The sad fact of life is that for every hundred reputable tradesmen who will carry out your home improvement project to your exact specifications, on time and at a reasonable price, there are a few rogue cowboys out there whose only objective is to part you from your money as quickly as possible with scant regard for their workmanship and thats if any work gets done at all!
However, there are many things you can do to avoid becoming a victim of these con artists.
Firstly, ask friends of yours for recommendations. They may have had work done by a particular tradesman previously who carried out a professional job at a fair price. If thats not possible, make sure the tradesman can prove their competence by finding out if they are trained to do the job and whether they are members of a particular trade association. The tradesmen themselves should be able to supply that information and give you contact details of associations with whom you can verify their claims or, alternatively, you can contact a particular trade association and ask them to recommend someone in your area.
Always ask for a written estimate or, better still, a fixed quote prior to work...