Here are steps to do so:

1 - Click on the center of the "big room" => deactivate the labels (Name, Dimensions and Area)

2 - Go to the Detail tab => using the polygon shape => outline the desired area (Kitchen, for example) => select the polygon => Properties bar => calculate area

3 - Using the Text tool => Add the desired name, dims and area.

4 - (Optional) Move the polygons to other tabs, so different rooms will appear with different colors.