The warning has been issued by Powys County Council’s Trading Standards Service to ensure residents and businesses are not conned out of money by the rogue traders.
The service has received reports that people are being approached and offered property repair services.
It has also seen examples of some cold callers who are unqualified conmen or woman, who charge extortionate amounts of money for little or no work, with prices also being very misleading.
They usually demand immediate payment while practice methods aiming at not being traced.
Clive Jones, the council's professional lead for trading standards, community safety and emergency planning, said: "In these cases the work conducted is usually extremely poor. Our advice is never to accept work from cold callers and remember the old saying that if it is too good to be true, it usually is.
"The best advice we can give householders or businesses is not to employ any unknown trader who cold calls, offering to carry out repairs or improvements to their property."
Anyone who thinks they may be a victim or thinks someone they know may have been, should get in touch with Citizens Advice Consumer Service on 03454 04 05 06.