ABSTRACT
Estoppel is a doctrine which is designed to protect a reliant party from the detriment which may flow where another party denies the truth of an assumption or expectation which they have encouraged the reliant party to believe. Estoppel doctrines are recognised under both common law and equity and whilst, traditionally, estoppel has served an evidentiary function, these days equitable estoppel is recognised as having a substantive operation in some circumstances. Classically, there are three classes of estoppel: estoppel by record, estoppel by writing and estoppel in pais (conduct). Common law recognises estoppel in writing and by record, and a limited form of estoppel in pais. Equity only recognises estoppel in pais where it is not already recognised by the common law. The rationale of equitable estoppel is to prevent ‘persons from being victimised by other people’, and, as in other areas of equitable doctrine, the basic rationale underlying equitable estoppel is the prevention of conduct which is unconscionable; in estoppel that will ‘commonly involve the use of, or insistence upon, legal entitlement to take advantage of another’s special vulnerability or misadventure ... in a way that is unreasonable and oppressive to an extent that affronts ordinary minimum standards of fair dealing’.1
