So, he charges dad full price for the good building materials, goes out and buys the cheapest stuff he can find, and pockets the difference. Once the old man is in his grave, the figures, he'll be able to unload the house on some unsuspecting purchaser before any cracks have appeared in the foundation.
Finally, the house is finished. He tours his father through the place, pointing out all the impressive-looking features. At the end of the tour, he hands his father the keys.
The old man hands them right back to him. "Son, my plan all along has been to work with you on designing a house that you will be happy living in. Take the keys. This is my gift to you, my legacy. It's your house now, son."