The sims 4 life and death

@bee_queen67 and @SharpGoblin  I went and checked this in my game, and yes, some Build and Buy mode items can be used as heirlooms, but not all of them.  For example, decorative landscaping rocks, or landscaping trees/bushes, as well as fences, doors, and windows did not come with the ability to become heirlooms.  However, indoor and outdoor lighting fixtures, the birdbath, and window boxes (with flowers), and items like beds and chest of drawers, the piano, mirrors, and more can be used as heirlooms.  There are so many items in the game that I don't have time to find out the status of all of them, but there did seem to be a nice variety of items that can become heirlooms. 😊