Permission granted for loft extension a size larger than "permitted development"?

[DaisyDoo]

Hi, just wondered if anyone has had experience of requesting for & hopefully being granted planning permission for a loft extention to be larger than the size that is allowed within permitted development? We are end of terrace so will get 40 cubic square metres unlike the 50 cubic square metres allowed to semi detached houses. This is in relation to Lewisham council where the process of finding out is only through applying with drawing etc so a cost of ?2k at least but they won't tell us if we'd be completely wasting our time and mone in applying for planning permission for this. Any advice/experiences shared most appreciated, thanks