hi there.. i am no expert.. but..
you will need to let us know more..i.e.
is it a flat or a detached house.. first floor or second or attic stairs..
i hope you find the answer... or you can call the council, and they can put you in touch with someone who knows.. in the planning office.. or visit thier planning web site.. and find answers there maybe.
See our project on building regulations and click through to the Planning portal web site where you can read all the regs. It is the Building Control Department of your local council you need to sopeak to, not the planning office.
If Building regs are all you need, you don't apply for them, you notify them that you are carrying out the works, pay the inspection fee, and then take their advice, because they have to sign the works off.
As to a steel, versus oak beam, they will advise on sizes for both, depending on the stresses applicable.
Your council website is usually very helpful.