A consultant is not required for ISO 27001 implementation, and in the majority of cases, companies implement this standard without the help of a consultant.

Hiring a consultant could be helpful if the deadline for ISO 27001 implementation is very short, or if the employees of the company do not have any spare time to dedicate to such a project.

Examples:
A small service company decided to hire a consultant for ISO 27001 implementation because of a very short certification deadline set by the major customer.

A software development company decided not to use a consultant since they have a project manager that can dedicate time to ISO 27001 implementation, and they will get the know-how for the implementation using online tools.