This topic has been raised a hundred times already, if not more. There is no real solution for what you want. Sorry, but you have to live with it.

Walls are drawn along their middle line, connected at their middle line.

You can draw the room first, then double-click the Walls button to surround the room with walls. This works well only if you have just one room.

More rooms? Then, for each wall, you have to add half the wall thickness to its length to get the correct inner spavce (= room).