- Proprietary estoppel operates as a ‘sword’ to enable a person, to whom a promise or an ‘assurance’ is given, to seek positive relief to enforce the assurance.
- Proprietary estoppel applies to any property including chattels, choses in action and can apply to any land.
- Proprietary estoppel may be applicable where the assurance involves testamentary dispositions as well as inter vivos dispositions.
Proprietary estoppel is an old concept with a name devised in reasonably recent times to describe a cause of action which has increasingly common application. ‘Proprietary estoppel’ was first referred to in the decision E R Ives Investment Ltd v High  2 QB 379. The Court adopted the then newly coined description of the legal principle in the text, Snell’s The Principles of Equity, (Sweet & Maxwell Ltd, 26th ed, 1966). The origins of proprietary estoppel can be traced to Dillwyn v Llewellyn (1862) 45 ER 1285 (‘Dillwyn v Llewelyn’), Ramsden v Dyson (1866) LR 1 HL 129 (‘Ramsden v Dyson’) and Plimmer v Wellington Corporation (1884) 9 App Cas 699.
Unusually for an estoppel, proprietary estoppel is not a legal principle that is deployed to achieve a negative, defensive position like a ‘shield’. Proprietary estoppel operates as a ‘sword’ enabling the person to whom an assurance is given (the ‘promisee’), to seek positive relief to enforce the assurance (Shymko v Lach  NSWSC 1096 at ).
Proprietary estoppel applies to any property, including chattels and choses in action (Cobbe v Yeoman’s Row Management Ltd  UKHL 55). It usually arises in the context of land. It can be any land, such as a house (McNab v Graham  VSCA 352 (‘McNab’), Moore v Aubusson  NSWSC 1466 and Ceko v Ceko  SASC 91 (‘Ceko’)). Proprietary estoppel is most likely to arise in relation to farm land (Guest v Guest  UKSC 27 (‘Guest’), Laird v Vallance  VSCA 138 (‘Laird’), Protheroe v Protheroe  NSWSC 188 (‘Protheroe’)) .
The principles of proprietary estoppel apply in circumstances where the assurance concerns testamentary dispositions as well as inter vivos dispositions (Flinn v Flinn  VSCA 109, Delaforce v Simpson-Cook  NSWCA 84) (‘Delaforce’).