Assuming this is a two-storey house, the three main problems you will have to solve to do this legally (that means with Building Regulations) are
1) the strength of the joists that support the floor
2) satisfying the fire escape regulations - there must be a corridor of escape and a 30 minute fire barrier between the loft conversion and the first floor below it (usually solved by having two layers of plasterboard on the ceiling)
3) having enough insulation in the walls and ceiling of the new structure.
Hence too many variables to begin to guess how much. Call in a company with expertise.