Regulatory and governance requirements include the need to consider cyber security throughout the lifetime of safety-related systems, from concept and development to operations and disposal. Whilst there is guidance that provides principles, such as the IET Code of Practice (CoP): Safety and Cyber Security, and application specific standards that require interaction between safety and security, there is little guidance on how to structure a holistic safety-security argument or co-assurance case. This guidance document provides the framework for developing such an argument. It is an extension of the technical principles presented in the IET CoP and applies a novel approach to developing a coassurance case using the Safety-Security Assurance Framework (SSAF).