You can move stuff by holding down on the objects you want to move, either on build mode or not.
Also! Did you know that you can make walls different colors in the same room?! And the exterior walls?! Just choose the “Wall” option rather than the default “Room” option while choosing wall styles, it’s in the upper right hand corner when you’re changing the walls.
Actually the right answer is, you can build another house but in the same and the only lot/land you have right now. If you watch a youtuber video, she build 3 house (separately), a very nice design I will say.