That depends entirely on the type of projects you are known for (read: are willing to take). Some software projects agreeably will be next to impossible to esti

That depends entirely on the type of projects you are known for (read: are willing to take). Some software projects agreeably will be next to impossible to estimate, others not so much. That being said I always bill per day of work, rarely ever a fixed sum (in fact only for good friends that are poor artists). So if the scope changes and there are more days, they know my rate and it will be their decision if they want to pay for that. Also they will have the insecurity of me not having time because I work on other things — risks that I clearly communicate before they agree to have me. I expect a certain amount of changes and feiction that is included in my original estimate, so this "extension" period is rarely ever needed and when I am faster the customer has to pay less than they expected which typically makes them happy. It doesn't always work that flawlessly, but it was never catastrophic and my customers value the up-front clarity.