Examines how the recent CA decision in "R v London Underground Ltd ex p Prokopp" ([2003] EWCA Civ 961, [2003] PLSCS 166) has provided a way out when a planning permission has been lost through expiry. Outlines the reasons why expiration is a common problem. Describes what developers normally do to keep permissions alive and the rare exceptions to the principle that if all conditions precedent to commencement are not complied with before construction work begins, implementation will be unlawful and, if close to the expiry date, the permission will be lost. In "Prokopp" the CA decision ruled that a s106 agreement entered into by London Underground, the developer, and Tower Hamlets LBC, the local planning authority, was implicitly upheld. The permission was no longer alive but enforcement action could not be taken because it would be irrational in the circumstances as the condition could not be complied with at the time it was drafted in 1997.