It is highly unlikely that this loft conversion will require planning permission as nowadays most conversions will be classed as permitted development. And as you are installing Velux windows it is doubtful that you would have to apply for planning consent unless your hose is a grade two listed building or is located in a conservation area.
However, it should be noted that planning permission is completely separate from the building regulations. And whether planning consent is required or not it is almost certain that you will need building regulations approval.
The issue here is whether the new loft space is deemed to be a habitable room. It has to be remembered that a habitable room must gain building regulations approval and importantly has to comply with the regulations regarding fire doors.
You don?t mention how the loft room will be accessed. A single room can be accessed by a compact staircase but not a ladder whereas if the loft is just to be used for storage tan a ladder is acceptable.
