Retro fit converted garage insulation

[Devoner]

Good evening,

In the process of purchasing a new property and planning works to be done. The original house is cob (so needs to breath) and it has a converted garage which is single block and according to the original planning application internal insulation and drylining which appears very thin as the total thickness is 100mm. The floor is solid and according to the original planning application 50mm screed over 100mm reinforced concrete over 1200 grade DPM over 50mm flooring grade EPS (25mm upstand at perimeter) over 100mm compact hardcore. Roof may have very thin insulation too. 

 

Surveyor recond it would be cheaper to knock down and start again but assuming 2.5k per sq meter (its 11sqm) it seems like a lot of cost and there must be cheaper alternatives. There is space to insulate the floor and the ceiling. I thought EWI although don't have much overhang but what do people reckon? The room will be a study/spare room.

 

Thanks in advance for your thoughts.