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When placing a wall, its default snapping point is its centre line.

To place the wall relative to a wall side and not its centre line use Ctrl + W key combination to toggle through the 3 options: Centre line, Outside wall edge, Inside Wall edge

You can also employ the use of the Constructional support tools. While placing a wall right click to display the context menu and select Constructional support These functions all have keyboard short cuts so you can implement just by pressing keys while placing a wall.