Slitherine

Can large units defending buildings claim to be self supported? (as they can in regular combat)

If so must they be is a 'deep' formation ?

If you are defending a building you are deployed around the perimeter so you cannot be self-supporting.

They are not in deep tactical, so no.

so if you occupying in deep tactical you could be self supporting?

If occupying yes you can as you have to be in tactical while occupying. Page 76 second paragraph after top bullet point.

For someone who hasn't studied building defense, "occupying" and "defending" are alternative situations.

A unit "defending a building" can't rear support itself since it is in defending, not deep tactical, formation, but it can be rear supported by an infantry unit outside that is properly positioned.