Should I begin building a separate house now, or is it permissible for me to invest surplus funds in projects to reduce the necessary years of saving?

A private dwelling is a right of the wife, and its minimum is a room with an independent entrance and exit, and facilities such as a kitchen and a bathroom. If this is available in your small house within your father's home, then you are not obligated to accede to your wife's request to move or build a separate house, as the obligation is to provide accommodation with these specifications. It is advisable to engage in understanding and dialogue with your wife to achieve mutual benefit.