I wouldn't include that in initial estimate but maybe explain that they may want to budget 0-30% for enhancements. You might also want to explain no project is

I wouldn't include that in initial estimate but maybe explain that they may want to budget 0-30% for enhancements. You might also want to explain no project is guaranteed to work. All you can do is minimize the risks and costs. by making as many mistakes on paper and mockup first and realize that ultimately it's the end users decide whether it's a good product or not. And anyone that says any different is promising rainbows and unicorns.