The prospective buyers of a waste treatment plant (“Reprotech”) wished to use the site to generate electricity from the waste produced. They asked the Chief Planning Officer of the local council whether this would be a material change of use, for which planning permission would be required, and were assured that it would not. After they bought the land, the council insisted that the Officer lacked authority to make such a determination and required a formal application for planning permission; this was met by local opposition. Must the buyers make such an application and suffer a reduction in the value of the land if the application is unsuccessful, or should the public bear the consequences of the unauthorised act of the Officer? This was one of the questions facing the House of Lords in R. v. East Sussex County Council, ex p. Reprotech (Pebsham) Ltd. [2002] UKHL 8, [2002] 4 All E.R. 58. Their Lordships denied that there had been a “determination” for the purposes of the Town and Country Planning Act 1990, s. 64, upon which Reprotech sought to rely, but the obiter discussion of the proper role of estoppel in public law is the focus of this note.