"Raise" is a term for full-time employees, whose salary depends on other factors (e.g., years served, position, etc), not actually on amount and quality of work done (don't kill me, there are many exceptions here).
Being a freelancer, you are most likely bound with terms of a certain agreement. Any amendments to that document are subject of a mutual agreement. You haven't specified why are you willing for "raise". As I said, years served is not an argument for freelance job.
Finding the Right Arguments
So, let me theorize and look in what situations you may require for raise review of the contract terms. In any case, you should bring those arguments to a meeting and mutually agree on any contract changes. See below what if your changes are rejected.
- The most common thing that happens very often for Fixed Price projects is changed scope. Your initial agreement should have a section for this matter and any additional work (outside the original scope) should be a subject of an addendum to your main contract. It's hard to do for the first time, but when you manage to do it once you will see how many problems you have solved.
- Also, on Fixed Price projects, the amount of work may change dramatically due to other reasons. It may be lower quality of dependency product (say, a third-party program library is working worse than expected, and you are having hard time fighting with the library's bugs). Gather any evidences, make your new estimate, and bring it to a meeting.
- On a Time-and-Materials project, you are actually selling your time. Its cost may vary. When? For instance, if you realize that you are doing more qualified work than expected before. You may split your invoice into two positions: normal work you have agreed initially to do (at normal rate) and another work based on a different rate. If it's rejected, you may reject to do a qualified part, but remember that doing a more qualified work is your ticket to the future. So try not just to throw it away.
- Yet another situation. You have agreed to do an average of, say, 20 hours a week with an hourly rate of $X. In addition, you are spending a certain amount of time for non-billable things, and they are free just because they are split into small chunks and are simply hard to measure. If you are continuously working for 5 hours a week, you may find yourself underloaded. Those "small things" are small in comparison with expected 20 hours a week, but it may grow if your billable time is 5 hours. Again, raise it to a meeting. It wouldn't look like you are asking for a raise; you just want more workload.
During a Meeting
So you have your arguments and you have scheduled a meeting for changes. What's now?
Don't panic. You, as well as your client, are willing to get the job done with best possible quality and at lowest possible cost. If you are not just asking something for yourself, but instead demonstrate that you two are "on the same boat", struggling with the same problems, your client will be much more compliant to your requests and suggestions.
Possible Outcomes
As a freelancer, you are an independent business entity. You take full responsibility on any consequences happened due to your decisions. As well as your client! If you suggested a contract change and they rejected it, you actually have two ways:
- Complete what you have signed for, at your cost (and get some reputation);
- Cancel the contract (and lose cancellation fees).
In any case, remember that reputation is the hardest to earn and easiest to lose.