How to avoid taxes on lake house

[Barney Davis]

My father in law owns a lake house in Louisiana that they do not use anymore. Obviously it’s a good time to sell and he’s thinking he can get $650k. He built it 15 years ago and honestly doesn’t remember what it cost him back then. His primary residence is in MS. He plans to sell the lake house and use that money to build another house on some land he owns. He would then probably sell his primary residence that he lives in now. He’s trying to figure out if he’ll owe the government a big chunk and wants to pay as little as possible in taxes. Any tax people out there with good advice?