The incident happened Saturday afternoon in the Township of Lake of Bays, which is located nearly 250 kilometres north of Toronto.
Ontario Provincial Police responded to a call from a property on South Camp Lake Road shortly after 3 p.m. ET.
Police say that several people were doing work on the cottage when "the roof of the cottage fell on one of the parties."
One person died.
The incident remains under investigation.
The District Municipality of Muskoka is made up of a handful of smaller municipalities including the Township of Lake of Bays.