This article will present a state-of-the-art method which is an amalgamation of several recent research studies from various authors to determine the required structural fire resistance period of buildings. The bespoke approach is based on the Monte-Carlo probability method, which considers not only a range of fully developed compartment fires, but also the phenomenon of travelling fires in increasingly open-plan spaces. An improvement is made on the random selection of compartments depending on the use and area of the individual compartments. A case study is presented in which the probability method is applied to a steel-framed office building in Scotland. The fire resistance period for the building is reduced from the code-required 120 minutes to 60 minutes.