Planning shouldn't be an issue as it's usually well within permitted development, they just require the outside to be 'in keeping'.
Building regs however are a different matter, and getting retrospective approval for loft conversions is usually a very expensive business. The two areas where they won't compromise are structural strength and fire regs. So if the floor hasn't been strengthened using RSJs, timbers of the right strength are very deep so you loose head height in the loft or ceiling height in the bedrooms below, they will want them putting in. Fire doors and fully enclosed stairs from the loft to ground floor, interlinked fire alarms.
Insulation and some of the fire regs can be avoided, but to do so you have to prove that the conversion was done at a time period when they were less stringent. However, if there are no stairs to the loft it was never converted (official view) and so will come under current regs. The only exception to that which will be taken as evidence of an earlier conversion would be a building regs application which was made but never signed off.
So from what you say, he's either in for a five figure bill and a lot of work, or forget having an official loft conversion.
Oh, and if it's used for anything other than storage without BC sign off as a conversion, his insurance will be void and they won't pay out in the event of an attempt to claim.
