Go to the quest and click on the little question mark in the top right corner. It will tell you what you need to do. Usually you have to click on a specific piece of furniture in your home such as a table for example, for the option to appear.
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.