Can I build an ADU in Yarmouth, Maine?
Yes. Accessory Dwelling Units are a permitted use in Yarmouth's residential districts (RR, LDR, MDR and others), and they can be inside, attached to, or detached from a single-family home. An ADU is approved under the town's separate Site Plan Review Ordinance (Chapter 702), which holds the detailed ADU standards we did not review.
| Allowed where | Permitted use in RR, LDR, MDR and most other districts that allow homes [2] |
| Attached vs detached | Within, attached to, or detached from the single-family home on the same lot [1] |
| Density | An ADU approved under Site Plan Review doesn't count as a separate unit for area and density rules [1] |
| Extra units on a lot with one home | Up to 2 additional units (one attached + one detached, or two in a detached building) [3] |
| Setbacks | Same as the zoning district's setbacks [4] |
| Before occupancy | Written proof of adequate water and sewer/septic before a Certificate of Occupancy [5] |
Where ADUs are allowed
Yarmouth defines an Accessory Dwelling Unit as a self-contained unit within, attached to, or detached from a single-family home on the same lot [1]. ADUs are listed as a permitted use in the Rural Residential (RR), Low Density Residential (LDR) and Medium Density Residential (MDR) districts [2], and also appear in the permitted-use lists for the commercial, water-oriented commercial and General Development districts.
How approval works
The definition says an ADU is approved under the Site Plan Review Ordinance (Chapter 702), and an ADU approved that way isn't counted as a separate unit for area and density requirements [1]. The zoning ordinance itself does not set an ADU maximum size, an owner-occupancy rule, or a parking requirement. Those details, if any, would be in Chapter 702, which we did not review. Manufactured homes and Tiny Homes may also serve as an ADU if they meet Chapter 702.
Additional dwelling units (Article II.EE)
Since January 1, 2024, a lot with one existing home may add up to two more units: one inside or attached, one detached, one of each, or two in a separate detached building [3]. Each unit must meet the district's setbacks [4]. Outside the Growth Area, each added unit generally needs the district's minimum lot area per unit; inside the Growth Area, a conforming lot may add the first two units without extra lot area. The section on adding a third unit to a two-unit lot doesn't apply where an ADU exists or was added after Jan. 1, 2024.
Water and septic
Before a Certificate of Occupancy, you must give written verification that each unit has adequate water and sewer or septic service. For septic, the local plumbing inspector must verify the system is adequate [5].
Shoreland and other rules
The dwelling unit allowances don't exempt projects from the Shoreland Overlay rules (Article IV.R), Subdivision, or Site Plan Review.
See how Yarmouth compares with 8+ other Maine towns on accessory dwelling units (adus) →
Next steps
- Ask Yarmouth Planning & Development (207-846-2401) for the Chapter 702 ADU application and its size, parking and occupancy standards.
- Check your zoning district and whether your lot is in the Growth Area or the Shoreland Overlay District.
- Plan the ADU to meet your district's front, side and rear setbacks.
- Line up proof of adequate water and sewer or septic (septic design by a licensed site evaluator if needed) before you ask for a Certificate of Occupancy.
- Apply for a building permit once Site Plan Review approval is in hand.
Common questions
- Can I build an ADU in Yarmouth, Maine?
- Yes. Accessory dwelling units are a permitted use in Yarmouth's residential districts (RR, LDR, MDR and others), inside, attached to, or detached from a single-family home. They're approved under the separate Site Plan Review Ordinance (Chapter 702).
- How big can an ADU be in Yarmouth, Maine?
- It's unclear from the zoning ordinance, which sets no ADU maximum size, owner-occupancy rule, or parking requirement. Any such details would be in Chapter 702, which the page did not review.
- Can I add more than one extra unit to my lot in Yarmouth, Maine?
- Yes. Since January 1, 2024, a lot with one existing home may add up to 2 more units: one inside or attached, one detached, one of each, or two in a separate detached building. Each must meet the district's setbacks.
- What do I need before an ADU can be occupied in Yarmouth, Maine?
- You must give written verification that each unit has adequate water and sewer or septic service before a Certificate of Occupancy. For septic, the local plumbing inspector must verify the system is adequate.
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
- Article I.D Definitions, Accessory Dwelling Unit, Chapter 701 Zoning Ordinance, p. 6
Accessory Dwelling Unit: A self-contained dwelling unit within or attached to a single-family dwelling or detached from the single-family dwelling located on the same parcel of land. An Accessory Dwelling Unit approved under the Site Plan Review Ordinance shall not be considered a separate unit for the purposes of applying the area and density requirements of this Ordinance.
- Article IV.G LDR Permitted Uses, Chapter 701 Zoning Ordinance, p. 117
Home Occupations (pursuant to II.J) Accessory Dwelling Units Affordable Housing Development
- Article II.EE.2.c Dwelling Unit Allowances, Chapter 701 Zoning Ordinance, p. 93
c. If a lot contains one existing dwelling unit, up to two additional dwelling units per lot are allowed, subject to the following:
- Article II.EE.3.c, Chapter 701 Zoning Ordinance, p. 94
c. For each dwelling unit allowed by this section, the setback requirements of the zoning district in which the lot is located apply.
- Article II.EE.4.a Water and Wastewater Standards, Chapter 701 Zoning Ordinance, p. 95
a. Prior to the issuance of a Certificate of Occupancy, written verification that each unit is connected to adequate water and wastewater services shall include the following: