Customers who want work carried out are being warned to obtain written quotations in advance.
West Yorkshire Trading Standards say many people are failing to protect themselves against future disputes which may arise when the work has been completed.
The warning follows a shock bill received by Linton Benjamin after he put his car in for a service at a garage.
The 46-year-old from Norwood Street, Shipley, is now facing a £700-plus bill - and the garage has kept his car until he pays.
"I wanted a service and a few other things doing on it including the heater," he said.
"They told me it would all cost £530 plus VAT and I said it was more than I wanted to pay because it is an old car and I wanted to get rid of it.
"Then the mechanic and two salesmen broke the figure down to £273 plus VAT.
"But when I came to collect the car the bill was £712. My legs just went - I couldn't believe it.
"I kept asking how much the bill was going to be and if it was going to be a lot more and all the time they told me not to worry about it.
"At no time after I had agreed the work for £273 plus VAT did any of the staff tell me the bill could be more.
"I said when they gave me the bill they should have consulted me and I wouldn't have agreed to it all being done.
"I will pay £428 of it but no more. They have said they will keep the car until I pay it."
The garage sales manager said that his company had a long record of good service to its customers.
"If we had charged the full amount for the time we have spent on that car, it would be horrific," he said.
He claimed the biggest job was to replace the heater, which took 17 hours.
The garage normally charged £30 an hour labour but had reduced that to £11.50 an hour for the work on the heater and £16 an hour for the other jobs, he said.
"We told Mr Benjamin at the beginning that we didn't know how much the labour would be for the heater and we kept him informed all the way through," said the sales manager.
"He has offered to give us £400 and pay the difference in instalments but I have got to draw the line somewhere.
"We will keep the car until the money is paid in full.
"We have tried our level best to accommodate this customer - we took on the job but we have lost money."
A West Yorkshire Trading Standards spokesman said: "Firstly it is important to remember the difference between a quote and an estimate.
"An estimate is just that and the final price can vary plus or minus about ten per cent.
"But a written quotation is a contract and if you enter into a contract with someone to do work at a specific cost, they cannot charge you more than the amount contracted to do otherwise it is a breach of contract.
"If the cost is going to change from the price in the original contract, they have to tell you in advance how much it is going to be and you have to agree to it first. That amounts to a new contract.
"It is also worth remembering that, although a verbal contract is as binding in law, it is more difficult to prove if a dispute arises later - that's why it's better to get a quote in writing."
He said a trader was entitled to "exercise a lien" which allowed him to retain the goods until the price on the contract had been paid.
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