If a man puts his house in a life estate for his children with the right to live there until death then builds a garage onto the house with his own money who owns the garage?

In most states, in the process of creating a life estate an owner of real property conveys the property by deed and reserves a life estate for himself. The owner has effectively conveyed the fee to the land and title is now in the grantee. You, the life estate holder, have only the use of the land and the right to occupy it. You do not have the right to cause any damage to the property. You may need to ask permission to build a garage since the owners may consider it undesirable to attach a structure to the existing dwelling. If you built a garage on property for which you own a life estate you may be able to move it off the property during your life but only in such a way so as not to cause any damage to the property. If not then it is part of the real estate. You should seek the advice of an attorney in your area who can review your reservation of your life estate and any other rights you reserved to yourself in your deed. However, the bottom line is that your children own the garage.