What determines if you get a flat or a house?

As far as I can tell, the family housing on-camp is in houses. With the exception of Rakah (which has houses), the the family housing on the leased properties are all flats (Dhahran Tower, Radium, Pearl when families start going in there). The houses on camp aren't necessarily much larger than some of the flats, though. The word on the street is that there is a severe shortage of housing on camp, and there is a severe shortage of three-bedroom accommodation in the leased properties.

My data point: family of five, promised a three-bedroom, coming from Canada, and we're in a two bedroom flat. Apparently, we're on a waiting list for a three bedroom somewhere.