Unusual side extension

[Bobbie Build]

I shall make this clear with a diagram.
I wish to build a side extension within PD. How big can I build a side two storey side extension onto the old single storey house?
Originally people lived in loft, there still is a gable first floor window.
House is pitched roof and extension would follow, idea is to include a stairs in extension allowing access into old loft room. Field to left is 7.5 metres away.

 

[Bobbie Build]

Single road 27m away, old house 96m2, 1980s extension 49m2, open porch 2m2. I can gain extra height as ground floor new extension may be two steps down out of old part of house.

 

[tony1851]

A two-storey side extension is not permitted development.

 

[Leofric]

If you are serious about this building work employ an architectural designer to prepare a scheme for planning and building regulations approval.