This might be a bit heartless, but I would say restrict the M or L sized houses to FCs only. Then on top of that, have said FCs require a certain number of active members online... both during purchase and regularly in play.

Again nothing crazy, maybe something like 8 online same time to purchase the L, 4 different accounts enter every 45 days or so to negate demolition.

End result makes more effort to have larger FCs if housing is your jam...and should severely negate the 1 person FC housing nonsense.