2026 Cost Guide · Updated September 2026

ADU Cost in Maine — Portland, Augusta, Bangor & Statewide

Maine enacted ADU reform legislation in 2022. Here's what ADUs cost across Portland, Augusta, Bangor, and statewide — with the deepest frost depth in the continental U.S. factored in.

$88K–$318K Detached ADU (Statewide)

$54K–$158K Garage conversion

HUD FY2026Adopted rent benchmark

LD 2003 (2022) ADUs by right statewide

MAINE
ADU Collection

Beautiful. Functional. Built for Maine Living.

🏠Increase Property Value

💲Generate Rental Income

👥Multi-Generational Living

🌿Sustainable Design

Maine ADU outdoor living and patio space

Planning & Inspiration

Planning Reference

Example 1-bedroom ADU floor plan, illustrative layout

Example Floor Plan

Illustrative 1-bedroom layout. Actual floor plans vary by ADU type, size, local code, and homeowner needs.

Maine ADU exterior at twilight, warm interior light glowing

Maine ADU and main house evening backyard view

Aerial view of Maine property with backyard ADU

Homeowners reviewing ADU permit documents

Permits & Fees

What permitting actually costs.

State housing law requires ADU allowance in covered residential areas, subject to current statute and municipal administration. The detailed figures below are representative planning ranges, not a statewide fee schedule or review-time guarantee.

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Building Permit Fee

$1,800 – $8,500

Varies significantly by city and project value.

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Plan Check / Review Fee

Included or 65% of permit fee

Some jurisdictions combine with the permit fee.

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Utility Connection Fees

$3,000 – $18,000

Water, sewer, and electrical connections.

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Impact / Development Fees

$0 – $8,000

School, park, and transportation fees where applicable.

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Design & Engineering

$4,000 – $20,000

Architecture, structural engineering, energy compliance.

Verified September 17, 2026 · Effective framework

30-A M.R.S. § 4364-B

30-A M.R.S. § 4364-B requires ADU allowance in covered residential areas. LD 1829 (2025) expanded density statewide; LD 2173 (2026) moderated parts of that expansion and extended the compliance deadline to July 1, 2027.

Legal classificationEffective framework

Governing framework30-A M.R.S. § 4364-B

Representative permit fees$1,800 – $8,500

Representative review time5–14 weeks

Local verificationRequired

Source: Maine Legislature — § 4364-B →

Verify the property before design

These are planning benchmarks, not a permit quote, eligibility ruling, or legal opinion. Confirm zoning, building, utility, site, private-covenant, fee, and timing requirements with the current local authorities. See the 51-jurisdiction law directory and national permit guide.

ADU Types

Which type fits your property?

01 / Highest Value

Detached ADU

$88K – $318K

New foundation and freestanding structure — highest cost, highest value. Greater Portland (Cumberland County) commands the top of this range; Augusta and Bangor sit toward the bottom.

Detached ADU Guide →

02 / Most Affordable

Garage Conversion

$54K – $158K

The most affordable Maine ADU option, using an existing structure. Cost-effective statewide, converting 40–55% less than a comparable detached new build with similar rental income.

Garage Conversion Guide →

03 / Fastest Timeline

Modular / Prefab

$72K – $238K

Factory-built construction offers the fastest timeline of any ADU type — a real advantage in Maine's 6-month outdoor construction season, where new-build foundation work is weather-constrained.

Prefab ADU Overview →

04 / Shares One Wall

Attached ADU

$85K – $265K

An addition sharing one wall with the main house — typically 20–35% less than detached construction. A strong option on smaller lots where a detached structure isn't feasible.

Attached ADU Guide →

05 / Year-Round Build

Basement Conversion

$58K – $178K

Maine's deep frost depth — the deepest in the continental U.S. — means most homes already have full basements, a genuine year-round build option unaffected by the outdoor construction season. Egress windows are required.

Basement ADU Guide →

Cost by Region

Every Maine market is different.

Greater Portland

Cumberland County · Portland · South Portland

$122K–$318K

$178–$368 per sq ft

Highest cost

Augusta / Central Maine

Augusta · Central Maine

$88K–$225K

$132–$285 per sq ft

Moderate

Bangor / Penobscot County

Bangor · Penobscot County

$88K–$205K

$132–$262 per sq ft

Most affordable

Cost Factors

Why Maine ADUs cost what they cost.

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Labor Rates

Local skilled trades wages typically account for 40–50% of total ADU project cost in Maine, the largest single cost variable. Greater Portland commands premium rates versus the rest of the state, where fewer specialty contractors mean more scheduling lead time.

40–50% of total cost

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Frost Depth Foundation

Maine has the deepest frost depth in the continental U.S. at 48 inches, requiring deeper foundations with more concrete and excavation than slab-on-grade construction in warmer climates.

$15K–$35K added cost

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Current ADU Legal Framework

30-A M.R.S. § 4364-B requires ADU allowance in covered residential areas. LD 1829 (2025) expanded statewide density requirements; LD 2173 (2026, Public Law Chapter 733) moderated parts of that expansion and extended municipalities' compliance deadline to July 1, 2027.

Effective framework · 30-A M.R.S. § 4364-B

At a Glance

Maine ADU costs — by type.

Detached ADU

$88K – $318K

$178–$368 per sq ft · Greater Portland skews highest

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Attached ADU

$85K – $265K

Shares one wall · 20–35% less than detached

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Garage Conversion

$54K – $158K

Most affordable · uses existing structure

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Basement Conversion

$58K – $178K

Year-round build · egress windows required

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Modular / Prefab

$72K – $238K

Factory-built · fastest construction timeline

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Effective framework: 30-A M.R.S. § 4364-B. State housing law requires ADU allowance in covered residential areas; LD 1829 (2025) expanded density statewide, and LD 2173 (2026) moderated parts of that expansion and extended the municipal compliance deadline to July 1, 2027. Permit fees and review timing remain planning estimates until confirmed for the specific property.

Calculate My ROI →

Return on Investment

Maine ROI planning inputs.

$1,000–$1,525

Adopted statewide 1-bedroom rent range

15%

National property-value planning input

10%–20%

Property-value uncertainty range

30%

Operating-expense planning reserve

Rent source

HUD FY2026 revised

One-bedroom FMR market basket

Exact HUD mappings

ROI-CM-1.4

Home-value source

Statewide Zillow ZHVI

June 30, 2026

One statewide input

Not a valuation

Payback period

Calculated live

Budget, ADU type, size and rent

Open calculator →

No static statewide promise

Full methodology

Transparent sources

HUD, Census PUMS, Zillow and FHFA

Review methodology →

Planning estimate, not a valuation

Save Money

5 ways to cut your
Maine ADU cost now.

These moves push savings further and prevent the most common costly mistakes homeowners make on Maine ADU projects.

Calculate My ADU ROI →

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Compact Detached ADU

Maine — statewide range

Property-Value Planning Input

15%

Convert before you build new

If your property has a qualifying garage or basement, conversions cost 40–55% less than a comparable detached new-construction ADU while generating similar rental income in most Maine markets.

Saves 40–55% vs. new construction

Get a minimum of three contractor bids

Contractor pricing for ADU projects in Maine varies 20–35% for equivalent scope. Three bids from contractors with ADU permit experience in your city is the most reliable way to avoid overpaying.

Pricing varies 20–35% for equivalent scope

Start your contractor search during permit review

Experienced ADU contractors in active Maine markets book 2–4 months in advance. Starting your search early means you can break ground the day your permit arrives.

Avoids months of schedule delay

Right-size for your market's rent ceiling

In most Maine markets, a 600–700 sq ft 1-bedroom ADU commands rents close to a 900 sq ft 2-bedroom — at significantly lower construction cost. Size to the rent curve, not the maximum allowed square footage.

Lower build cost at near-equal rent

Explore financing options before you start

HELOC, cash-out refinance, and ADU-specific loan programs have different costs and qualification criteria. Choosing the right option can save real money over the life of the project.

Can save $10,000–$40,000 over the project

Frequently Asked Questions

Maine ADU answers.

How much does an ADU cost in Maine?

ADU costs in Maine range from $54,000 for a basic garage conversion to $318,000 for a large detached new build in the most expensive markets. The most common project — a 600–700 sq ft 1-bedroom detached ADU — falls in the mid-range for your region. Always get three contractor bids before finalizing your budget.

What ADU law applies in Maine?

30-A M.R.S. § 4364-B requires municipalities to allow ADUs in covered residential areas. This framework was significantly expanded by LD 1829 (2025) and then partly moderated by LD 2173 (2026, Public Law Chapter 733) — see the next question for how those two laws relate. Local zoning, building, utility, site, and private-covenant requirements may still apply; verify the current rules for the property before design or construction.

How did Maine's LD 1829 and LD 2173 change ADU rules?

LD 1829 (2025), signed by Governor Janet Mills and effective around late September 2025, significantly expanded Maine's housing density rules: it required municipalities to allow at least 3 dwelling units on any residential lot (up to 4 in growth areas or lots served by water and sewer), capped minimum lot size at 5,000 sq ft in serviced urbanized areas, allowed ADUs on nonconforming lots without increasing the nonconformity, and removed owner-occupancy requirements. LD 2173 (2026), sponsored by Rep. Amanda Collamore (R-Pittsfield) and signed April 16, 2026 as Public Law Chapter 733, then moderated parts of that expansion: it extended municipalities' ordinance-compliance deadline to July 1, 2027, reinstated rate-of-growth ordinances (capped at 105% growth over a 10-year average) that LD 1829 had removed, and doubled the minimum lot size in non-growth areas served by water and sewer from 5,000 to 10,000 sq ft.

How long does it take to get an ADU permit in Maine?

The representative ADU permit-review planning timeline for Maine is 5–14 weeks. This is not a statewide deadline or guarantee. Timing can change with the local authority, project complexity, corrections, utilities, site conditions, and application completeness; confirm the current timeline locally.

What is the best ADU type for my Maine property?

If your property has a qualifying garage or full basement, start there — conversions cost 40–55% less than detached new construction while generating similar rental income in most Maine markets. If conversions aren't viable, a detached new build maximizes rental income and property value.

What is the cheapest ADU to build in Maine?

Garage conversions are the least expensive option, typically $54,000–$158,000, using an existing structure. Basement conversions are also cost-competitive statewide at $58,000–$178,000, thanks to Maine's deep frost depth requiring most homes to already have full basements.

What is the ROI on an ADU in Maine?

The adopted Maine one-bedroom planning range is $1,000–$1,525 per month, derived from the HUD FY2026 revised FMR market basket under ROI-CM-1.4. Payback is calculated from the homeowner's budget, ADU type, size, rent range, and a 30% operating-expense reserve. Property value uses a 15% national planning input with a 10%–20% uncertainty range; it is not a valuation.

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Planning Disclaimer: Cost, fee, and timeline ranges are approximations for education and early planning; actual requirements and costs vary with the property, local authority, utilities, site conditions, design, and contractor. The Maine legal classification and framework shown here were verified on September 17, 2026, but they do not determine parcel eligibility and are not legal advice. Confirm current zoning, building, utility, private-covenant, fee, and timing requirements directly with the applicable authorities before design or construction. Obtain at least three contractor bids before committing to a project budget. Last updated September 2026.