I made the same experience. Normally you should be able to bill most of the project specific (especially domain knowledge) learning as well.
The answer for not billing depends, imho on the following: • Could the learning be applied to other projects as well? • If you don’t overshoot the budget is there a possibility to do more work for the client in the future? • Could I use the leaning in a side project or even make a product out of it? If one of those questions is true I cut most of the learning time and don’t bill it. If it’s the second you can slowly add the time later on, by rounding up hours in your favor. In the end it’s finding a balance between not making the project a loss for you and making the customer happy

writen by Benedikt
great answer Benedikt. In the end you need to renegonitiate the deal with the costumer and try to make it so that both are happy right?

writen by Tiago Ferreira
If you had a budget, then that’s a good idea as well. Outcome depends on the customer, though

writen by Benedikt