Limiting the risk of external fire spread between buildings is one of the essential fire safety requirements in the UK following from the performance objectives set in the 2010 Building Regulations, Approved Document B. Although, in principle, any engineering solution that fulfils this requirement is considered acceptable, the methods given in BR 187 (UK guidance document for external fire spread) are commonly applied and accepted by the authorities. The BR 187 methods can be used to determine either a required separation distance between two buildings for a given area of unprotected facade, or a maximum allowable unprotected area of the facade for a given distance between the two buildings. The safety level which is achieved by the BR 187 methodology is unknown and the beneficial effects of sprinklers are only taken into account indirectly and without clear justification. In this paper a methodology and calculation tool are presented that allow the building’s designer to directly calculate radiation intensities along the site-boundary or the opposite facade, and to evaluate the probability of fire spread between buildings due to high radiation values. Through probabilistic considerations and Monte Carlo simulations it is argued that the inclusion of sprinklers in a building significantly reduces the risk of reaching high radiation values at the opposite facade or at the site boundary, resulting in a higher level of safety as compared to a traditional design compliant with BR 187 where a fraction of the facade has been fire rated. Consequently, opting for sprinklers results both in a safer design and allows a larger fraction of the facade to be unprotected. As a case-study the methodology is applied to a high-rise office building in London, UK.