Usually you wont find a flag pole on private property in the cities and it's clearly far, far, far, far, far away from e.g. US levels of showing the flag, but it's hard to picture a Schrebergartenkolonie[1] without a german flag (even the wikipedia picture show one: [2]). Same is true for private properties where flag poles are more common (rural or costal regions).
I agree it's not a custom, but it's not definitely no taboo.
"The German flag, long weighted by the country's postwar reluctance about open displays of national pride, is flying again, an expression of exuberance as Germany plays host to the World Cup" [1]
Either it's illegal or a social taboo. And to my knowledge neither of them are true.