These days, large plots are hard to source in new suburbs - and they are becoming more expensive each day. As a result, building a double storey house is becoming more popular. This allows you to still have big rooms, the open feeling, and a backyard for your kids. A double storey house means the ability to have an extra room, and perhaps a double garage.
A multi-storey house allows you to create a separate zone in the house - like having the living and entertaining areas downstairs, while the upstairs can be the sleeping area. And the view from a first floor window tends to be more flattering.