Can I build an ADU in Topsham, Maine?
Yes. According to a Topsham Codes Office handout (we could not review the zoning ordinance text itself), an ADU is allowed as long as it meets the standards in Section 225-54 of the Zoning Ordinance. Under the 2021 building code, a unit with its own complete living facilities inside a single-family house generally makes the building a two-family dwelling, so expect to meet two-family construction standards.
| Zoning status | Allowed if it complies with §225-54 (not listed in the use table) [1] |
| Building code effect | An ADU with independent living facilities generally reclassifies a single-family home as two-family under the 2021 IRC [2] |
| Building code exception | Existing in-house ADU space created before April 7, 2025 and rented to no more than 3 outsiders [3] |
| Septic | Maine Subsurface Wastewater Disposal Rules apply; sizing depends on bedrooms and dwelling units [4] |
Source note
This is based on a Codes Enforcement Office handout, an official town summary rather than the ordinance text itself. We could not download Chapter 225 (Zoning), so the size, parking, owner-occupancy and attached-versus-detached rules in Section 225-54 aren't covered here.
Zoning
The handout says an ADU isn't listed in the zoning use table, but its requirements are in Section 225-54, and "as long as the ADU complies with 225-54, it is allowed" [1]. The ordinance's definition of a dwelling unit specifically includes accessory dwelling units [5].
Building code
The town points out that the 2021 International Residential Code covers one- and two-family dwellings. Adding a unit with its own complete living facilities to a single-family house would generally change it to a two-family dwelling [2]. There is one exception: if the ADU space is inside an existing dwelling, was created before April 7, 2025, and is rented to no more than 3 outsiders, two-family dwelling standards don't apply [3]. The handout also says one electric meter can serve both units, as long as each unit's occupants can always get to their own breakers.
Utilities
If the property has a septic system, the Maine Subsurface Wastewater Disposal Rules set system and tank size based on bedrooms and number of dwelling units [4]. For public sewer or water, contact the Topsham Sewer District or the Brunswick-Topsham Water District.
Ask the Codes Office for the current text of Section 225-54 before you design the unit.
See how Topsham compares with 8+ other Maine towns on accessory dwelling units (adus) →
Next steps
- Ask the Topsham Codes Enforcement Office (207-725-1723) for the current text of Zoning Ordinance §225-54 (ADU standards).
- Have your designer check two-family dwelling requirements under the 2021 IRC.
- If you're on septic, hire a Maine licensed Site Evaluator to check septic capacity for the added unit.
- If you're on public utilities, call the Topsham Sewer District (207-729-3612) and the Brunswick/Topsham Water District (207-729-9956).
Common questions
- Can I build an ADU in Topsham, Maine?
- Yes. According to a Topsham Codes Office handout, an ADU is allowed as long as it meets the standards in Section 225-54 of the Zoning Ordinance. We could not review the ordinance text itself, so ask the Codes Office for the current text.
- Does an ADU change my building code classification in Topsham, Maine?
- Generally yes. Under the 2021 building code, a unit with its own complete living facilities inside a single-family house generally makes the building a two-family dwelling, so expect two-family construction standards.
- Is there an exception for existing in-house ADUs in Topsham, Maine?
- Yes. If the ADU space is inside an existing dwelling, was created before April 7, 2025, and is rented to no more than 3 outsiders, two-family dwelling standards don't apply.
- Do I need to upgrade my septic for an ADU in Topsham, Maine?
- The Maine Subsurface Wastewater Disposal Rules set system and tank size based on bedrooms and number of dwelling units. For public sewer or water, contact the Topsham Sewer District or the Brunswick-Topsham Water District.
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
- ADU Regulation handout, Local Zoning, ADU Regulation (Codes Office handout), p. 1
An Accessory Dwelling Unit (ADU) is not listed in the use table but requirements are detailed in section 225-54. As long as the ADU complies with 225-54, it is allowed.
- ADU Regulation handout, Residential Building Code (IRC 2021), ADU Regulation (Codes Office handout), p. 1
Creating multiple independent dwelling units in a structure classified as a single-family dwelling would generally change its classification to a two-family dwelling or a multiple-unit dwelling, which are defined separately.
- ADU Regulation handout, Residential Building Code (IRC 2021), ADU Regulation (Codes Office handout), p. 1
(Exception: If the ADU space is within an existing dwelling, was created prior to April 7, 2025, and is rented to no more than 3 outsiders, compliance with two-family dwelling standards is not required.).
- ADU Regulation handout, Septic System, ADU Regulation (Codes Office handout), p. 2
The rules dictate septic system size and septic tank size based on factors such as number of bedrooms and number of dwelling units.
- ADU Regulation handout, Local Zoning (Dwelling Unit definition), ADU Regulation (Codes Office handout), p. 1
The term includes accessory dwelling units and manufactured housing, but not recreational vehicles.