You can change it. Just like everything else, just tell them you're doing a remodel, and you can change anything you want from the size, to their plot, and everything inside- including all their must haves that you originally need.
Yes, you can add extra room. When you go to remake the house, Lottie will ask if you want it to be larger or the same size as what you currently have. If you select larger, you get to select a new layout and can add room.
