I remember seeing those around Minnesota as well when I was a kid.
We had them in the UP, too, when I was growing up in the fifties. Flat topped basements with tarpaper decks and a triangular stairwell with door, sewer vent stack, and chimney protruding above the deck. Some on small subdivision lots before zoning codes or building permits, some on propertied hogged out of the rural wilderness.
I had forgotten about them, as they gradually got built out to small post WWII sub 1000 sf ranch style houses. As a kid, I always assumed they were built by returning GI’s working low paying jobs. No kids that I knew at the time lived in one. I sort of felt sorry for the occupants at the time, although I was probably well under ten years old then.
Sort of makes me a little nostalgic for simpler times.