Hi, I started an addition project about a year ago. Now we are done! My contractor sent me a final bill with the remaining fee due PLUS his extra work (about $3000)! In it he said was a couple little thing he did like remove a couple forms under the house and frame extra 12"x12" niches to the new addition. I asked him to give me quote for them but he never did. But he did it. I always knew that it was not for free but I thought that I will give him a $300-$500 when done. I tried to settle with him but he only want the full amount. Should I take the dispute to the CSLB or court ? Does he has a case.
Thanks for your advices!

Just from the tone of your post, it sounds like you gave permission for the additional work to be done, without a written estimate of the cost.

"I always knew that it was not for free" is the part I'm reading this opinion into.

If this work was required for the work to be completed correctly, I believe he would prevail if the case goes to court.

Just my opinion.

Did you have a written contract for the original addition work? If you did, it may have language about changes and how they are to be handled.
If not, if the contractor is licensed you may have leverage through the state licensing board.
If not licensed you may get help from the attorney general, consumer protecton office.
He may be willing to negotiate if you have other options.
If there are no written contracts, it is your word against his. It appears to come down to the fact that both knew about the change, the issue is the value of the change.
If you go to arbitration or court, you have no more than a 50-50 chance of prevailing--that is if there is no written contract. Negotiate and get rid of it as fast as you can.

Yes, I did ask him to let me know how much he would charge to have the niches put in. But never did he gave me a price for it then I came home one day and he was more than half done with it.
He is a license contractor and I did have an original contract for the addition. I have been told that any change of order have to be written ? Am I wrong to assume that ?

Yes you are mistaken about change orders. A large home builder would have used a change order but it sounds like you're working with a smaller outfit plus you did not stop that work or demand and estimate before he went further. This would apppear to be tacid permission.