A builder had to change his quotation after commencement of work.

No professional should change the quotation while carrying out a job unless the work needed has changed by the circumstances of what is found keeping the customer fully informed of the need for change is the most important thing so this can be shown at the time, but all circumstances should of been covered in the first place if given the right information,A true expert will have the knowledge to prevent this from happening and thus avoid the need to re quote and an uncomfortable situation for both parties

you are really between a rock & a hard place if you allowed him to carry on but altered the design that's on you, what you really should have done is tell him what you wanted and waited till he had requoted it before giving him the go ahead, it really bad practice on his part to not inform you that any changes would effect the final price , your obviously concerned what you need to ask him is what the final price will be now not at the end then you can decide were to go, then you need to talk to him tell him your concerns try and come to some compromise but you need the final price before you can make that decision. good luck Alex

Before he started the extra work he should have had a chat with you first to make you aware of the situation. I'd ask him to stop now before he finishes the job and try and work something out. Fingers crossed you both get it sorted without any issues.