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HMRC internal manual

Employment Income Manual

Travel expenses: general: choice of route: example

During the course of a business trip an employee takes the opportunity to visit his mother. If the cost of his journey would otherwise be deductible, the fact that he makes a social call on the way does not affect that. However, if the social call involves a significant detour, that detour is not in the performance of the duties of his employment and is not attributable to attendance at a particular workplace. So the cost of the detour is not deductible.