By the sounds of it, you won't need planning permission as it should be a permitted development. I am assuming it will be to the rear of the property and built similar to existing house. Look at planningportal.gov.uk for more information. If it is permitted I would always recommend sending plans into the council and get in writing that you do not need planning permission.
You will of course need Building Regulation approval, and would need engineer calcs for the new beam to support the load.