Accessory Dwelling Units in Southern Maine: Zoning, Design, and Construction Considerations

Accessory dwelling units, commonly called ADUs, are self-contained residences located on the same lot as a primary home or qualifying multi-unit structure. They may be created within an existing house, attached as an addition, converted from a garage or accessory building, or built as a new detached structure.

Across Southern Maine, ADU projects are governed by a combination of Maine statutes, local zoning ordinances, building codes, utility requirements, and site-specific conditions. Though projects range in complexity, the most reliable process begins with zoning research, continues through coordinated design and permitting, and ends with construction, inspections, and occupancy approval.

For homeowners working with Southern Maine builders or home builders in Southern Maine, the following considerations provide a framework for evaluating an ADU project within approximately 50 miles of Saco, including Portland, South Portland, Saco, Biddeford, Brunswick, Bath, and nearby communities.

Maine’s statewide ADU framework

Maine’s primary ADU requirements are established under 30-A M.R.S. § 4364-B and explained in the Maine Office of Community Affairs Housing Legislation Guidance.

In general, municipalities must allow at least one ADU on a lot containing a single-family dwelling or qualifying multi-unit structure where residential uses are permitted. An ADU may be located:

• Within an existing dwelling,

• Attached to or sharing a wall with the primary structure,

• In a new detached structure built primarily as an ADU.

One ADU is exempt from local density calculations, and an additional parking requirement may not be imposed specifically for the ADU. Municipalities may still regulate setbacks, height, exterior design, lot coverage, shoreland zoning, flood hazards, historic resources, and other objective standards.

The state technical building code establishes a minimum ADU floor area of 160 square feet under the current 2026 guidance. Local ordinances may establish higher minimums or maximums. For example, Saco’s published ADU guide identifies a 400-square-foot minimum and a 1,000-square-foot maximum, while other communities use different thresholds.

Because state requirements and local ordinances continue to be updated, it is often helpful to confirm the current standard with the applicable code enforcement or planning office before finalizing a design.

Local zoning and permitting considerations

Portland

Portland properties are evaluated under the city’s zoning ordinance, building regulations, and applicable overlay requirements. The City of Portland Zoning Administration and Permitting and Inspections Department are the primary resources for determining the current approval path.

A Portland ADU review generally considers:

• Base zoning district,

• Lot configuration and nonconforming conditions,

• Building height and setbacks,

• Maximum floor area,

• Detached accessory structure placement,

• Historic district requirements,

• Flood hazard or slope conditions,

• Water, sewer, electrical, and mechanical systems.

A building permit and applicable trade permits are generally required. Depending on the property and scope of work, additional zoning, historic, site, or flood-related review may apply.

Saco

Saco provides a detailed Accessory Dwelling Unit Guide and identifies ADU standards under Section 230-701 of the city code.

Published Saco requirements include:

• One ADU per individual building lot,

• A 400-square-foot minimum,

• A 1,000-square-foot maximum,

• A permanent foundation,

• Compliance with applicable setbacks, lot coverage, and height requirements,

• Shared driveway access for a detached ADU,

• Exterior compatibility with the primary residence,

• Public water and sewer connections where those services are available,

• A certificate of occupancy before habitation.

Saco’s guide also states that ADUs are not intended for short-term rental use and must be rented for more than one month at a time. Since the city is reviewing additional ordinance changes related to statewide housing legislation, the Saco Housing page should be checked along with the Code Enforcement Department before permitting.

Biddeford

Biddeford’s ADU resource page identifies the Code Enforcement Office as the local authority for ADU permitting and enforcement.

The city’s published standards have historically included:

• One ADU per qualifying single-family lot,

• Attached, internal, or detached configurations,

• A 300-square-foot minimum,

• A maximum based on the primary dwelling’s floor area and a local square-footage limit,

• Common ownership with the primary residence,

• Specific entrance, stair, window, and exterior design standards.

Because some local materials may reflect earlier ordinance language, current requirements should be confirmed with Biddeford Code Enforcement. Plans are typically expected to include a site plan, floor plans, elevations, structural information, utility documentation, and required plumbing and electrical applications.

Brunswick, Bath, and nearby Midcoast towns

Midcoast municipalities apply the statewide ADU framework while retaining local standards for site conditions, dimensions, utilities, and design.

Brunswick has published ADU materials through its ADU Boost Program. Bath’s Accessory Dwelling Unit Codes Handout addresses size limits, sewer connections, fire separation, electrical service, and parking.

Detached units in Midcoast communities may require additional site review where the project involves shoreland zoning, historic resources, flood hazards, substantial grading, or utility extensions. State guidance distinguishes prohibited discretionary approval of an ADU as a use from staff-level site plan, historic, or environmental review. That distinction should be evaluated early.

Design considerations for Southern Maine properties

ADU design begins with the site rather than the floor plan. A measured survey or accurate plot plan is used to identify lot lines, existing structures, driveways, wells, septic components, utility routes, wetlands, slopes, and applicable setbacks.

Several design decisions require particular attention:

• Attached versus detached configuration: An internal conversion may reduce exterior site work but create challenges with fire separation, sound control, ceiling height, egress, and mechanical distribution.

• Building placement: Detached units are typically positioned behind or beside the primary dwelling, subject to front, side, and rear setbacks.

• Scale and exterior character: Roof pitch, siding, trim, window proportions, and entry orientation are often coordinated with the existing home and neighborhood.

• Universal design: Single-level living, zero-step entries, wider doors, curbless showers, lever hardware, and accessible kitchen clearances may be incorporated where long-term adaptability is being considered.

• Daylight and ventilation: Habitable rooms require compliant glazing, emergency escape and rescue openings where applicable, mechanical ventilation, and adequate heating.

• Snow and moisture management: Roof drainage, ice dam control, flashing, foundation drainage, air sealing, and vapor management are especially important in Maine’s climate.

Guiding Builders’ drafting and design services can support the development of measured drawings, floor plans, elevations, construction details, and permit-ready documentation.

Structural and building-code requirements

Whether an ADU is converted from an existing space or built new, the structure must meet the applicable Maine building and energy codes.

For a detached unit, structural planning typically includes:

• Frost-protected foundation, slab, crawlspace, or basement design,

• Soil bearing and drainage conditions,

• Snow load and roof framing,

• Wind and lateral resistance,

• Header, beam, and point-load sizing,

• Floor framing and subfloor assemblies,

• Exterior wall insulation and air sealing,

• Roof ventilation and water management.

For an attached or internal ADU, the existing structure is evaluated for foundation capacity, floor loading, stair geometry, egress, fire-resistance-rated assemblies, and the separation of dwelling units. Smoke alarms, carbon monoxide alarms, fire blocking, plumbing protection, and emergency access are coordinated in the permit drawings.

An electrical service review is also necessary. The ADU may require a dedicated panel or subpanel, load calculation, new circuits, and upgrades to the existing service. Heating and cooling may be provided through a separate heat pump, a dedicated mechanical system, or a shared system designed for the added load.

Water, wastewater, and utility connections

Water and wastewater capacity are central permitting issues. Maine law requires written verification of adequate services before an ADU receives occupancy approval.

For public systems, documentation may include:

• Confirmation of available water capacity,

• Sewer connection approval,

• Connection fees,

• Service sizing,

• New or upgraded underground piping.

For private systems, the project may require:

• A septic evaluation,

• A licensed site evaluator,

• An HHE-200 application,

• Verification of design flow,

• Well yield and potability testing,

• Plumbing inspector approval.

The anticipated wastewater flow can depend on the ADU configuration, number of bedrooms, fixtures, appliances, and whether the unit is attached or detached. Utility planning is therefore completed before the floor plan is treated as final.

A coordinated ADU construction process

A typical ADU project progresses through the following stages:

Planning an ADU with a Southern Maine general contractor

ADU construction combines zoning analysis, technical design, utility coordination, code compliance, and detailed field execution. For homeowners in Saco, Biddeford, Portland, and surrounding communities, early coordination reduces the likelihood of redesign caused by setbacks, septic limitations, structural conditions, or incomplete permit documentation.

Guiding Builders’ project portfolio includes residential construction, renovations, drafting, and complex project coordination throughout Southern Maine. Homeowners considering an ADU can contact Guiding Builders to discuss site feasibility, design development, permitting requirements, and the construction process.

Local ordinances and state requirements change periodically. Before construction begins, project-specific requirements should be confirmed with the applicable municipality and qualified design and construction professionals.