Town Permit Guide

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Can I build an ADU in South Portland, Maine?

South Portland, Cumberland County · Ordinances checked 2026-10-01 · Reviewed · 12 cited sections

Allowed with a permit

Yes. South Portland permits one accessory dwelling unit (ADU) per lot in every residential zone, inside the house, as an attached addition, or as a detached building, with a permit from the Code Enforcement Officer. On lots under 10,890 sq ft the ADU can be up to 800 sq ft (or 90% of the main home, if smaller), no extra parking is required, and an ADU approved under the current rules cannot be used as a short-term rental.

ApprovalPermit from the Code Enforcement Officer (Planning Board minor amendment if the lot has an approved site plan) [1]
Residential zones where allowedPermitted in all residential districts (RF, AA, A, RC, G, VR, RT, WR) [2]
Max size, lot under 10,890 sq ft800 sq ft or 90% of the main home, whichever is smaller [4]
Max size, lot 10,890 sq ft or larger10% of parcel area, 1,200 sq ft, or 90% of the main home, whichever is smaller [5]
Minimum size190 sq ft [6]
Number per lot1 [7]
Max height, detached ADU28 feet [9]
Extra parkingNone required [10]
Short-term rental useNot allowed for ADUs approved under Ord. #4-22/23 [11]

Where and how

ADUs require a permit from the Code Enforcement Officer; if the property already has a Planning Board-approved site plan, a minor amendment is needed instead [1]. The residential land use table lists ADUs as permitted in all eight residential districts [2]. An ADU can be built inside an existing home, as an addition attached to it, or as a new or detached structure [3]. Detached ADUs have extra placement rules: exterior mechanical equipment must stay out of setbacks and at least 10 feet from a property line, no windows (other than roof openings) within 5 feet of a lot line, and the ADU entrance generally not within 10 feet of a lot line.

Size and design

On lots under 10,890 sq ft, the ADU is capped at 800 sq ft or 90% of the main home's floor area, whichever is smaller [4]. On larger lots the cap is the smallest of 10% of the parcel, 1,200 sq ft, or 90% of the main home [5]. The minimum is 190 sq ft [6]. Only one ADU is allowed per lot [7], and a detached ADU may be no taller than 28 feet [9]. If the ADU changes a building's footprint, the addition must meet the zone's setback and coverage rules [12]. Design standards also apply, such as a less-prominent ADU entrance, a matching roof pitch and materials, privacy screening, and a 3-foot walkway to the sidewalk.

Ownership, parking and rentals

There is no owner-occupancy requirement in the text, but the ADU and the main home must stay in common ownership; the ADU can't be sold off separately [8]. No additional parking is required [10]. An ADU approved under these rules may not be used or registered as a short-term rental [11], and on a single-family lot neither the house nor the ADU may be a short-term rental once the ADU is approved.

State law

The 190 sq ft minimum is higher than the 160 sq ft state minimum described in the statewide summary; state rules may override it. Starting July 1, 2027, state law bars towns from requiring Planning Board approval for an ADU, which may affect the minor-amendment route for site-plan properties.

See how South Portland compares with 8+ other Maine towns on accessory dwelling units (adus) →

Next steps

  1. Hire a Maine licensed land surveyor for a mortgage loan inspection plan of the lot (may be waived for ADUs built entirely inside an existing structure).
  2. Prepare a marked-up plan showing the ADU, floor areas, parking, plus building plans/elevations and a short narrative on how the design meets Sec. 27-1578.
  3. Sign the owner certification that the ADU (and on a single-family lot, the house) will not be used as a short-term rental.
  4. Submit the ADU application to Code Enforcement; start construction within 3 years of approval.

Common questions

Can I build an ADU in South Portland, Maine?
Yes. South Portland permits one accessory dwelling unit per lot in every residential zone: inside the house, as an attached addition, or as a detached building. It needs a permit from the Code Enforcement Officer.
How big can an ADU be in South Portland, Maine?
On lots under 10,890 sq ft, an ADU can be up to 800 sq ft or 90% of the main home, whichever is smaller. On larger lots the cap is the smallest of 10% of the parcel, 1,200 sq ft, or 90% of the main home. The minimum is 190 sq ft.
Does an ADU need extra parking in South Portland, Maine?
No. No additional parking is required. A detached ADU may be no taller than 28 feet.
Can I rent my ADU on Airbnb in South Portland, Maine?
No. An ADU approved under the current rules may not be used or registered as a short-term rental, and on a single-family lot neither the house nor the ADU may be a short-term rental.
Do I have to live on the property to have an ADU in South Portland, Maine?
The text has no owner-occupancy requirement. However, the ADU and main home must stay in common ownership, so the ADU can't be sold off separately.

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.

State sources: 30-A M.R.S. §4364-B (Accessory dwelling units) · 30-A M.R.S. §4364 (implementation dates) · 25 M.R.S. §2463-B (sprinklers in ADUs) · LD 2173 (P.L. 2025, c. 733) status and chaptered law · Maine Office of Community Affairs, Housing Legislation Guidance (March 2026)

What the ordinance says

Exact text from South Portland's published ordinances. Section numbers are as printed in the source.

  1. Sec. 27-1577(a), Zoning (Code of Ordinances Chapter 27), p. 394
    Accessory dwelling units (ADUs) require permit approval by the Code Enforcement Officer. ADU applications on a property with an existing Site Plan approved by the Planning Board shall require a minor amendment pursuant to Sec. 27-142.
  2. Sec. 27-404, Table A (Allowable Land Uses in Residential Districts), Zoning (Code of Ordinances Chapter 27), p. 114
    Accessory Dwelling Units (ADUs)* P P P P P P P P §27-1576 et seq.
  3. Sec. 27-1578(a)(iii), Zoning (Code of Ordinances Chapter 27), p. 396
    (iii) Incorporating an ADU into a new structure or constructing a detached ADU structure, subject to the following:
  4. Sec. 27-1578(b)(i), Zoning (Code of Ordinances Chapter 27), p. 397
    (i) The modified floor area for ADUs on parcels under 10,890 sq. ft. shall not exceed 800 square feet or 90% of the primary dwelling modified floor area, whichever is smaller;
  5. Sec. 27-1578(b)(ii), Zoning (Code of Ordinances Chapter 27), p. 397
    (ii) The modified floor area of ADUs on parcels of 10,890 sq. ft. or larger shall not exceed 10% of the parcel area, 1,200 sq. ft., or 90% of the primary dwelling unit modified floor area, whichever is smaller.
  6. Sec. 27-1578(b)(iii), Zoning (Code of Ordinances Chapter 27), p. 397
    (iii) No ADU shall have less than 190 square feet of modified floor area.
  7. Sec. 27-1578(e), Zoning (Code of Ordinances Chapter 27), p. 397
    (e) Only one ADU is permitted per lot.
  8. Sec. 27-1578(f), Zoning (Code of Ordinances Chapter 27), p. 397
    The dwellings on the property and the ADU must be held by a common ownership in part or in whole. An ADU may not be sold such that the owner or owners of the principal dwelling no longer maintain partial or full ownership of the ADU.
  9. Sec. 27-1578(h)(iv), Zoning (Code of Ordinances Chapter 27), p. 398
    If the ADU is constructed as a detached structure from the primary dwelling, the maximum height of the detached structure shall not exceed 28 feet.
  10. Sec. 27-1578(i), Zoning (Code of Ordinances Chapter 27), p. 398
    (i) No additional parking spaces shall be required for an ADU.
  11. Sec. 27-1577(b), Zoning (Code of Ordinances Chapter 27), p. 394
    Notwithstanding the provisions of 1 M.R.S. § 302 or any other law to the contrary, commencing on the enactment date of the amendments to this Chapter as evidenced by Ordinance #4-22/23, no accessory dwelling unit reviewed and approved pursuant to an application to the Code Enforcement Officer under this section and Sec. 27-1578 shall be used, occupied or registered as a short-term rental for short-term rental guests, as those terms are defined in Chapter 14, Article XVII, Sec. 14-801 of the Code, as may be amended.
  12. Sec. 27-1578(d), Zoning (Code of Ordinances Chapter 27), p. 397
    Where the creation of an accessory dwelling unit changes the footprint of the structure in which the ADU is located, the addition to the structure must conform to the setback and lot or building coverage requirements for the zoning district in which it is located, except as provided in Section 27-1578(a).
This page summarizes South Portland's posted ordinances as of 2026-10-01. Ordinances are amended, and individual lots can have overlays, subdivision covenants, or HOA rules that are stricter. Confirm with Code Enforcement (Department of Planning & Development) (207-767-7603) before you build. Spot an error?