It depends, most of the time if I get attached I may just keep adding on for a couple generations before moving if at all, one of my oldest legacies I always let the spares stay with the parents and the heirs got a new house. However I still hung out there and added onto the house, while another I would add on as family members died cause it felt weird to take their space, especially playing on short ha. My sims are gonna have so many babies they'll probably be moving more.