Turning Big floor into smaller pieces

So i started my first project yesterday and decided to make a house. I used one big piece for the for the floor and the house is pretty big with lots of rooms and I want to be able to change each individual rooms material of the floor without changing the entire map to one thing. (Kitchen to tile, Hallway to Wood, Ect.) Is there anyway I can like draw or cut out lines to break it into smaller pieces??