Accessory Dwelling Unit (ADU) Rules for Bath, ME

ADU State Law

What can you build in Maine?

• –Maine established statewide minimum ADU rules under 30-A M.R.S. §4364-B (Accessory Dwelling Units). These were recenlty expanded under LD 1829 (Public Law 2025, ch. 385), which was signed June 20, 2025 and goes into effect late-September 2025.
• –At least one ADU is allowed for any single-family dwelling in an area where residential uses are permitted.
• –The ADU may be Internal, Attached, or Detached.
• –A lot's nonconformity does not prevent an ADU, as long as the ADU does not make the nonconformity worse.
• –One accessory dwelling unit (ADU) is exempt from density limits and minimum lot-size requirements (§4364-B(4)(A)). Additional ADUs trigger the multi-unit density rules.
• –Local setback and dimensional requirements apply. For Internal or Attached ADUs, these cannot be more restrictive than those for the principal single-family home.
• –Municipalities may not require additional parking beyond what's required for the primary dwelling.
• –The ADU must be connected to adequate water and wastewater services.
• –If located in shoreland zones, the ADU must comply with shoreland zoning requirements.
• –The state law does not set a maximum size for ADUs, but municipalities may establish a size limit.
• –Lots located in the watershed of a water source serving the City of Lewiston or City of Auburn are exempt from the state ADU law.
• –Private covenants, such as HOA rules, may restrict or prohibit ADUs.
• –Municipalities may also regulate or restrict short-term rentals of ADUs.
• –LD 1829 (Public Law 2025, ch. 385) was signed June 20 2025 and, as a non-emergency law, takes effect **September 24 2025** (90 days after the 132nd Legislature adjourned). Key provisions include:
• –At least **three dwelling units per lot** (including ADUs) are now By-Right statewide, and **up to four units** must be allowed on lots in designated growth areas or on lots served by both public water *and* sewer.
• –An **ADU may now be added to a multi-unit structure or lot**. The single-family-only limitation has been removed.
• –Municipalities may not require fire sprinklers for an ADU unless the structure contains three or more total dwelling units (ADU included).
• –**Owner-occupancy may no longer be required** for either the primary dwelling or the ADU.
• –State density caps: ≤ 5,000 sf minimum lot size and ≤ 1,250 sf per unit (first 4) in growth areas with utilities; alternate caps apply elsewhere (§4364-A(2-A)).
• –For the new 3-/4-unit allowance, towns may not impose setbacks, height, or frontage stricter than for a single-family house (§4364-A(3)).
• –Planning-board review **cannot be required** for projects that add ≤ 4 units within one structure (§4364-A(5-A)).
• –After plumbing sign-off, towns may not demand extra waste-water documentation before issuing a C.O. (§4364-A(4) & §4364(5)).
• –Statute is enforceable on **Sept 24 2025**, but towns have until **July 1 2026/2027** (depending on charter) to amend local ordinances (Sec. 23).