Can I build an ADU in Mount Desert, Maine?
Yes. On a lot with a single-family home outside the Shoreland Zone, Mount Desert allows one accessory dwelling unit with a Code Enforcement Officer permit, and you don't need extra lot area for it. It can be inside the house, attached, or in a new building, and must be at least 190 sq ft but no more than 75% of the main home's living area.
| Number allowed | 1 per lot with a single-family home, outside the Shoreland Zone, no extra lot area needed [1] |
| Attached vs detached | Inside the house, attached, or in a new structure [2] |
| Minimum size | 190 sq ft [3] |
| Maximum size | 75% of the main home's living area [3] |
| Approval in village, residential, shoreland residential, rural and village commercial districts | Code Enforcement Officer permit [5] |
| Approval in Shoreland Commercial and Conservation districts | Planning Board conditional use approval [5] |
What's allowed
If your lot has a single-family home and sits wholly outside the Shoreland Zone, you can add one accessory dwelling unit without needing a bigger lot [1]. It can be part of the house, attached to it, or a new building [2]. A separate garage that contains a dwelling unit counts as a dwelling unit [6].
Size and services
The unit must be at least 190 sq ft and no more than 75% of the main home's living area [3]. It needs an adequate water supply and must meet town and state wastewater (septic or sewer) standards [4]. If you convert an existing garage or accessory building, that building's existing setbacks apply [9].
Approval by district
The use table lists an ADU as needing a Code Enforcement Officer permit in the Village Residential 1 and 2, Residential 1 and 2, Shoreland Residential, Rural/Woodland and Village Commercial districts [5]. Shoreland Commercial and Conservation need Planning Board conditional use approval, and Resource Protection allows it only by Planning Board special exception (table note 8), and Stream Protection only with a setback variance (note 4) [5]. In Village Residential 3, an ADU must be staff housing only [8].
Shoreland Zone lots
The density break applies only to lots wholly outside the Shoreland Zone [1]. On a lot that is wholly or partly in the Shoreland Zone, every additional dwelling unit must meet the full dimensional requirements, such as lot area, on its own [7]. In practice that can mean you need twice the minimum lot area.
Not addressed
We found no owner-occupancy rule and no ADU-specific parking requirement in the ordinance. The cover of our copy lists amendments through May 5, 2026, the most recent annual town meeting, so it appears current; still confirm with the town before you build.
State law
Maine's ADU law is summarized separately on this page. Mount Desert's 190 sq ft minimum is larger than the 160 sq ft state minimum, and after July 1, 2027 towns can't require Planning Board approval for an ADU, so state law may override those parts of the ordinance.
See how Mount Desert compares with 8+ other Maine towns on accessory dwelling units (adus) →
Next steps
- Find your district on the Town of Mount Desert Land Use District Map and check whether any part of your lot is in the Shoreland Zone (within 250 ft of tidal water, a great pond or wetland, or 75 ft of a stream).
- Measure the main home's living area so you can show the ADU is between 190 sq ft and 75% of it.
- Get water and wastewater approval: if you're not on public sewer, have the Plumbing Inspector review a septic design or plumbing permit application.
- Apply to Mount Desert Code Enforcement / Planning (207-276-5731) for a CEO permit with a scaled site plan showing all structures and setbacks.
Common questions
- Can I build an ADU in Mount Desert, Maine?
- Yes. On a lot with a single-family home outside the Shoreland Zone, Mount Desert allows one accessory dwelling unit with a Code Enforcement Officer permit, and no extra lot area is needed. It can be inside the house, attached, or in a new building.
- How big can an ADU be in Mount Desert, Maine?
- At least 190 sq ft and no more than 75% of the main home's living area. It needs adequate water and must meet town and state wastewater standards.
- Do I need an owner to live on site for an ADU in Mount Desert, Maine?
- We found no owner-occupancy rule and no ADU-specific parking requirement in the ordinance. Confirm with the town before you build.
- Can I build an ADU on a shoreland lot in Mount Desert, Maine?
- It's harder. The no-extra-lot-area break applies only to lots wholly outside the Shoreland Zone. On a lot wholly or partly in it, every additional dwelling unit must meet the full dimensional requirements on its own, which can mean twice the minimum lot area.
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. 6B.11.3.3, Accessory Dwelling Units, Land Use Zoning Ordinance, p. 54
Accessory Dwelling Units. For any existing or new lot with a Residential Dwelling One-Family that is wholly outside the Shoreland Zone, one accessory residential dwelling unit is allowed per lot without an increase in the minimum lot area requirement.
- Sec. 6B.11.3.3(a), Land Use Zoning Ordinance, p. 54
Accessory residential dwelling units can be within the Residential Dwelling One-Family, attached to it, or in a new structure.
- Sec. 6B.11.3.3(b), Land Use Zoning Ordinance, p. 54
An accessory dwelling unit must be a minimum of 190 square feet and may be no more than 75 percent of the living area of the primary dwelling unit.
- Sec. 6B.11.3.3(c), Land Use Zoning Ordinance, p. 54
The accessory residential dwelling unit must have adequate water supply and shall meet town and state standards for wastewater disposal.
- Sec. 3.4, Permitted, Conditional, and Excluded Uses by District (table), Land Use Zoning Ordinance, p. 14
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- Sec. 3.4, table note (c), Land Use Zoning Ordinance, p. 14
A separate garage is an accessory structure. A separate garage with a dwelling unit
- Sec. 6B.11.4.4, Multiple structures (Shoreland Zone), Land Use Zoning Ordinance, p. 55
If more than one residential dwelling unit, principal governmental, institutional, commercial, or industrial structure or use, or combination thereof, is constructed or established on a single parcel that is wholly or partially within the Shoreland Zone, all dimensional requirements shall be met for each additional dwelling unit, principal structure, or use.
- Sec. 3.4, table note (e), Land Use Zoning Ordinance, p. 14
Accessory residential dwellings in the VR3 District must be for staff housing only.
- Sec. 6B.11.3.3(d), Land Use Zoning Ordinance, p. 54
For an accessory residential dwelling unit permitted in an existing accessory building or secondary building or garage as of the implementation date, the required setback requirements in local ordinance of the existing accessory or secondary building apply.