This publication is intended to provide guidance to people involved in actions that affect hazel dormice (Muscardinus avellanarius) in connection with built development in the UK. This includes decision-makers reviewing planning applications and ecological consultants involved in any of the following activities: surveys; design and implementation of mitigation measures; preparation of licence applications; and post-development monitoring. The book accompanies the simultaneous publication of the 3rd edition of the Hazel Dormouse Conservation Handbook.