In many cases (and for a variety of reasons) an owner or contractor may want to deviate from the architectural drawings. Some common reasons include discovery of an unforeseen condition, availability of certain products or materials, design changes, and feasibility. In some cases this deviation will cost additional money due to increased time or materials cost to the contractor. It is also possible that the change will result in a savings to the owner.
Whatever the case, when changes to the contract amount need to be negotiated there is potential for disagreements to arise between the contractor and owner. As with any interaction where both sides have legitimate claim to some portion of the truth, a peaceful resolution is all in the approach.
Keep the following things in mind as you seek a positive resolution:
It is reasonable for the owner to ask (nicely) for a written description of any change to the architectural drawings and a written summary of how the contractor arrived at the additional amount. In contractor-speak, this is a "Change Order Request", or COR. The COR should include a description of the discovered conditions, proposed remedy, and the proposed cost including overhead and profit. The owner may want to ask for a breakdown of time and materials for the proposed remedy. He or she can then sit down with the original contract, the architectural drawings, and of course the existing work to decide whether the proposed work should be covered under the contract or is truly above and beyond. The architect may also be of some assistance in evaluating the validity of the COR (perhaps an hour of time would be worth the expense).
Asking for an explanation of the costs in writing will often inspire a contractor to be reasonable in his or her requests. There is obviously a gray area when it comes to interpreting the results of this query. You can run to Lowes to check the proce of a 2x10, but who determines whether a time estimate is "reasonable"?
Some things to look for:
The owner don't want the contractor to walk off the job or to cut corners because he or she is angry and/or losing money on the job. The contractor doesn't want the owner telling 1000 other renovators about his or her bad experience. The key will be open communication and the impression that both sides are honestly working towards an agreeable solution.
Neither side wants to end up in court, but that could be a potential last resort if an agreement can't be reached. Litigation will likely extend the project timeline.