extension build price shell

[shane]

hi all
my name is shane , iam after some info on price for building an extension in the south east of england, i only want the base walls windows and roof, as i am an electrician and plumber , i will then get someone to do the plastering afterwards and then finsih the job myself in terms of carpentry, can some one please tell me what i should expect to pay?
the build is 80 sq m two storey garage on ground two beds and corridor on 1st floor with pitched tile roof.
many thanks
if i can help you with electric prices i will
thanks
shane