In addition to what the others have mentioned, there is the "volunteer labor factor" to consider.
Expect non-tradesman to be performing a large part of the work, like hanging drywall, setting plumbing finish, etc., all in the name of saving money.
This can really slow down the job and can be a scheduling nightmare (things that aren't ready for you to do your job).
We've had a church job that was (generously) scheduled to take 3 months take 15 months.
