If you are going to employ a builder, ask them the following:
Have you served an apprenticeship?
What are your formal qualifications?
Often, you find the answer is that they have just picked it up as they have gone along in life. These are the people to avoid. You must only use qualified and experienced builders who can show you good examples of their work. You would not take the risk with an unqualified person carrying out an operation on your body or an unqualified mechanic operating on your car....so why would you take the risk with a house?