How do you pay your rehab contractor?

[Philip Accardo]

I am meeting with a few contractors this week, I am trying to find one that I can trust to do complete rehab work on rental properties and flips. How do most of you work out your compensation agreement with the contractor? Do you pay them to assess the property? Do you have them quote you a flat fee for the entire project or hourly? I am leaning more towards a quote because I can budget it in my expenses better. Have you ever had a contractor quote you a price and it ended up costing more due to unforeseen circumstances? When interviewing contractors what questions do you ask them?

Thanks
Phil