The paper presents an overview of the main results of the European Commission Network of Excellence “HySafe” activity to estimate an allowable hydrogen permeation rate for automotive legal requirements and standards. A slow, long term hydrogen release such as that due to permeation from a vehicle into an inadequately ventilated enclosed structure is a potential risk associated with the use of hydrogen in automotive applications. Due to its small size hydrogen permeates through the containment materials found in compressed gaseous hydrogen storage systems and is an issue that requires consideration for containers with non-metallic (polymer) liners. Various rates have been proposed in draft legal requirements and standards based on different scenarios and the assumption that permeated hydrogen disperses homogeneously in a garage like enclosure. This paper focuses on the development of a methodology to estimate an allowable upper limit for hydrogen permeation in automotive applications, by investigating the behaviour of hydrogen when released through permeation with a focus on European scenarios. The background to the activity is explained, worst credible scenarios are identified, a methodology proposed and a maximum hydrogen permeation rate from road vehicles into enclosed structures is estimated.