"Even old dog learn new tricks"

select the create room icon and click on a wall

I've never know this trick.
Thanks!
@stef.salmon From your picture I don't understand way you use cavity walls in place of normal walls? Except the case when the cavity really exist on the original house.
To reproduce an old house with many different thickness wall is not an easy task but there is tricks and friends [:)]
I know that while I've done it for around 20 old buildings.