Ray is correct. You don't need Planning because you aren't changing the appearance of the house from the outside, or the category of usage of the property (for example, converting a house into flats). But you do need Building Regulations approval from Building Control department at the local council, because your proposed works impacts the safety of your house. It won't be as simple as just running a staircase. By doing this, the implication is that you are converting the loft into a habitable space. You will therefore have to show drawings for structural alterations (the fitting of the staircase to the joists and any other cutting of joists, and that the strength of the existing joists is good enough to take the load), thermal insulation, and escape from the loft in the event of fire. It is possible to DIY the Plans, but you might be better off talking to a loft conversion company.