are villages allowed to build close to each other? if not, then you'd have ample battlegrounds in between villages...
Well, "Common sense"
Since common sense seems not so common on the interwebs i'll say what I think.
build atleast 50 blocks from a place where some one live, and 30 blocks from a playermade structure that is not inhabited.
Honestly, 100 blocks is completely reasonable, and has been the standard for the last couple of servers. There's no good reason to be intrusive about where you build.
We've also had to extrapolate on that standard of common sense though. Don't build somewhere just to block further progress of another village, don't immediately make a boundary that you have no chance of building on in the near future, etc. Common sense isn't as common as we'd like, but our ops usually have it.