The building regulations we updated in April 2002 with the effect that all replacement glazing now has to meet the same standards as new build for thermal performance, ventilation, safety and means of escape. The latter being particularly relevant where homeowners build and extension to the rear of their property where the building is close to the a neighbours boundary. Building control may insist that any windows facing on to the boundary line be protected with a fire rating.
Given that fire windows are fixed field this will lead to design constraints that the home owner and the architect must take into account at the design stage. The exact requirements as for whether a fire resistant window is required will depend upon the distance from the boundary and the m2 area of the room to which the fire window is fitted. It is worth noting that a fire resistant window will not be a means of escape as its primary purpose is to act as a barrier to the progress of flames, smoke and heat and hence must remained closed.
Further information can be found at the planning portal website by clicking Here