Assuming I understand... Walls are always determined from their middle point, so if you thicken a wall, it will grow in both directions.

• You can set [font=courier new]Preferences - New wall thickness[/font] before you double-click with the room tool to create walls. But that does not work if you want a room with different thickness walls.
• What is easier, may be to adjust both thickness and position of each wall afterwards to fit the room. (In your ase, the left wall X values should be some -10 cm compared to the original value.)