A Fixed staircase can be installed without going through an application for building regs.
The main reason you apply for building regs is due to structural changes within your property. This could be when you are converting your loft, extending your property, removing a structural wall or moving a bathroom etc.
You can install a staircase if you wanted to for the storage into your loft without regs but this would remain as an attic and not a loft conversion. That’s the difference.
The moment you think about a loft conversion, you will need building regulations approval.
I hope this helps
Thanks
Brendan