Worth speaking to the council before you start, some are better than others so know your facts and don't let them fob you off - most cases the 70m3 volume for permitted development clears planning and 30m2 clears building control - I kept my 'garage' under both and cost me nothing in applcation fees plus I have letters from the council saying its permitted, very handy when you come to sell the house.
I built mine out of blocks, rendered to match the house, galv. tin sheet roof, 6m long from local farm barn builder (less to joints so no leaks). If you go down the steel route the boundry will be your problem, tin cladding to walls might be better than ply, block doesn't rust and IMHO is a better long term solution...
The old dear next door did ask if I could get a car arund the side of the house to put in the 'garage' but nothing says you have too Been up 10 years, full of junk and never had a car near it...