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Building an ADU in Portland, Maine

City code and Maine law · Last reviewed October 10, 2026

Portland, Maine's Land Use Code allows accessory dwelling units (ADUs) on every lawful lot with a legal residential use, and up to two ADUs per property. An ADU can be a new detached building, an addition, or space inside an existing building. Portland requires no off-street parking. You need a building permit from the Permitting and Inspections Department, applied for online, and new ADUs pay city impact fees. [1][2]

Portland at a glance

Where ADUs are allowed
On all lawfully conforming and nonconforming lots with legal residential uses [1]
Number per property
Up to two [1]
Types
New detached building, building addition, or inside an existing building [1]
Max size
2/3 of the gross floor area of the largest principal unit Detached ADUs together no larger than the principal structure [1]
Detached ADU height
18 ft, or 25 ft if built above a garage [1]
Side and rear setbacks (detached)
Same as small accessory structures: 5 ft rear in RN-1 to RN-6 5 ft side in RN-1, RN-2, RN-5 and 3 ft in RN-3, RN-4, RN-6 [1]
Front yard
Detached ADUs can't sit between the house and the front lot line [1]
Parking
No off-street parking required [1]
Lot coverage and density
ADUs are exempt from maximum lot coverage and minimum lot area per unit [1]
Building permit fee
$25 for the first $1,000 of work plus $15 per additional $1,000 [5]
Impact fees
New ADUs pay city impact fees Parks, transportation, wastewater [2][6]
Short-term rental
A mainland ADU can be licensed as a non-owner-occupied short-term rental for up to 5 years from its Certificate of Occupancy [8]
On this page
  1. At a glance
  2. What you can build in Portland
  3. Size and height
  4. Setbacks
  5. Parking
  6. Fees
  7. Garage conversions and fire sprinklers
  8. Renting it out and short-term rentals
  9. Permit steps
  10. Who to call
  11. Questions people ask

What you can build in Portland

Section 6.6.2(A) of Portland's Land Use Code (Chapter 14 of the City Code) says ADUs "shall be permitted on all lawfully conforming and nonconforming lots with legal residential uses." You can build them "as new detached accessory structures, building additions, or within existing lawfully conforming or nonconforming structures." Adding an ADU can't make a structure more nonconforming unless the section allows it.

"Up to two ADUs shall be permitted per qualifying property." The city's zoning FAQ says ADUs are "smaller and subordinate to the principal dwelling unit(s) on the property" and that creating one requires a building permit.

Portland adopted a rewritten land use code (ReCode) that took effect December 4, 2024, and the current code PDF is marked effective August 24, 2026. One limit: ADUs are not permitted within a cottage court residential development.

Maine's state law also requires towns to allow at least one ADU with a single-family home. Portland's rules allow more. See our Maine ADU law guide. Looking for the other Portland? See our Portland, Oregon ADU page. [1][2][3][4]

Size and height

Under Section 6.6.2(A):

[1]

Setbacks

In residential zones, detached ADUs, "regardless of size," follow the side and rear setbacks for small detached accessory structures in Article 7. In the residential neighborhood zones (Table 7-A), those are:

Detached ADUs "shall not be permitted between a principal structure and a front lot line." Island zones have their own dimensional table. Look up your zone on the city's zoning map and confirm the setbacks with Zoning Administration. [1]

Parking

Portland doesn't require parking. Section 18.1.2 of the Land Use Code says: "Off-street vehicle parking is not required." Some larger developments need a Transportation Demand Management Plan, but the code doesn't list that for a single ADU. [1]

Fees

Portland's building permit fee schedule (revised July 2026) charges by cost of work:

Impact fees: the city's zoning FAQ says "New ADU's are subject to payment of the city's impact fees." The 2026 schedule charges single-family and two-family units $1,461 per unit for parks and recreation and $2,802 per unit for transportation, plus a wastewater fee by water meter size (for example $2,448 for a 5/8-inch meter). Permitting and Inspections decides which category applies, and you pay before the Certificate of Occupancy. The FAQ also says an ADU may need a new sewer line. [1][2][5][6]

Garage conversions and fire sprinklers

The city's zoning FAQ says "An attached or detached garage may qualify for conversion to an Accessory Dwelling Unit." The code allows ADUs within existing lawful conforming or nonconforming structures, as long as the conversion doesn't increase the nonconformity.

The same FAQ warns that creating an ADU "may require installation of a new sewer line and installation of a fire sprinkler system." Under state law, 25 M.R.S. 2463-B, sprinklers are not required for an ADU unless it is within or attached to a structure with more than 2 dwelling units (counting ADUs), or other fire codes require them. Ask the city's plan reviewers which rule applies to your building. See our garage conversion guide for the general steps. [1][2][7]

Renting it out and short-term rentals

Section 6.6.2(A) doesn't require the owner to live on the property. Any rental listed for 29 days or less must be registered as a short-term rental, with the license number posted in every online ad.

Since September 1, 2025, a mainland ADU "can be licensed as a non-owner occupied short term rental for up to 5 years from the date of the Certificate of Occupancy." You renew by December 31 each year and include a copy of the Certificate of Occupancy. After five years, the ADU must join the waitlist for a non-owner-occupied license or become a registered long-term rental. Call the Licensing and Housing Safety Office at 207-756-8131 with questions. [1][8]

Permit steps in Portland

  1. Find your zone and rules

    Look up your property on the city's zoning map and read Section 6.6.2(A) and Article 7 of the Land Use Code. [1][3]

  2. Ask the zoning staff

    Book a 30-minute remote Ask the Zoning Staff meeting and attach your drawings, or email zoning@portlandmaine.gov. [2][9]

  3. Apply online through Citizen Self Service

    Create a CSS account and submit the building permit in the portal, with two PDFs named "Drawings" and "Documents." See the Building Permits page. [10][11]

  4. Pay the invoice and go through review

    After intake staff confirm the application is complete, pay the invoice. Staff then review zoning, building code and life safety; new detached ADUs also get planning review, and properties in historic districts get historic review. [11][12]

  5. Build, inspect and get the Certificate of Occupancy

    Once the permit is issued, construction may begin. Schedule the required inspections, pay any impact fees before the Certificate of Occupancy, and pay the $100 Certificate of Occupancy inspection fee. [5][6][12]

Who to call

Questions people ask

How many ADUs can I build in Portland, Maine?

Up to two per qualifying property, on any lawful lot with a legal residential use. [1]

How big can an ADU be in Portland, Maine?

Up to 2/3 of the gross floor area of the largest principal unit on the lot. All detached ADUs together can't be larger than the principal structure, and detached ADUs are limited to 18 feet tall (25 feet over a garage). [1]

Do I need a parking space for an ADU in Portland, Maine?

No. Portland's Land Use Code says off-street vehicle parking is not required. [1]

Can I rent my Portland ADU on Airbnb?

A mainland ADU can be licensed as a non-owner-occupied short-term rental for up to 5 years from its Certificate of Occupancy, renewed each year by December 31. After that it must join the waitlist or become a long-term rental. [8]

What does an ADU permit cost in Portland, Maine?

The building permit is $25 for the first $1,000 of work plus $15 for each additional $1,000. New ADUs also pay city impact fees, and the Certificate of Occupancy inspection is $100. [2][5]

Sources (13)
  1. City of Portland Land Use Code (Chapter 14), effective 8/24/2026, City of Portland, Maine. Accessed October 10, 2026.
  2. Zoning Frequently Asked Questions, City of Portland, Maine Permitting and Inspections Department. Accessed October 10, 2026.
  3. Zoning Administration, City of Portland, Maine. Accessed October 10, 2026.
  4. Title 30-A, §4364-B: Accessory dwelling units, Maine Legislature, Office of the Revisor of Statutes. Accessed October 10, 2026.
  5. Building Permits and Inspections Fee Schedule (revised 7/2026), City of Portland, Maine Permitting and Inspections Department. Accessed October 10, 2026.
  6. Impact Fee Frequently Asked Questions (January 2026), City of Portland, Maine. Accessed October 10, 2026.
  7. Title 25, §2463-B: Fire protection in accessory dwelling units, Maine Legislature, Office of the Revisor of Statutes. Accessed October 10, 2026.
  8. Short-Term Rental Registration, City of Portland, Maine. Accessed October 10, 2026.
  9. Ask the Zoning Staff Meetings, City of Portland, Maine. Accessed October 10, 2026.
  10. Building Permits, City of Portland, Maine. Accessed October 10, 2026.
  11. Instructions for Submitting Permit Applications Online, City of Portland, Maine Permitting and Inspections Department. Accessed October 10, 2026.
  12. Residential Permit Process, City of Portland, Maine Permitting and Inspections Department. Accessed October 10, 2026.
  13. Permitting & Inspections, City of Portland, Maine. Accessed October 10, 2026.

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