When do I have to appoint a co-ordinator?

The co-ordinator must be appointed on all notifiable projects

A notifiable project is:

When the construction phase of a project is "Planned" to last for over 30 working days.  a working day is classed as a working shift (it might be a 6 or a 12 hour shift).

So if you are planning to work 24/7 in 2x 12 hour shifts then you will reach 30 working days within 14 calander days. it is the intended duration that matters, Weekends and holdays are not included if they are not going to be worked on

or where the construction phase is planned to involved no more than 500 person days in total.

The appointment must be made at the very start of the design and planning process

It is accepted that clients may explore and discuss the business case for development and investigate likely constraints on construction work – but a co-ordinator must be in place before concept design work begins, or any strategic development brief is prepared.

Designers (architects, structural engineers and others) are prohibited from beginning such work until they have been given details of the CDM co-ordinator who has been appointed for the project.