A national survey by Which? suggests at least 2.5 million people have had a dispute with a builder or decorator in the last three years, with a quarter forced to take formal action.
The watchdog found the most common disagreement was over the quality of the work, with other major gripes including traders not turning up when agreed, delays over completion, and properties being left untidy.
The report comes as news emerged of an 83-year-old disabled man from west London who was charged £14,400 for just five hours work to have his household drain cleared.
Harold Stern told the Telegraph he was "shocked" after being told the charges by an emergency plumbing firm from Hertfordshire.
A quarter of people surveyed by Which? said they failed to receive a written quote from their builder or decorator before work commenced, and 4% were asked to pay the full amount for the job upfront.
Which? conducted a survey of 2,545 adults and extrapolated from their results on the basis that the UK’s population of over 16-year-olds is 48.8 million.
One of the most effective ways to avoid the cowboys is to use a service such as TrustATrader.com - a directory of tradespeople you can trust because their place on the website depends upon their quality of service.