Maybe he's been dreaming about building it for a helluva long time and couldn't swing it when he was 40 but still wants to do it to achieve his goal?

A three-level house may be a bit much for that scenario, but I could see some justification for rather elderly people to build a new house.  If you have something that is wheelchair accessible, easy to cook and clean in, energy efficient, low maintenance etc. etc.-well, it wouldn't take saving very many months of nursing home bills to pay for that new house, and have a superior quality of life to go with it.