when does a proposal become binding

Your "proposal" becomes binding when you include all the necessary information in your project agreement and it's approved by the employer.

so best to make sure you have all the information from the employer before agreeing? and if the employer changes the project details/specification then the agreement becomes invalid? Is this correct?

Yes, you should have all the information and details and whatever else is necessary for you to complete the job. That's so you'll know exactly what you have to do, how you have to do it and the time it will take to complete it. If the employer changes the project AGREEMENT, you have to approve it before it becomes binding. This potentally can become a back and forth issue where both of you just can't come to a definate agreeement. To avoid any problems make sure everything is spelled out EXACTLY in the agreement. Also include a statement, saying, "Anything above and beyond what is specified in this project agreement will constitute additional charges." Once the project agreement is approved, and you start work, if the employer then changes the details, and specifications, which at times does happen, -it's referred to scope creep - you are only bond on what is specified in the agreement. Anything above that, IF the employer agrees to additional charges, that can be added to the agreement (and approved by the employer.)

ok thanks Stacy, very useful link.