Be careful how and when you enter into this conversation. Your reputation is at stake.
Good communication with the homeowners throughout the course of the work will completely eliminate surprises and conflicts when you go to collect your pay. Make sure he or she understands the work being done and the challenges you are faced with.
If the over run couldn’t be avoided, talk to the customer and explain what the situation was. Often, a job runs over due to unforeseen problems that crop up as you are working. If it was totally out of your control the customer should pay. If you uncover additional work to be done, show your customer as soon as possible and talk to him about the problem that faces you both. Show proof of any unexpected material costs by showing him the invoices.
If you have a good honest reason for the over run the customer will agree, or at least be willing to split the difference with you. Remember this old adage: “Volume solves all problems!” Keep your eyes on getting a referral from every customer.
Lastly, don’t lose your temper! If negotiations grind to a halt and you’re willing to walk away with your original estimate, just do it. Tell them that you will stand by your word. Your customer will be happy and your reputation will be intact. Ask for a referral and chalk it up to experience.