I want to add a side extension to the garage.  The extension sits on the gable end side of the garage and the garage resides at the back of the property.  As I need an internal roof height of 2.5m (need that as it's going to be a gym room) I appreciate I will have to submit a planning application.   I want to know whether that will then mean I have to comply with current building regs, eg 100mm insulation in the cavity.   I was hoping to use 50 or 75mm insulation in the cavity so as to keep costs down.

 

Thanks

The majority of new builds use 75 mil insulation So no problem there 

Latest Approved Doc L has U=0.18W/m2K in extension wall. Looking at approx 150mm of mineral wool or 100mm 

PUR

Building Regulations : First thought - is this extending an existing detached garage? Does it take the floor area over 30m2? If no then the resulting building is still exempt (assuming built mainly from non-combustible material if within 1.0m of the boundary).

 

If yes then it's not exempt and so will require regs. However if it is still less than 50m2 and stand alone, then the energy efficiency requirements don't apply so the choice of insulation is yours to make.

 

Planning: If it's at the back (and no road behind) then max height of 4.0m for dual pitched roof with eaves no higher than 2.5m. Otherwise 3m (presumably for flat roof or single slope). However if it's within 2.0m of a  boundary then height should not be more than 2.5m.