Many people project manage a build themselves. If it's a relatively small job and they are knowledgeable about the building process, that's fine.
But you don't have to watch too many episodes of Grand Designs before you work out that an experienced project manager can save money in the long run. As well as making sure that each trade turns up at the right stage of a project, they'll find suppliers and get the right supplies in on time, the right amount and on budget.
If you're unsure we'll give you an overview of what the project manager's role would be on your project so that you can decide whether to do it yourself or not.