Casco Bay Islands Property Tax: Two Very Different Systems Under One Ferry Line

Long Island and Chebeague Island: Their Own Towns, Their Own Assessors

Long Island's secession from Portland followed Portland's 1990 property-tax revaluation, which sharply raised taxes on the island; residents argued the tax burden was out of proportion to the municipal services the island actually received, and the town became its own incorporated municipality on July 1, 1993. Chebeague Island's secession, on July 1, 2007, had a different trigger entirely: after 184 years as part of the town of Cumberland, islanders pushed to secede primarily to protect their own small elementary school amid Cumberland-district downsizing pressure. The Maine Legislature authorized Chebeague's secession through a specific public law (PS Law, Chapter 47), following a statewide legislative vote on April 5, 2006.

What that means practically: Long Island has its own town office and assessor, and Chebeague Island has its own town office and assessor, each independently setting its own mill rate and issuing its own tax bills, entirely separate from Portland and from each other. No current, independently confirmed mill rate for either the town of Long Island or the town of Chebeague Island was found for this page, and this page will not invent one -- a wrong number on a page meant to inform a real purchase decision is worse than no number at all. Anyone evaluating a property on either island should contact that island's own town office directly: the town of Long Island (townoflongisland.us) or the town of Chebeague Island (townofchebeagueisland.org) for the current mill rate, assessed value, exemption rules, and appeal procedures.

Great Diamond, Little Diamond, and Cliff Island: Taxed by the City of Portland

Great Diamond Island, Little Diamond Island, and Cliff Island have no secession story at all -- unlike Long Island and Chebeague, they have always been part of the City of Portland, and property on all three is assessed and taxed by Portland's own city assessor's office, the same office that handles every mainland Portland neighborhood. That means owners on these three islands are subject to Portland's own citywide mill rate, not an island-specific one.

Portland's own confirmed FY2026 mill rate is $11.98 per $1,000 of assessed value, down from FY2025's $15.01, following a citywide property revaluation that raised assessed values by roughly 43%. That combination matters: a lower nominal mill rate does not automatically mean a lower tax bill, because it is being applied to a much higher assessed value than before. Many Portland-area owners, including on these islands, have seen their actual dollar tax bill move differently than the headline rate alone would suggest -- so treat the $11.98 figure as the applicable nominal rate for Great Diamond, Little Diamond, and Cliff Island, not as a shortcut to estimating any specific bill.

Why These Islands Make a Bad Place to Guess at a Tax Bill

Real estate on all five of these islands is genuinely thin and heavily seasonal. Little Diamond Island had as few as three total housing units per one demographic source, and Great Diamond, Little Diamond, and Cliff Island all lack a reliable, independently confirmed current median sale price -- these are markets where a handful of annual sales can make any typical figure statistically meaningless. Assessed values on a handful of unique waterfront or historic properties, a redeveloped former military post like Diamond Cove, or a tiny year-round cottage on Cliff Island can vary enormously from one parcel to the next, in a way a single town or citywide average rate cannot capture.

That is exactly why this page states the systems and the one confirmed number (Portland's mill rate) rather than projecting a dollar bill for any specific property. Before assuming any tax figure for a Long Island, Chebeague Island, Great Diamond Island, Little Diamond Island, or Cliff Island property, verify the current assessed value and mill rate directly with the relevant office: the town of Long Island's or Chebeague Island's own assessor for those two towns, or the City of Portland assessor's office for the three Portland-administered islands.

A Note on Peaks Island, and Maine's General Tax Framework

Peaks Island, the largest and most-visited of the Casco Bay islands, is also part of the City of Portland and is covered in depth, including its own property tax page, elsewhere on this site -- it is mentioned here only as context, since it shares Portland's assessor's office with Great Diamond, Little Diamond, and Cliff Island. Like every Maine municipality, Long Island, Chebeague Island, and Portland each fund local government primarily through property taxes billed at a locally set mill rate, and Maine also offers a statewide Homestead Exemption concept, generally available to owners who make a qualifying property their permanent legal residence for a required period, which reduces the taxable value used to calculate the bill. Exact eligibility rules and dollar amounts are set by the State of Maine and administered locally, and they can change, so this page describes the concept generically rather than stating current numbers.

What This Means Before You Decide

Put together, honestly: there is no one Casco Bay islands property tax rate. Long Island and Chebeague Island are each their own incorporated town with their own assessor and their own mill rate, neither of which was independently confirmed for this page. Great Diamond Island, Little Diamond Island, and Cliff Island are taxed under the City of Portland's own system, where the confirmed FY2026 mill rate is $11.98 per $1,000 of assessed value, down from FY2025's $15.01 following a roughly 43% citywide revaluation -- a lower nominal rate that does not necessarily mean a lower bill once the higher assessed values are applied.

None of this is a reason to avoid any of these five islands -- it is the real structural picture of how tax obligations actually work here, which is genuinely more complicated than on a single mainland town. Before making any purchase decision, confirm the current assessed value, mill rate, exemption eligibility, and appeal procedures directly with the relevant office -- the town of Long Island, the town of Chebeague Island, or the City of Portland assessor's office -- and consult a qualified Maine tax professional or attorney for anything specific to your situation. Nothing on this page is legal or tax advice.