Thank you for your question. I look forward to working with you to provide you the information you are seeking for educational purposes only.
An estimate is just that, a guess at the approximate cost for the job. Once the work is started, estimates can legally change and are not binding contracts
to the amount quoted unless the document you have states it would not change in price specifically. The reason for this is in these jobs, the business does not know what they will encounter until they actually begin the work and if they find other things that need to be done or encounter issues that were not expected, then the costs can change.
You can send them written notice demanding they contact you for each additional item they have to do before they do it for you to approve the work, but legally the estimate is not something you can use to force them to hold to that price when they find other issues that need to be done to make it work. (Unlike say painting a house, where they see the house and can give a concrete contract for the cost because they see exactly what has to be done).
I truly aim to please you as a customer, but please keep in mind that I do not know what you already know or don't know, or with what you need help, unless you tell me. Please consider that I am answering the question or question that is posed in your posting based upon my reading of your post and sometimes misunderstandings can occur. If I did not answer the question you thought you were asking, please respond with the specific question you wanted answered.
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Also remember, sometimes the law does not support what we want it to support, but that is not the fault of the person answering the question, so please be courteous.
This is NOT the practice of law nor is it legal advice to you, it is merely educational information for you to use to seek out a licensed attorney in your state to get actual legal advice from them. Please use sites such as http://www.martindale.com or http://www.lexmundi.com or http://www.hg.org to find a local attorney to get actual legal advice in all matters.