Rent out a loft room without building regs?

Hi,

I'm looking to buy a house which I intend to let out to students. There are 3 bedrooms, a lounge and a dining room. I would rent out the lounge as a bedroom to make it a 4-bedroom let. However, one of the bedrooms would be a loft conversion WITHOUT building regs. It was done a long time ago, and was not done to regs. There is a velux window, radiator, power, a fixed staircase (albeit quite a narrow wooden one with gaps in the vertical between the steps) and good height clearance at the top of the staircase. However, I suspect (still to confirm this from the current owners) the necessary steel RSJs are not in place and there is no fire door.

I've been advised that getting the retrospective building regs would be virtually impossible. I'm hoping though that I may be able to add a fire door and put in a better staircase. What are the laws regarding renting out a loft room without building regs as a bedroom to an individual? It seems that it is quite regularly done - indeed, this very property has rented out as a 4-bed to students in the past. Is it illegal? If not, would I be taking risks?

Any help is much appreciated,

Thanks