The job is already under way, perhaps even complete, when this one hits. Suddenly your contractor informs you that the agreed-upon price has skyrocketed. He blames the discovery of structural problems, like a missing beam or termite damage, or design changes that you made after the job began.

The additional fees might very well be legit, but some unscrupulous contractors bid jobs low to get the work and then find excuses to jack up the price later. If you’re unsure whether your contractor is telling the truth about structural problems, you can get an impartial opinion from a home inspector, the local branch of the National Association of Home Builders, or even your local building department.

How to protect yourself: Before signing the contract, make sure it includes a procedure for change orders — mini-contracts containing a work description and a fixed price for anything that gets added to the job in progress. The extra work, whether it’s related to unforeseen building issues or homeowner whims, can proceed only after the change order is signed by both homeowner and contractor.