The successful integration of human factors (HF) into the design, construction and operation of any hazardous installation is essential in order to optimize operability and personal safety. It is also critical for ensuring that the risks of a major accident are as low as reasonably practicable (ALARP). Furthermore, it is a requirement under the COMAH Regulations to identify and address any HF issues for the site. In September 2002, Atkins Consultants began working with a client to address HF for a new chemical plant being built in the UK. This had been prompted by HF specialists within the HSE who, in reviewing the pre-construction safety report, identified a number of areas where specialist HF input was required. The work Atkins conducted on behalf of the client involved the development and implementation of a human factors integration plan (HFIP) to specify how HF activities would be integrated within the project lifecycle. The areas covered included operator selection and training, control room design, DCS interface assessment, human error analysis, procedure review and workload assessment. This paper describes HSE's role in assessing and promoting HF for COMAH sites, and details the analyses conducted by Atkins to satisfy the requirement of the regulator, ultimately helping to optimize plant safety. It highlights the advantages of ensuring human factors issues are considered from an early stage in plant design.