Ok. Thank you very much.You need the Island Designer app! The game doesn't tell you how to unlock it but you just need to keep progressing the "main" story. If you want to know specifics:
Once you move someone in from the campsite, you'll get the option to do an island eval with Isabelle. You need to get a 3 star rating, which is near-impossible to get until you have 8 villagers living in your town. Once you get the 3 star rating, you'll get Island Designer the next day.
I really want that app. Laying the floor will help me so much, with sorting out where i want to put new plots.