Estoppel is a long-established doctrine in English law which, put very simply, prevents a person who agrees one set of facts with another individual going back on their word. For instance, if you have been told that you no longer owe a debt, but the creditor later insists on payment in full they may be estopped from enforcing their right to repayment.
Estoppel is not, however, a simple doctrine and there are a number of different ways in which it can operate. The link between the different types of estoppel is hard to determine and can make it difficult to provide general advice. While the application of the doctrine can be a beneficial way of defending your rights or a prior agreement, legal advice is always recommended to ensure that you protect your position with the maximum effect.
Common types of estoppel
Such is the complicated nature of the doctrine of estoppel, a definitive list of the kinds of estoppel that are common to the law of England largely eludes. Broadly, however, estoppel can be divided into four distinct groupings:
1. Estoppel by representation;
2. Promissory estoppel;
3. Proprietary estoppel;
4. Estoppel by convention.
Estoppel by representation
This is sometimes referred to as an evidential rule, and its principles are used when a case is brought to court. It can be used to prevent a plaintiff denying that a statement or set of facts are true where they had previously represented that they were. The plaintiff would be estopped from doing so if the defendant had acted to his detriment believing the plaintiff’s original statement was honest. This rule prevents people from saying one thing out of court, inducing someone to rely on it, and then go into court to deny what they said was true.
Estoppel by representation cannot be used as the basis of litigation. It is commonly referred to as being a ‘shield, not a sword’. However, its very operation can help build a case. For instance, someone says that they have passed property to you and you rely on that representation by making improvements to the land and buildings that they said were yours. If the transfer does not occur and the owner makes attempts to recover the property, you may be able to rely on the fact that they represented that the property had passed to you, to estopp them from denying that fact and defend the claim.
Promissory estoppel is often seen as an extension of estoppel by representation. This is because it includes many of the elements required for the latter.
To establish promissory estoppel, you need to establish three elements:
1. An unambiguous promise by words or conduct;
2. A change of position in reliance on those words or behaviour. Detriment does not have to be proved.
3. Injustice if the promise were to be revoked.
Again it is a ‘shield, not a sword’ so cannot be used as a basis of an action, but could be used as a defence.
One such example could be in the context of a landlord/tenant relationship. As part of the lease, the tenant is obliged to pay rent. The tenant has been complying with their obligation and paying rent timeously. However, the tenant falls on hard times, and the landlord tells them they will not charge them rent until they are back in employment. After a couple of months, the tenant gets a new job but does not restart rental payments. The landlord attempts to sue for payment of the rent for the whole period. While they would be entitled to the rent from the point that their tenant found employment, they would be prevented from seeking the rent for the period that their tenant was unemployed as they had represented that they would not charge rent, and the tenant acted on that.
Proprietary estoppel occurs where someone relies on the belief that they have, or will have, an interest in land that belongs to someone else. This reliance will mean that the person to whom the land is promised expends money or otherwise acts to their detriment in the belief that the property or land is, or will be, theirs.
For instance in the case of Inwards v Baker  2 QB 29 a father promised that he would leave land to his son in his will, and encouraged him to build a house on it. His son did so and lived there for 30 years. However, on the father’s death, it transpired that the land had not been transferred in the will. The executors were estopped from evicting the son.
Proprietary estoppel can provide the basis for a case, where someone seeks to enforce their right to land that was promised to them. This sets it apart from estoppel by representation and promissory estoppel. It means that you can bring a claim to be granted an interest in land, and not just defend yourself from having claims brought against you.
For instance, in the case of Thorner v Major  UKHL 18, the claimant had worked for 30 years for his father’s cousin – Peter – without payment, on the understanding that he would inherit the farm on Peter’s death. However, when Peter did pass away, he left no valid will. The claimant raised an action asserting that he was entitled to the farm under the law of proprietary estoppel. The unpaid work that he had done was carried out on the understanding that one day he would inherit the land. The House of Lords agreed that it would be ‘unconscionable’ for him not to inherit and the farm was passed to him.
Estoppel by convention
Estoppel by convention extends the boundaries of estoppel to include stopping people from arguing a point because of the way that they have acted over time. Instead of focusing on the behaviour of one party who induces another to act in a certain way, estoppel by convention looks at the way in which both parties to a contract have acted due to an assumed state of the law or facts. If consistent over time these actions could amount to a convention.
The critical requirements of this type of estoppel are that:
1. The assumption is shared by the parties;
2. The party which claims the benefit of the convention must have relied on it. Reliance can be as little as being influenced by it in some way.
3. It must be inequitable for the party to act contrary to the convention so that he is estopped from doing so.
Importantly here estoppel by convention does not require inducement by one party, nor detriment through reliance from the other. On that basis, an assessment of inequity will become a large part of the consideration.
Other types of estoppel
Although these are the four significant categories of estoppel, it is not an exhaustive list. Estoppel is the root of the doctrine of res judicata, for instance. This is where a party is prevented from re-litigating an issue which has already been determined and is also known as ‘cause of action estoppel’ or ‘issue estoppel’.
Why you should seek legal advice
Estoppel can be an extremely useful mechanism in litigation, and in asserting rights. It is far from straightforward though. There are many different types of estoppel, and as a doctrine, it has wide application. Proprietorial estoppel, for instance, can act in a way which protects your property rights or compensates you should a promised bit of land not be forthcoming. The operation of this doctrine is complicated, and legal advice is recommended to consider whether you have rights that can be enforced, or defended by estoppel.