Abject apologies glennw, i should have realised. OK, in Revit Architecture when drawing a wall a usuer can set the properties / constraints and one of the options is " Location Line". The location Line determins whether the wall length is measured from the inside face of the wall or the exterior face of the wall.
To save getting bogged down in explanation and end up misleading the forum i have provided this link to the ACAD help file topic.
http://help.autodesk.com/view/RVT/2015/ENU/?guid=GUID-F26DB6DA-A0EC-424D-B656-3BDF47607F4F
I appreciate CA is a differnt product but was just wondering if i have missed a setting somewhere when trying to learn how to use it.
Hope this helps.