The exterior/interior doors in player housing properties aren't connected and don't allow a visitor to "knock." Some properties are also large or separated enough that a knock wouldn't be heard anyway. I think that it would be a good, useful feature for homeowners to have some kind of bell system attached to their property signs, that alerts the owner to a potential guest trying to reach them.
This would have a secondary benefit of facilitating meetings with homeowners, which would then presumably happen inside the pertinent home.
I am pro-doorbell.
Incidentally knocking works correctly in the current Gellemede temple property (1? 2?) (I realize however this is not universally the case)