In the world of computer calculations, a safety factor is a strange thing: loads and stresses are calculated to 0.01% precision - and then a 'safety margin' of 50-100% is added to create the final design. In the past, the safety factor was usually a simple number, included in the code permissible stresses for convenience, but modern codes have complex systems of partial safety factors. How much of this is science and how much is 'black art'? How have the values of safety factors been decided and how have they changed over the years? The history of how the existing safety factors have evolved is important background knowledge for those involved in developing new codes of practice. It is also important for engineers involved in appraising existing structures. There are many issues to consider in such an appraisal1 but if a change of use is planned, the engineer needs to understand the original design at an early stage and estimate what floor loadings it was designed for. To do this, the engineer needs to know the permissible stresses and design safety factors that were in use at the time of the original design. An assessment of all aspects of safety in structural design would be long and complex. For simplicity, this paper focuses on the safety factors for bending, tension and compression in steel and concrete, as these usually govern the sizes of members in design and quantifying bending, tension or compression resistance of structural elements is usually the simplest way to establish what loads it was originally designed for. This paper considers how UK building code safety factors have changed from the late 19th century through to the 21st century.