This is when you do not wish to have any dealing with the building contractor or sub contractor. A project manager is a qualified person who can oversee the build for you and deal with all the ordering of materials and taking care of the contractors.
It is their responsibility to make sure that everything is done properly to the required standard and according to building regulations.
The role of a project manager can be done by an architect or a building engineer if they are nearby and they can visit the site on regular basis to check on the work done.
However they do not need to be a graduate in the construction industry to be classified as a project manager but need to be experienced enough to know what needs to be done and what is required.
If you do choose someone who is not qualified by certificate you have to satisfy yourself that they are competent enough to do the job.
Obviously do ensure that you check all the insurance matters of the contractors yourself as a matter of percussion.
And finally, do have a look around the site yourself from time to time to make sure that the build is going ahead as you expected it to be.