May 6, 2021

Is an ADU In The Cards For You? - A Breakdown of Greater Portland’s ADU Requirements, Town-by-Town

May 6, 2021

Is an ADU In The Cards For You? - A Breakdown of Greater Portland’s ADU Requirements, Town-by-Town

Is an ADU In The Cards For You? - A Breakdown of Greater Portland’s ADU Requirements, Town-by-Town

Accessory dwelling units (ADUs) are becoming more and more commonplace, but the zoning laws regarding these additional buildings differ from town to town. In order to make your life a bit easier, BrightBuilt is happy to provide some insight to help get you started. Whether you’re looking for a guesthouse for the in-laws, a rental unit, or just wanting to improve the value of your property, we hope we can help put you on the right path! To start off, we’ll look at towns close to our home in southern Maine.

The requirements for Portland and its surrounding towns are unique for each municipality, but they do have some in common. For example, the owner of the property must occupy either the principal dwelling or the ADU, though some towns explicitly state the required length of time the owner must reside during the year. Besides Portland, all the towns and cities discussed allow for only a single ADU to be built on a lot. They all also require some sort of permit, whether for design approval, building, or occupancy. For further clarification on permitting, please reach out to your local planning department for exact details.

Click on the city/town’s names to see more information on their website.


Maximum Size: Two-thirds the size of the primary unit on the property and the height of the ADU may not exceed the height of the principal dwelling

Interior, Attached, or Detached: All three are allowed

As the largest city in Maine, Portland has found it worthwhile to be hospitable to ADUs. Residential properties that feature a single-family home are allowed to have up to two accessory dwelling units. These ADUs can be built within the principal building (such as a basement), within a pre-existing detached building (such as above a garage), or can be built detached (such as our Jordan!).

Regarding size, if you’re planning to construct two ADUs on your property, the combined footprint of the two accessory units must not be larger than the primary structure.

Portland, like the rest of the municipalities we’ll be looking at, requires that the owner of the property occupy either the principal or accessory dwelling unit. The other unit, however, may be rented out if desired. One exception to that rule is that no ADU can be used for short term rentals on Peaks Island.

South Portland

Maximum Size: “No more than forty percent (40%) of the living area of a principal dwelling unit, nor more than 800 square feet, nor less than 300 square feet, nor have more than two (2) bedrooms.”

Interior, Attached, or Detached: Interior or attached only

Across the Fore River is South Portland, where rules on ADUs are a bit more restrictive. The number of ADUs is limited to one per residential lot and fully detached ADUs are not allowed, so any accessory unit must exist within the interior of the principal dwelling or it must be “attached to the principal dwelling by a fully enclosed breezeway.”

Parking is also a necessary feature to take into account in South Portland. You’ll need to provide two on-site parking spots in order to have your ADU application approved.



Maximum Size: “At a minimum, your accessory unit can be 300sqft, while at a maximum, it may be 900 square feet or 35% of the total floor area of the principal dwelling unit, whichever is greater.”

Interior, Attached, or Detached: All three!

Feature-wise, ADUs in Biddeford are limited to having, at most, two bedrooms. Rooftop decks are not allowed and if a balcony is part of your design, then it cannot face the interior side lot lines.There are further regulations depending on whether your ADU is internal, attached, or detached. For instance, the facade of a detached ADU that faces an alley or public street must have no less than 10% of its area be windows.

An off-street parking space is required for your ADU, unless legal on-street parking or publicly supplied off-street parking is located within 1,000 feet.


Maximum Size: 40% of the living area of the principal dwelling or 1000 sqft, whichever is less.

Interior, Attached, or Detached: All three!

In addition to the restriction on maximum square footage, ADUs in Sanford are required to have no more than three bedrooms. Sanford also allows for ADUs to be built only on lots that are zoned as Rural Residential, Rural Mixed Use, or Single Family Residential, so be sure to double check how your property is classified.


Maximum Size: 800 sqft

Interior, Attached, or Detached: Attached or detached

If you’re planning on building a detached ADU in Saco, you’ll have to be sure that your new structure is located to the side or behind the principal dwelling on the property. Whether or not your ADU is attached or detached, there is a requirement that it’s design must visually harmonize with the surrounding neighborhood, so keep that neon paint for the inside!


Maximum Size: 800 sqft

Interior, Attached, or Detached: Attached only

Folks in Westbrook are limited to having only two individuals occupy their ADU. Perfect for in-laws, but maybe not the best for an accessory frat house.


Maximum Size: Must not be greater than 750 square feet or 35 percent of the floor area of the principal structure, whichever is greater.

Interior, Attached, or Detached: All three!

Although you still need a permit to build an ADU in Brunswick, you luckily are not required to submit your plans for Design Review. However, you will need to keep in mind that you are not allowed to change the design of the facade of the primary dwelling when you add on your ADU. Brunswick also allows for ADUs to be built on not only single-family lots, but also two-family residential and commercial lots.



Maximum Size: See specific ordinance requirements.

Interior, Attached, or Detached: All three!

Designing and building in Scarborough requires a bit more attention to ordinances than some of the other locales we’ve looked at. Here, your ADU’s “must be designed and constructed in the style of a garage, barn, storage building, carriage house, accessory cottage, or similar structure customarily located on the same lot with a single-family residence.” The accessory unit most also “retain and respect the existing streetscape, character of the neighborhood, and preserve the single-family appearance, architectural style, and character of the original dwelling and property.”

In addition to these architectural demands, you must provide at least one off-street parking space for your ADU and you are limited to having may the maximum of two people to occupy the ADU at one time. If you’re planning on renting either the ADU or primary dwelling, the rental period must be for at least 28 days. Scarborough does, however, allow the owner of the property to only reside on the property for a minimum of three months, so vacation homes and seasonal residences are perfect for adding on an ADU.


Maximum Size: 600 sqft

Interior, Attached, or Detached: Interior or attached

Over in Windham, your ADU must have a separate entrance from the primary structure and the number of required parking spaces will depend on your local requirements. Occupancy is limited to three or fewer individuals.


Maximum Size: N/A

Interior, Attached, or Detached: All three!

Property owners in Kennebunk that are looking to build an ADU are required to have their primary or accessory unit be the principal residence for at least six months of the year. In addition to that regulation, the town allows for only one of the units to be rented. The other must remain as a primary residence. However, beyond those standards and the need to show that the ADU is connected to either sewer or septic, Kennebunk is fairly relaxed when it comes to adding on an accessory apartment.


Maximum Size: “Shall not exceed 100% of the gross floor area of the single family dwelling to which it is accessory, not including garages, porches, decks or unfinished areas, or 850 square feet of gross floor area, whichever is less.”

Interior, Attached, or Detached: Attached

The one additional provision that is required for ADUs in Falmouth, besides the size regulations and that the unit must be attached to the principal dwelling, is that the primary unit and the ADU must share a main entrance. That entrance may lead to a foyer with separate doors to each individual unit. Any other entrances of the ADU must appear as subordinate to that shared doorway.



Maximum Size: 900 sqft

Interior, Attached, or Detached: All three!

Regarding the design of your ADU, Yarmouth requires that it conform to the “building forms, height, construction materials, colors, landscaping” of the primary residence and to that of the neighborhood. In addition, the orientation and positioning of the ADU must be chosen so as to minimize any changes to the landscape and maintain existing “heritage or significant trees and shrubs”

The ADU may be a studio or it can have up to two bedrooms. One parking space is required for a studio or one-bedroom ADU and two parking spaces if the accessory unit has two bedrooms.

Yarmouth does place some restrictions on where an ADU can be built. Not only must the ADU share a lot with a single-family residence, but it can not be permitted on a lot with a non-conforming use or on a lot with mixed uses.

North Yarmouth

Maximum Size: 40% of the living area of the primary dwelling unit

Interior, Attached, or Detached: All three!

Much more simple than the ordinances of their southerly neighbors, residents in North Yarmouth are limited to having just one bedroom in their ADU, with it being intended for, at most, two occupants.

We hope you enjoyed these cliff notes on local ADU ordnances. If you’re keen to get started on your own ADU, let us know how we can help!

For more information about our newest ADU, the Sterling, check out the link below!

Also: Check out this virtual tour of our 2 bedroom, “Option A” floor plan.