Dormers - how to calculate volume please help!!

[claref]

Hi, I'm planning a dormer to my chalet bungalow which on my calcs is either just over or just under 50 cubic metres.

I'm trying to calculate the additional roof space created by previous owners changing a sloping roof to a gable end. They effectively added on a pyramid shape (It was an odd roof originally and had two perpendicular sloping roofs joined) If I calculate the volume by by creating a cube as planning portal suggests then halving it and halving again it gives me a different volume to when I calculate the volume directly as a pyramid. Which calculation will planners use?

Its quite crucial as my planned dormer is right at the limit allowed by PD and I know I would be refused planning permission.

One other question, the previous owners integrated the former flat roofed garage into the house making an extra room and adding a pitch roof the space of which forms part of the upstairs roof space - do I include this as part of the 50 cubic metre allowance or is it considered separately.

[DevilDamo]

There is a roof volume calculator on the planning portal website, which is quite good...

Google "planningportal.gov.uk/uploads/volcalc/volcalc.html"