Yes – but a porch or conservatory built at ground level and under 30m2 in floor area is generally exempt provided that the glazing complies with the safety glazing requirements of the Building Regulations (i.e. Approved Document K: Protection from falling, collision and impact).

You should not construct a conservatory where it will restrict ladder access to windows serving a room in the roof or a loft conversion, particularly if any of the windows are intended as a way of facilitating escape or rescue if there is a fire.