long and short (IANAL):
you're ok if no-one complains.
If you can get away with 12 years, you get automatic "retrospective" permission.
However, if you avoid buolding regs then it wont be insured and it makes the house harder to sell.
An outbuilding can be declared on your buildings insurance, just like a garage. And since when will an outbuilding make it difficult to sell a house ? especially as it is well away from the property as stated by the OP.
You need to ensure that the proposal meets the exemption criteria to make sure building regs etc do not apply.
The key is that if you do something that should have building regs approval or planning permission and does not, then that will devalue your property due to the liability, this is the point I was getting at.
Similar for insurance, you would need to make sure they are aware that the room in question has said electrical equipment permanently installed inside.
Its very interesting the number of drive re-furbs and extensions you see happening now which should have planning permission, also the same for knocking down internal structual walls/adding openings which should have building regs.
The key is care and checking to ensure you dont fall into the trap, a little time spent and all that.