Albert Barnes Commentary 1 Chronicles 29:3

"Moreover also, because I have set my affection on the house of my God, seeing that I have a treasure of mine own of gold and silver, I give it unto the house of my God, over and above all that I have prepared for the holy house," — 1 Chronicles 29:3 (ASV)

Of mine own proper good - that is, from his own private estate. He makes the offering publicly in order to motivate others by his example (1 Chronicles 29:5).