Start by ensuring that you have entered into a contract of agreed terms, including terms such as use of power and toilet facilities.
Ask the contractor for a schedule of works, a health and safety proposal and a method statement for any significant
parts of the work such as demolition and excavation.
Notify your home insurance provider and obtain a copy of the contractor's insurance to ensure that there is no gap.
Ask for evidence that the contractor has complied with all statutory requirements, such as building regulations.
The best advice is to create what lawyers call the paper trail. Record everything in a duplicate
book especially amendments in the work and the cost implications, and instructions at on site meetings.
The contractor probably has many years of successful trading behind them, which means that there should be no
problems with the majority of jobs. However, if you are not happy with anything ask immediately and preferably speak to the foreman or manager.
The standard that the contractor will follow, in the absence of other directions at tender
stage, is to meet the reasonable standards of the industry for similar cost work done in similar
circumstances. It may not be as good as you had expected or indeed could achieve yourself, but
it does not necessarily mean that it is incorrect or unacceptable. Raise any concerns that you have about the standard of work with the contractor. If you remain unhappy with the outcome professional advice and opinion may be needed to confirm real problems.