Brighton's taxpayers have been warned to return their self-assessment forms before the September 30 deadline by local chartered accountants.
Paul Feist, chairman of the Brighton group of the South Eastern Society of Chartered Accountants, said people missing the deadline would have to calculate their own bill.
Mr Feist, a partner at Feist Hedgethorne in Brighton, said they risked making expensive mistakes.
He said: "The danger of doing it yourself is you could end up out of pocket by paying too much or face a big bill next year if you've not paid enough."
More than 35,000 people in Brighton and Hove are subject to self-assessment.
Mr Feist said: "People who have to calculate their own tax need to know what they're doing or they should seek professional help."
Increased earnings and perks could all change tax liabilities, he said.
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