In my country (Germany) afaik a little over 10% of the population are at least millionaires. So I think the likelihood to have a millionaire living next door is quite high.
Anecdotal (but nevertheless true): I have (at least) two millionaire neighbours (less than 3 doors/gates away). Both won the lottery - one euromillions and one national lotto draw.
And if we are talking average income, then everyone on this side of the village is a millionaire simply because of those two. However, most of the rest of us seem to struggle daily.
There is some chance that the bank owns their house, or at least some sizeable fraction of it. Mortgages are more common than houses that are paid off.
And if we are talking average income, then everyone on this side of the village is a millionaire simply because of those two. However, most of the rest of us seem to struggle daily.