Who should write mandatory documents in organization?

In fact, both consultant and process owners are involved in the mandatory documents’ elaboration:

• The consultant provides the initial structure for the documents (i.e., what needs to be included in them regarding the standard)
• The process owners provide the customizations required based on their knowledge of the process and business needs
• The consultant evaluates if customizations do not compromise compliance with the standard, and suggests adjustments if necessary

For further information, see:

• 8 criteria to decide which ISO 27001 policies and procedures to write https://advisera.com/27001academy/blog/2014/07/28/8-criteria-to-decide-which-iso-27001-policies-and-procedures-to-write/
• How detailed should the ISO 27001 documents be? https://advisera.com/27001academy/blog/2014/09/22/detailed-iso-27001-documents/