I assume this means you are billing by the job/project rather than by the hour right? If the client adds an additional 10% requirements to the job and you do n

I assume this means you are billing by the job/project rather than by the hour right? If the client adds an additional 10% requirements to the job and you do not adjust your billing then the client is reducing your pay rate. Upping your bill to reflect the new requirements of the job is not giving yourself a raise. If you don't want to renegotiate the contract then you can still bill for the project as spec and then by the hour for deviations from spec.