If the client added new functionality or has been asking for new features you could approach him for additional fee for the additional functionality. Typically

If the client added new functionality or has been asking for new features you could approach him for additional fee for the additional functionality. Typically features and 'nice to have' items creep into the scope of a project as it progresses. I would recommend incremental development, review, invoicing, payment so basically you're delivering modules/phases to the client so he can see/approve/pay for the work as things progress so you don't have too much time invested before the client sees the work and you see that he is paying. If you just totally missed the mark with your quote and can't do the work for the amount you quoted it's probably better to stop work right away and be prepared to refund any up front payment they provided. Then approach them to renegotiate the project fee/scope to an amount you are comfortable with. Or perhaps scale back some of the functionality to an initial version of the application for the fee they have authorized. In any case if it's going to take a lot of your time to complete without proper compensation either scale it back to a scope that matches the agreed amount, get a new fee/scope approved, or honestly say the scope/features are beyond your original quote and you aren't going to be able to complete the work. Just don't get hung up on a project that's going to eat up your time and doesn't provide you with proper compensation. You will resent working on it and I expect eventually you would give up on it and/or the client wouldn't receive the quality he was expecting.