Change orders are legal, written changes to your construction contract.  Unfortunately, some dishonest contractors will lure you into signing a contract with them by "low-balling" the price of your pool, and will later make changes to your project during construction in order to recoup money they would lose if they stuck to the original contract. Be sure that all changes to your contract are legitimate and the cost changes are reasonable. Before you sign a contract with a contractor, ensure that he has a process and policy for executing change orders.  Do not allow any changes to your construction contract without written agreement, or you may be asked to pay much more than the actual cost of the change.