Can I build an ADU in Harpswell, Maine?
Yes. Harpswell allows one accessory dwelling unit on any lot with a single-family home, inside the house, attached, or in a separate building, without extra lot area. The ADU can be up to 750 sq ft, the owner must live in either the house or the ADU, and it needs a Code Enforcement permit plus proof the septic system can handle both units.
| ADUs allowed per lot | 1, on a lot with a single-family home (no extra lot area needed) [1] |
| Owner occupancy | Owner must live in the main house or the ADU [2] |
| Maximum size | 750 sq ft of floor area (attached, internal or detached) [3] |
| Setbacks for additions or detached ADUs | Same property line and road setbacks as other structures [4] |
| Standard setbacks outside the shoreland zone | 20 ft from lot lines; 40 ft from the traveled way for principal structures [7] |
| Septic | Written proof to the Plumbing Inspector that the system is sized for both units [6] |
| Permit for one- and two-unit residential use (non-shoreland) | Code Enforcement Officer permit [8] |
The basic rule
Outside the shoreland zone, the Basic Land Use Ordinance lets you add one ADU to any lot that has a single-family home, without meeting the 40,000 sq ft per-unit lot size [1]. The Definitions Addendum describes an ADU as a self-contained unit with living, sleeping, bathing and cooking facilities, inside or attached to the house or in a freestanding building, that stays secondary to the main home [9].
Standards
- Owner occupancy: the owner must live in either the main house or the ADU [2].
- Size: a maximum of 750 sq ft of floor area, whether attached, internal or detached [3].
- Appearance: an attached or internal ADU, including its outside entrance, must keep the look of a single-family home.
- Setbacks: additions and freestanding ADUs must meet the normal property line and road setbacks, and a detached ADU can't block the front of the main house [4][5]. Outside the shoreland zone that means 20 ft from lot lines and 40 ft from the road's traveled way for principal structures [7].
- Septic: before permits are issued you must show the Plumbing Inspector that the septic system is (or will be) sized for both units, and that any existing system works properly [6].
The ordinance doesn't set a parking requirement for ADUs.
Permits
One- and two-unit residential uses need a permit from the Code Enforcement Officer outside the shoreland zone [8].
Shoreland zone
The Shoreland Zoning Ordinance has no ADU section. It requires each additional dwelling unit on a lot to meet all dimensional requirements, including the 40,000 sq ft lot area per unit [10]. It separately allows an owner-occupied "in-law" apartment for a relative, with a building permit and annual registration, that must be removed when the relative leaves [11]. State law says one ADU per lot is exempt from density limits and towns can't ban ADUs in the shoreland zone, so state law may override the per-unit lot-size rule for ADUs there.
State law timing
Maine's ban on ADU owner-occupancy requirements takes effect July 1, 2027. Until then, Harpswell's owner-occupancy rule applies.
See how Harpswell compares with 8+ other Maine towns on accessory dwelling units (adus) →
Next steps
- Call the Harpswell Code Enforcement Office (207-833-5771) to confirm whether your lot is in the shoreland zone, which changes which ordinance applies.
- Have a licensed site evaluator check or design a septic system sized for both units, and get the Plumbing Inspector's sign-off before applying.
- Submit a permit application with a scaled site plan showing the ADU, its floor area (750 sq ft max) and setbacks to the Code Enforcement Office.
- Be ready to show that you, the owner, will live in either the main house or the ADU.
Common questions
- Can I build an ADU in Harpswell, Maine?
- Yes. Harpswell allows one accessory dwelling unit on any lot with a single-family home, inside the house, attached, or in a separate building, without extra lot area. It needs a Code Enforcement permit plus proof the septic system can handle both units.
- How big can an ADU be in Harpswell, Maine?
- Up to 750 sq ft of floor area, whether attached, internal or detached. Additions and freestanding ADUs must meet the normal property line and road setbacks.
- Do I have to live on the property if I build an ADU in Harpswell, Maine?
- Yes, for now. The owner must live in either the main house or the ADU. Maine's ban on ADU owner-occupancy requirements takes effect July 1, 2027.
- Are ADUs allowed in Harpswell's shoreland zone?
- It's different. The Shoreland Zoning Ordinance has no ADU section and requires each additional dwelling unit to meet all dimensional requirements, including the 40,000 sq ft lot area per unit. State law may override that for ADUs, so call Code Enforcement.
Maine's statewide ADU law
Under 30-A M.R.S. §4364-B (first passed as LD 2003 in 2022), a town must allow at least one accessory dwelling unit (ADU) on a lot with a single-family home anywhere homes are allowed. Towns had to comply by 2024.
Rules in effect now:
- It can be inside the house, attached, or a new detached building.
- One ADU per lot is exempt from density limits.
- No extra parking can be required beyond what the house already needs.
- Setbacks are the same as for the house.
- Minimum size: the state building codes board set 160 sq ft in 2025. Towns may set a maximum size.
- Before you can move in, you must show the ADU has adequate water and septic/sewer.
- In the shoreland zone ADUs must meet shoreland rules, but towns can't ban them there outright.
- Sprinklers aren't required unless the ADU is in or attached to a building with more than 2 units.
Coming July 1, 2027 (LD 1829 of 2025, pushed back by LD 2173 in 2026): towns can no longer require the owner to live on the property, ADUs must also be allowed next to 2- and 3-unit buildings, and towns can't require planning board approval for an ADU. Towns may still regulate short-term rentals in ADUs. Private covenants and HOA rules can be stricter.
What the ordinance says
- Sec. 11.12 Accessory Dwelling Unit (ADU), Basic Land Use Ordinance (2026), p. 20
One accessory dwelling unit (ADU) may be added to any lot with a principal one-unit residential structure without having to meet the minimum lot area per dwelling unit requirement for the ADU, provided that it meets all of the following standards:
- Sec. 11.12(a), Basic Land Use Ordinance (2026), p. 20
(a) The owner of the property shall occupy either the principal dwelling unit or the ADU;
- Sec. 11.12(b), Basic Land Use Ordinance (2026), p. 20
(b) Any ADU within or attached to the principal structure or within an accessory or freestanding structure shall have a maximum of seven hundred fifty (750) square feet of floor area;
- Sec. 11.12(d), Basic Land Use Ordinance (2026), p. 20
(d) Any addition to the principal structure or an accessory structure to create the ADU shall comply with the property line and road setbacks.
- Sec. 11.12(f), Basic Land Use Ordinance (2026), p. 20
(f) The location of a freestanding structure on the property for use as an ADU shall comply with the property line and road setbacks and shall not obscure the front of the principal structure;
- Sec. 11.12(g), Basic Land Use Ordinance (2026), p. 20
(g) Written evidence shall be provided to the Plumbing Inspector before the issuance of any permits needed to create the ADU that a subsurface wastewater disposal system(s) to serve both the principal unit and the ADU is or will be sized to meet the design sewage flows for the two units in accordance with the Maine Subsurface Wastewater Disposal Rules; and
- Sec. 11.3.1 Setback, Basic Land Use Ordinance (2026), p. 16
All principal structures and expansions of existing principal structures shall be set back at least forty (40) feet from the edge of the traveled way and at least twenty (20) feet from all lot lines;
- Sec. 13.3 Table 1, row 15.1, Basic Land Use Ordinance (2026), p. 30
15.1 One and two–unit residential uses including CEO PB10 CEO CEO No CEO No driveways
- Definitions, Accessory Dwelling Unit (ADU), Definitions Addendum (2026), p. 3
Accessory Dwelling Unit (ADU) – A self-contained dwelling unit located within or attached to a one-unit residential structure, or within an accessory or freestanding structure on the same lot as a principal one-unit residential structure for use as complete, independent living quarters, with provision for living, sleeping, bathing, and cooking that complies with the standards of Section 11.12 of the Basic Land Use Ordinance.
- Sec. 15.1.5, Shoreland Zoning Ordinance (2025), p. 25
If more than one (1) residential dwelling unit, principal governmental, institutional commercial or industrial structure or use, or combination thereof, is constructed or established on a single parcel, all dimensional requirements shall be met for each additional dwelling unit
- Sec. 15.22 "In-Law" Apartments, Shoreland Zoning Ordinance (2025), p. 46
An apartment for a relative related by blood, marriage or adoption to the record owner(s) of the property may be added to any dwelling unit provided that (a) the property is owner-occupied; (b) a building permit is obtained; (c) all of the requirements of this Ordinance, with the exception of shore frontage, road frontage, and minimum lot size, are met; and (d) all requirements of the Maine Subsurface Wastewater Disposal Rules in accordance with 22 M.R.S.A. § 42, as may be amended from time to time, are met.