I've Always Wondered.....

I LOVE the show, "Extreme Makeover Home Edition" Seriously, I do. Each week the story seems to get better than the week before. But I've always wondered something...... How do they manage to build such a large house/yard on what may have previously been very small property???
Do they buy existing/neighboring properties and tear them down? Is having a big property a requirement for the show in the first place? Do they build up really high to compensate?
Does anyone know the answer to this? Or want to venture a guess????