I amafraid this is a bit of a tricky one to answer from little information.
as the local Planning Building Control would be involved from the planning stage. They would want to see plans for the extension, prepared by a builder or Architect. showing walls, proposed openings (size of the opening) loads above the opening , if there is a an upper storey to the building. All this is in the calculation for the beam (RSJ) . The Local Council Planning Office should have information with regards to this.
There are companies who would do this for you for as little as £45.00 plus vat.
It could be worth that money being spent.
I hope this helps you.