From Nookipedia, the Animal Crossing wiki

The Windmill is a landmark that first appears in Animal Crossing: City Folk. When a total of 1,000,000 Bells is donated into the Town Fund (after the fountain has been built), the human residents in town will have a chance to vote (by talking to Pelly in the Town Hall) on whether a Windmill or a Lighthouse should be added to town. The winning choice will appear near the beach after a week of voting, and Tortimer will appear next to it on the day of completion. Neither the Windmill nor the Lighthouse appear to have any effect on gameplay.

The Windmill as it appears in the public works project list in New Leaf.
Function Public Works Project (New Leaf)
Main appearances
Other appearances
The Windmill in City Folk

It is possible to switch from the Windmill to the Lighthouse (or vice versa) by donating a further 1,000,000 Bells to the Town Fund after the initial structure is built.

In Animal Crossing: New Leaf, the Windmill returns as a public works project. It costs 372,000 Bells to install and is suggested by Sisterly (Uchi) villagers.

Town (Main Street)CityAnimal IslandTortimer IslandHarv's IslandMystery Islands
Player's house (Attic)Villager housesBridgeBulletin BoardEvent Plaza (Town Tree)FountainInclineLighthouseSignpostWindmillWishing Well
Transit Facilities
AirportBus stopPierTown GateTrain Station
Civic Facilities
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General Store
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Happy Home Designer
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Pocket Camp
CampsiteCabinMarket Place (Nookling GlobalAble Sisters To GoKicks)Breezy HollowLost Lure CreekOK MotorsSaltwater ShoresShovelstrike QuarrySunburst Island
Implied locationsBoondoxCategory