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Can I build an ADU in Portland, Maine?

Portland, Cumberland County · Ordinances checked 2026-10-01 · Reviewed · 13 cited sections

Allowed with a permit

Yes. Portland allows up to two ADUs on any lot with a legal residential use. An ADU can be inside the house, an addition, or a new detached building, and it can be up to 2/3 the size of the main unit. Detached ADUs can be 18 feet tall (25 feet over a garage), can't sit between the house and the front lot line, and need no parking. You'll need a building permit; we found no owner-occupancy requirement in the Land Use Code.

Where allowedAll lawfully conforming and nonconforming lots with legal residential uses [1]
Number allowedUp to 2 ADUs per qualifying property [2]
Max size2/3 of the gross floor area of the largest principal unit [3]
Detached ADU total sizeCombined detached ADUs can't exceed the principal structure's floor area [4]
Detached ADU height18 feet; 25 feet if built above a garage [5]
Detached ADU setbacks (residential zones)Small-accessory-structure side/rear setbacks regardless of size (e.g., 5 ft rear; 3-5 ft side in RN zones) [6]
Front yardDetached ADUs not allowed between the house and the front lot line [7]
ParkingNone required (citywide) [8]
Lot coverage / lot area per unitADUs are exempt [9]
Short-term renting an ADUTreated as non-owner-occupied for STR rules; an ADU registered within 5 years of its certificate of occupancy is exempt from the citywide cap [10]

Where and how many

Section 6.6.2(A) says ADUs are allowed on all lawfully conforming and nonconforming lots with legal residential uses [1]. Up to two ADUs are allowed per qualifying property [2]. An ADU can be a new detached accessory structure, a building addition, or space inside an existing structure, but adding one can't make a nonconforming structure more nonconforming. The code defines an ADU as a dwelling unit subordinate in size to the principal residential structure, located inside it or in an accessory structure.

Size and height

An ADU can be no larger than 2/3 of the gross floor area of the largest principal unit on the lot [3]. The total square footage of detached ADUs on a lot can't exceed the gross floor area of the principal structure [4]. Detached ADUs are limited to 18 feet in height, or 25 feet if built above a garage [5]. ADUs are exempt from maximum lot coverage and minimum lot area per dwelling unit [9]. When converting an existing nonconforming structure, the design must consider neighbors' privacy where practicable.

Setbacks and placement

In residential zones, a detached ADU of any size uses the side and rear setbacks for detached accessory structures under 250 sq ft [6]. In the RN zones that's 5 ft from the rear line [12] and 3 or 5 ft from the side lines, depending on the zone [13] (see Sheds). Detached ADUs can't go between the house and the front lot line [7].

Parking, occupancy and approvals

Portland doesn't require off-street parking for any use [8]. We found no owner-occupancy requirement for ADUs in the Land Use Code. Building an ADU needs a building permit, and a certificate of occupancy is required before anyone moves in. In the Coastal Flood Resilience Overlay, ADUs are classified as "critical" uses and may face extra flood-resilience standards. If you plan to rent the ADU on Airbnb, Chapter 6 counts ADUs as non-owner-occupied short-term rentals [10]. Those are capped citywide, but an ADU that got its certificate of occupancy within 5 years before registering is excluded from the cap [11]. ADUs aren't allowed in cottage court developments.

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

Next steps

  1. Confirm your lot has a legal residential use and check your zone and any overlay (historic district, shoreland, Coastal Flood Resilience) on the city zoning map.
  2. Size the ADU at no more than 2/3 of the main unit's floor area; for a detached ADU, plan for 18 ft max height (25 ft over a garage), keep it out of the front yard, and meet small-structure side/rear setbacks.
  3. Apply for a building permit with Permitting & Inspections (207-874-8703) with floor plans, a site plan and elevations; separate electrical and plumbing permits apply.
  4. Get a certificate of occupancy before anyone moves in; if you plan short-term rental use, register it with the city (a newly built ADU is exempt from the citywide STR cap for 5 years after its certificate of occupancy).

Common questions

Can I build an accessory dwelling unit (ADU) in Portland, Maine?
Yes. Portland allows up to 2 ADUs on any lot with a legal residential use. An ADU can be inside the house, an addition, or a new detached building. You'll need a building permit.
How big can an ADU be in Portland, Maine?
An ADU can be up to 2/3 of the gross floor area of the largest principal unit on the lot. Detached ADUs can be 18 feet tall, or 25 feet if built above a garage.
Does Portland, Maine require parking for an ADU?
No. Portland doesn't require off-street parking for any use, ADUs included. Detached ADUs can't sit between the house and the front lot line.
Do you have to live on the property to have an ADU in Portland, Maine?
The page found no owner-occupancy requirement for ADUs in Portland's Land Use Code. A building permit is required, and a certificate of occupancy is needed before anyone moves in.
Can I Airbnb an ADU in Portland, Maine?
It's restricted. Portland treats ADUs as non-owner-occupied short-term rentals, which are capped citywide. An ADU registered within 5 years of its certificate of occupancy is exempt from the cap.

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 Portland's published ordinances. Section numbers are as printed in the source.

  1. Sec. 6.6.2(A)(1), Land Use Code (Code of Ordinances Chapter 14, ReCode), p. 98
    1. Accessory Dwelling Units (ADUs) shall be permitted on all lawfully conforming and nonconforming lots with legal residential uses.
  2. Sec. 6.6.2(A)(3), Land Use Code (Code of Ordinances Chapter 14, ReCode), p. 98
    3. Up to two ADUs shall be permitted per qualifying property.
  3. Sec. 6.6.2(A)(5), Land Use Code (Code of Ordinances Chapter 14, ReCode), p. 98
    5. An ADU shall be limited to a gross floor area of 2/3 of the gross floor area of the largest principal unit on the lot.
  4. Sec. 6.6.2(A)(5), Land Use Code (Code of Ordinances Chapter 14, ReCode), p. 98
    The aggregate square footage of detached ADUs on a lot shall not exceed the gross floor area of the principal structure.
  5. Sec. 6.6.2(A)(7), Land Use Code (Code of Ordinances Chapter 14, ReCode), p. 98
    7. Detached ADUs shall be limited to a maximum height of 18 feet, unless constructed above a garage, in which case the height of the structure shall be limited to 25 feet.
  6. Sec. 6.6.2(A)(8), Land Use Code (Code of Ordinances Chapter 14, ReCode), p. 98
    8. In residential zones, detached ADUs, regardless of size, shall be subject to side and rear setbacks for detached accessory structures less than 250 square feet, as established in Article 7.
  7. Sec. 6.6.2(A)(9), Land Use Code (Code of Ordinances Chapter 14, ReCode), p. 98
    9. Detached ADUs shall not be permitted between a principal structure and a front lot line.
  8. Sec. 18.1.2, Land Use Code (Code of Ordinances Chapter 14, ReCode), p. 335
    Off-street vehicle parking is not required.
  9. Sec. 6.6.2(A)(6), Land Use Code (Code of Ordinances Chapter 14, ReCode), p. 98
    6. ADUs shall be exempt from maximum lot coverage and minimum lot area per dwelling unit requirements.
  10. Sec. 6-150.1 (Owner-Occupied), Buildings and Building Regulations (Chapter 6) – incl. short-term rental registration, p. 38
    Accessory dwelling units are not considered owner- occupied units for purposes of short term rental registration and regulation.
  11. Sec. 6-153(b), Buildings and Building Regulations (Chapter 6) – incl. short-term rental registration, p. 47
    Mainland accessory dwelling units that are registered as non-owner occupied short term rentals and that were constructed and received a certificate of occupancy within 5 years before the unit’s registration date are excluded from the 1.5% limit set forth in this paragraph.
  12. Table 7-A, Rear setback (min.), Land Use Code (Code of Ordinances Chapter 14, ReCode), p. 116
    Detached accessory structures 5 ft. 5 ft. 5 ft. 5 ft. 5 ft. 5 ft. (≤250 SF footprint)
  13. Table 7-A, Side setback, interior (min.), Land Use Code (Code of Ordinances Chapter 14, ReCode), p. 116
    Detached accessory structures 5 ft. 5 ft. 3 ft. 3 ft. 5 ft. 3 ft. (≤250 SF footprint)
This page summarizes 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 Permitting & Inspections Department (zoning, building, code enforcement) (207-874-8703) before you build. Spot an error?