Oh yes! But first of all, there is a fundamental problem with ownership. If she builds a property on your land, then you will be deemed to own it, since any building belongs to the person on whose land it is erected. I assume that this isn’t what you intend? If you do nothing there is a danger that the money she puts into the build will effectively become a gift to you, you will be deemed to own the property for Inheritance Tax purposes, and indeed if you sell it, the capital gain will be yours, not hers, and will not be covered by private residence relief.