Fire regulations state that all walls and doors in your house and loft conversion, should be able to resist fire for 30 minutes. The reason for this, is that there should be a fire protected path from your conversion to the outside. Which they call a ‘Means of escape’
Old regulations used to allow a low level window and ledge to be installed in the loft as means of escape, but it proved to be too dangerous to escape the house via the loft. So now the escape route should be through the house.
Main stairways need to be protected by installing fire rated doors on both storeys. When installing fire doors, people should remember that the hinges should also be fire rated. Interlinked smoke detectors are an essential fire safety item and must be installed on each level.
Floors and walls also need a 30 minute fire protection period. This can be achieved by using two layers of plaster board. Even though your loft conversions forms a third storey, the structure needs to be separated from the rest of the house.
In the event of a fire, the means of escape would be the main staircase, which must lead directly to an external door. This door will also need to be enclosed and give a 30 minute fire protection period and smoke alarms will need to be fitted on each floor.
If the staircase ends in an open plan room, this space can be fitted with a sprinkler system., and a fire door will be fitted, separating the ground floor from the first floor. The means of escape from the loft in this case, would be via a window on the first floor.
For any more information, there is a great guide to fire regulations on the Direct Gov website.