, gather the following information:
- Parcel/Map/Block/Lot Number
- Lot size
- Zoning district
- Any overlay zones (e.g., historic or coastal zones).
Step 2
Learn What You Can Build
Using the ADU Wizard (see above) and your local planning and building website, learn about what you can build.
Make sure you learn about:
- Maximum size (most towns allow 600–1,000 sq ft)
- Height limits
- Setback and lot coverage requirements
- Whether garage or basement conversions are allowed
Statewide rules to keep in mind:
- Single-family homes are allowed only one ADU.
- Multi-family properties (such as duplexes, triplexes, and larger apartment buildings) are allowed to add:
- Up to two detached ADUs total, and
- Additional internal or attached ADUs, provided the total number of new units does not exceed 25% of the existing number of units on the property.
Local ordinances may further regulate or expand these allowances, so it is important to check with your municipal planning department for exact rules and any exceptions.
Step 3
Learn About Special Zones
Shoreland Zoning Districts
Resource Protection Districts (RP)
The most restrictive district. ADUs and other types of development may be limited or prohibited depending on site conditions.
Limited Residential, Limited Commercial, and General Development
Other Shoreland districts with varying levels of restriction.
Other Special Zones
Flood Hazard Areas (FEMA-designated)
Regulated by: FEMA, local floodplain ordinances
Description: Includes coastal and riverine floodplains (e.g., AE, VE zones). Portland and many coastal communities have these.
Implications: Elevation requirements, development restrictions, and insurance impacts. Building an ADU may be cost-prohibitive or require special permitting.
Check Flood Maps: FEMA Map Service Center
Historic Districts and Designated Historic Properties
Regulated by: Local Historic Preservation Commissions, Maine Historic Preservation Commission
Description: Many Maine towns (e.g., Portland, Bath, Hallowell) have designated Historic Districts or individually listed buildings.
Implications: Exterior changes often require review or Certificate of Appropriateness. ADUs may face design compatibility requirements.
Reference: Maine Historic Preservation Commission
Wildfire Risk Areas
Regulated by: Maine Forest Service
Description: Maine has low to moderate wildfire risk overall, but some inland or rural forested areas may face higher risk.
Implications: Less strict than California’s fire hazard zones, but defensible space may be encouraged for new structures.
Reference: Maine Forest Service
Resource Protection Districts
Regulated by: Local municipalities under Shoreland Zoning
Description: Sensitive environmental areas—like fragile wetlands, steep slopes, or wildlife habitat—may be designated RP zones.
Implications: Typically the most restrictive zone in a municipality; new structures (including ADUs) are often not allowed
Growth Areas vs Rural Areas (Under Comprehensive Plans)
Regulated by: Local zoning and land use codes
Description: Many Maine municipalities define “growth” and “rural” areas in their comprehensive plans to manage sprawl.
Implications: ADUs may be encouraged in growth areas and discouraged or limited in rural zones depending on infrastructure and policy goals.
Step 4
Learn Other Key Rules
Owner Occupancy: Maine’s state law (LD 2003) does not require the property owner to live on-site for an ADU to be permitted.
Short-term Rentals: Some towns regulate or restrict short-term rentals in ADUs, especially in areas with high housing demand or limited inventory. For example, Portland requires registration and limits the number of short-term rental units per property. Confirm your town’s requirements before listing an ADU on a platform like Airbnb.
Septic/Well: If your property is not connected to public sewer or water systems, you may need to update or expand your septic system or water supply to accommodate the additional unit. This usually involves hiring a licensed site evaluator and getting approval from your local plumbing inspector or the Maine Department of Health and Human Services. Properties with limited capacity may face design or size constraints.
Step 5
Contact Utilities
Reach out to providers (sewer, water, electric) early to understand connection requirements and fees. Water districts in Maine commonly require separate meters and connections.
Step 6
Meet With Local Staff
Each town handles ADUs a little differently. Contact your local planning department early in the process.
Ready to ADU?
Navigate project phases using this section on each page.
Need help with ADU rules?
Check out these resources for help.
