Obtain a minimum of three quotes and make sure that quotes are based on the same specification.
Try to obtain fixed quotes rather than estimates, which are a rough price that is subject to change
Insist on a revised quote for any additional work - before that work is carried out
Beware of very low quotes. Tradesmen who provide these may not understand how much work is required and could try to claw money back later
Make sure the quote clearly shows the tradesman's name, full address and landline telephone number
Ask if the work is guaranteed and if guarantees are insurance-backed in case the builder goes out of business
Agree on start/completion dates and payment arrangements before work starts.
Most importantly - check any prospective builder's feedback and references thoroughly before committing to anything!