If you go with a professional project manager, it shouldn't add 50% to the cost, not even close. Some contractors will tack on 20 - 25%, but that's the highest I've seen get added to site prep... or most other phases of construction for that matter. And it's usually well worth that if you knew what went on behind the scenes to pull off a good project, which I expect you will be quite familiar with by the end of all this!
When we manage a project for instance, we're working with a crew that knows us, knows what to expect and what we expect out of them. We act on a client's behalf because we have the experience to know how to do that, typically FAR more than our clients do.
There is typically little to no cost raise to a client because the operator is getting repeat business from our company, so they drop their price to a 'contractor's rate', much like lumber yards and places like that will do. That's the way a lot of professionals will do it. You get the ones that charge a bit more too, but it's a rare one that will attempt a 50% mark up! Especially on an island. That said, you won't get those rock bottom prices that people must be half crazy to offer either
But you do get to relax and know you aren't going to get left with a nightmare.
Some people want to do it all on their own though, and I respect that too. It's good education in the long run.
Melissa Fletcher
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"Make yurts, not war" Bill Coperthwaite, 1973