Contractor wanting more money.

[Ricardo Meza]

I have a contractor asking for more money because of an estimating shortfall on a design/build project.  

He's saying he underestimated the electrical and plumbing.  I'm not inclined to pay him more, but the project seemed more challenging for him than I initially thought it would be.  He seemed to have issues with the County inspector, not following the plans, jumping ahead of inspections, and piped into the wrong underground drain line.  I feel bad for my contractor, but I don't know why I would pay for his mistakes.  I've already given him more time to finish, and I did the HVAC for him (for free).  I'm just curious what's the best way you've dealt with contractors who've made estimating mistakes, and if you'd consider a bonus payment, at all? And why? 

I'm in contracting myself, but I still don't know how to squash all the emotion out of this business.  Any tips on keeping my blood pressure low would be appreciated too!  I found myself telling him no, but left the discussion open for future discussion.  I'm not going to bring it up again, but I'm sure he'll remember.  What would you do?

Thank so much!