By which I mean a room that contains an area that is not the room

I set up a boundary of walls as the outer perimeter, but within that placed a smaller group of contiguous walls that I assumed would therefore not be part of the outer room. But when double clicking in the appropriate area, the resulting room fills the entire outer perimeter with no notice whatsoever taken of the smaller inner walled area, now completely covered by the large room, which was rather surprising since I thought that by default a room would not cross a wall.

So how can I do this? Create a room with such a 'hole'? In fact I'm just trying to define an area of grass that has something else in the middle (i.e. no grass). So how can I do this?