As that link shows, planning consent is not always necessary but compliance with BUILDING REGULATIONS (as they said in their post) IS. The poster never mentions planning!!
What I don't understand is how any decent builder would have done such a conversion without complying with Building Regs and having the appropriate permission and inspections by the local building control dept. Or was it a dodgy, "board the loft & call it a room" DIY job, which is why you are having trouble now?