Police have received a number of reports about suspected rogue traders in Chesterfield.
Men visited elderly residents on Bursdon Close and Langtree Avenue yesterday and offered to carry out work to guttering or roofing.
The men claimed they had found further problems which needed fixing and increased their price by a substantial amount and said they would return the following day.
PC Kelly Brown, of Derbyshire Constabulary, said: "We always strongly advise residents not to agree to any work by strangers who turn up at their homes unexpectedly and offer to do repairs or maintenance work.
"If you do think the work is needed, check with a few reputable companies and get other quotes first.
"This can really help prevent you paying over the odds for poor quality repairs.
"Don't feel pressurised to answer your door to cold callers - remember you don't have to and if you don't recognise the person then don't open it.
"If they are selling something or offering to do any work on your house or garden, tell them through a locked, closed door that you are not interested and ask them to leave.
"If they persist or you feel threatened, tell them you will call the police and report it immediately."