It's up to each FC, but they could do something where if you donate, you get access, if you don't, then you can't use it. Something like leader pays for 25% of the cost up front and the other 75% is donated by the members. Maybe have some threshold that when met, gives access to rooms.

Donate 100k, get 1 room
Donate 200k, get 2 rooms

This of course assumes FC housing is one mansion with like 40 rooms. If you want access to the FC house, then you have to help build it, either with money or mats. That's how I would do it anyway. Though most FCs have more points than they know what to do with, SE could be using housing as a huge gil sink.