As soon as you use it for any purpose other than storage it becomes a livable space, then it is subject to building regs, which are quite strict.
We are about to have ours converted, work starts tomorrow and we asked the same questions, can we get around building regs and if so how. We found out, no you can't, for the following reasons:
If you have a fire, you will not be insured if your loft is used but does not meet building regs. If the roof collapses due to lack of support, you will not be insured if it is because of the loft. Etc.
You need to inform neighbours both sides of work if it involves touching the party walls, as they are jointly owned. If you are not following building regs they can object. Work that is not done according to building regs/with correct planning permissions/authority will not be covered in event of accident/damage to party walls so you will be liable for cost.
It will decrease the value of your property if not done correctly. When you come to sell, you will have problems if you cannot evidence the work has been done to building regs. Prospective buyers can now insist you take an indemnity out on the work done (or something) to protect them if something goes wrong.
If some-one complains and building control come around to check it out, they can insist that you either finish it, to building regs standard, or undo everything that has already been done.
You need for building regs to be met:
fire doors throughout your house (although that might just be for a flat as in our case)
Fire door at either the top of the stairs, or the bottom (not allowed open plan unless you are creative and that is expensive).
Integrated mains powered smoke alarms linked to kitchen on all floors.
Insulation and soundproofing requirements are stricter now.
You need at least one vented window (skylight is ok).
Stairs need to have specific head clearance on landing below, and treads on the stairs need to be a specific width.
Floor needs to be re-enforced to take the additional load bearing weight of regular use/heavy objects - usually involves steels, but not always.
Lighting and other electrics have to be part p certified.
Its a tall expensive job.
You can get a Haines Manual if you want more detail about DIY lofts. Very useful for tips on insulation etc if you are going to it yourself.