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HMRC internal manual

Employment Income Manual

The general rule for employees' expenses: relief is limited to expenses actually incurred: example

An employee receives from his employer a payment to meet the cost of hotel accommodation when he has to stay away overnight on business. However, instead of staying in a hotel, he sleeps in his car. He is taxable on the amount he receives from his employer and is not entitled to a deduction for overnight accommodation because he has not incurred any expense.