Measuring Loft Allowance Cubic Volume Increase

[chappers]

And by 'roof space' would that exclude any inaccessible voids created by carcassing out walls in front of eaves etc?

yes as they were part of the original roof space, you haven't reduced the amount of roof space just partitioned some of it off.
Wouldn't have thought anyone was going to quibble over the odd few m3.
It's to prevent huge box dormers on larger properties being built under PD