If stopway is economically possible, the ASDA lengths to be provided are preferably those for the balanced field length. The runway length is the take-off run required or the landing distance required, whichever is the greater. If the take-off distance/ASDA required is greater than the runway length so determined, the excess may be provided as stopway, usually at each end of the runway
CAP168 Appendix 3C Page 2