Depends on several factors. As an end terrace do you have a gable end (the side wall goes straight up to the roof) or does it come in from the side wall at an angle to join the front and back slopes (like 3 sides of a pyramid). If you need a gable building up and the roof extending, this cost lots more. If you need a dormer window (ie the attic isn't big enough and you need to extend it over the back bedrooms), this adds thousands too.

So a basic one could cost maybe 10-15k depending on your area's rates and the work involved. But wanting a gable end, dormer window, and a bedroom on my end of terrace made quotes jump to £38k inc vat !!! So i decided not to bother :-)

looking to strengthen the floor, walls and get a basic staircase from a cupboard its a 3 bed modern end terr, looking for a rough price, it wont be a bedroom so no planning req thanks xx

im in scotland