This is a very important question to consider before the work begins.  Perhaps you will want to have some additional financing available, “just in case” there are unforeseen costs.  Remember that in most cases, your contractor has no way of knowing what lurks below the surface. Unforeseen issues may arise, such as:

• Septic tank or sewer lines
• Other underground utilities (Electric, Sewer, Cable TV)
• Fill soil that will not bear the weight of the pool
• Rock or hard-pan conditions that require special digging equipment
• Underground water

In order to be competitive, a contractor must assume that you, the homeowner, have more knowledge of the property than anyone else. No one likes surprises, including a good contractor. To mitigate unexpected costs during construction, be sure you let your contractor know of any conditions you are aware of before construction begins. Also, ask your contractor what costs might be assessed if problems are encountered. Most importantly, if a contractor says prior to signing a contract, “We don't need to worry about that right now," you might want to look for somebody else.