Extension costs

[wainy]

Looking to extend the house in the near future, extend behind the garage and build over it, adding extra space in kitchen and 2 bedrooms to the side of the house. However also had the idea of extending the bground floor only at the back (say 2m) to give even more space to the kitchen and dining room. Would this be more expensive £/m2 compared to "normal" extensions because of probably steel work involved?