Do I need a permit for deck construction or replacement in Unincorporated Mahnomen County?

Most likely yes. Deck construction or replacement usually requires a permit in Minnesota. Confirm the specifics with Unincorporated Mahnomen County.

Minnesota's permit rule exempts only decks and platforms not more than 30 inches above adjacent grade that are not attached to a structure with frost footings and are not part of an accessible route (Minn. R. 1300.0120, subp. 4). Minimum footing depth for frost protection is 5 feet in the northern counties (Zone I) and 3 1/2 feet in the rest of the state (Zone II) unless an engineer shows otherwise (Minn. R. 1303.1600). Whether a building permit is issued at all depends on whether the city, township, or county covering the address has adopted and enforces the State Building Code (Minn. Stat. 326B.121); check the DLI local code lookup first. This deck is attached to the house or will have a roof. Where a small deck exemption exists at all, it covers only small, low, freestanding decks that do not serve the home's required exit door, so an attached deck falls outside it. A ledger bolted to the house is the most common point of deck failure, which is why departments inspect the footings, framing and ledger connection before the decking goes on. Adding a roof makes the deck a covered structure that is reviewed like a patio cover or addition, including roof framing and, in high wind areas, uplift connections.