Homeowners are doomed by just a few simple words: "We'll just touch this room up a bit...."
The inability to accurately predict project scope should not be regarded as an error in need of correction. If people can summon the will to do something, and organize the resources to at least start it, then its success is merely a matter of more resources. The unpredictability of construction projects should be acknowledged in the beginning. Much effort can be wasted on trying to control scope when it should be focused on improving the outcome.
Easy for me to say.