Under what circumstances might change requests to the project's scope be denied? How can i handle a customer's wishes if the scope change is not approved.
Short answer: The Project Manager's (PM's) job is to handle the project's requirements, not to handle the customer's unfunded wishes.
Scope changes always affect schedule, cost, quality, resources, and/or risk. A PM will not approve a change that negatively affects those constraints unless it is necessary in order to meet other, higher-priority constraints.
Obviously, if adding a bell means going over budget and falling behind schedule, the PM will disapprove it. On the other hand, if the customer will not accept the project because the whistle toots with the wrong tone, then the PM may approve spending more to tune the whistle.
Remember, these are business decisions. At some point, the penalties for cancelling a project may cost less than the cost of completing it.
The above deals mostly with changes requested internally. If the customer requests a change, the PM will present the customer with the effects (cost, schedule, ...) of the change. The customer then bears responsibility to sponsor or reject the change.
As always, this is a business decision. Even if the customer will bear the cost, extend the schedule, and forgive any realized risks or impacts on quality, the available whistle-tuners may have been committed to another project (inadequate resources), or the loss of reputation (even though the customer specified the wrong tone) may hurt the business in the long run.
To "handle the customer," get out your requirements traceability matrix and present the effects of the change on the scope. Then present the effects of the change on cost, schedule, etc.
Copyright (C) 2012, Richard Wheeler. Permission granted for use not involving publication.