I played around with Hearthfire a little, and as far as I'm aware the only requirement is that you become Thane of the Hold where the land plot is located. No level requirments. One word of warning however: On occassion when you come back to your house, you will occassionally find bandits or Giants attacking it. If you have a spouse, sometimes they will get kidnapped by Bandits and you'll have to rescue them. Sometimes skeevers will tunnel into your basement. I'm not trying to talk you out of it, I just want you know about it before diving into the house building. Have fun.