Rehab overrun question

[Jean Laurin]

If I get a quote for a rehab from a contractor, they work the job, and then the job goes way over budget, what happens?  Is the contractor responsible for some or all of the cost, or does it all fall on the investor? 

This is probably a super-newbie question, but I haven't heard anyone talking about this yet.  I'm still learning, so maybe my confusion is jut because I don't know what causes cost over-runs...

On the PB Podcast, I've heard about getting an itemized bit or statement of work, breaking-down labor vs materials, and paying after stages of work are completed, but I haven't heard who's responsible for cost over-runs.  Are over-runs something that are typically defined and addressed in a contract before the work starts?  And... is it different if the over-run was on a part of the rehab that was defined in the statement of work, versus if the contractor found a major unrelated problem like dry-rot or something?

Thanks in advance for clearing this up for me!

Jean Laurin