The 24-month rule determines when a contractor can deduct the cost of travel to a client site. Travel to a temporary workplace is allowable, but a workplace stops being temporary (and becomes a permanent workplace, so travel there is non-deductible ordinary commuting) once the contractor has spent, or expects to spend, more than 40% of their working time there over a period exceeding 24 months.
The rule is about expectation, not just elapsed time. As soon as it becomes known that an engagement will exceed 24 months at one site, travel to it ceases to be allowable from that point, not from month 24. So a contractor who signs an 18-month contract that is then extended to 30 months loses the deduction from the moment the extension is agreed, because the expectation has crossed the threshold. This is the single most-misclaimed contractor expense.
The rule interacts with IR35 status. For an inside-IR35 engagement, home-to-client travel is generally not deductible at all, because each separate engagement is treated as a separate employment and the client site is a permanent workplace for it. Outside-IR35 contractors keep the temporary-workplace relief, subject to the 24-month rule. The statutory hooks are ITEPA 2003 ss.337 to 339, with HMRC guidance at EIM32000 onward.