As a project manager dealing with contractors I hated to hear the words “out of scope”, not because of the obvious reasons ( i.e. I failed to include the requirement in the RFP ), I didn’t like hearing it because it was essentially telling me ( the client ) that I don’t have control over my own project and the contractor is unwilling.
Be nice, explain to them that it’s not within the current set of deliverables and it requires additional development time.
I think we ( as freelancers ) fall into the trap of considering every client is looking to shake you down. Of course, as freelancers/contractors we will always run into the client that intentionally makes the phase vague enough to inject added functionality later in order to get it free; your contract should be good enough to set the expectations clearly, stand strong with the deliverables you agreed to ( if this is an ongoing issue you need to review your contractual process, specifically your requirements definition and scoping. As well as improving your ability to spot a bad client ). Your contract is your control over the project don’t forget that.
Again, the boundaries are already set, that’s why you have a contract, re-enforcing the contractual boundaries by saying an item is “out of scope” can only be perceived as an unwillingness to add to the current version or even worse work with them on a future release. So stop worrying and start selling with confidence.
Try to sell when your working with a client, instead of saying “out of scope”, say something like, “since this/feature/item is not included within this phase’s deliverables we could either add it to a future version or add it to this release if it’s a requirement. Including it in this phase will require … “, then talk about how it may affect deadlines and costs. That’s not the only approach, if you’re in a flat rate project with a strict budget talk about dropping a low priority feature to include this new one; I love this because I have the opportunity to drop one of the more complex features that have little return for the client.
In a nutshell, be nice and know your place on a project.