Where a promisee already owes the promisor a legal duty, then in theory performing that duty should not in itself be a consideration – if the promisee does nothing more than they are already obliged to do, they are suffering no detriment and the promisor is only getting a benefit to which he or she was already entitled.
However, in recent years the courts have discovered consideration in the performance of an existing duty, causing some controversy in the process. Existing duties can be divided into three main categories: public duties, contractual duties to the promisor, and contractual duties to a third party.
Existing public duty
Where a person is merely carrying out duties they are legally obliged to perform, doing that alone will not be considered.
However, where a promisee is under a public duty, but does something which goes beyond what they are bound to do, that extra act can amount to consideration as was illustrated in the case of Glasbrook Brothers v Glamorgan County Council (1925).
Existing contractual duty to the promisor
The position on contractual duties and consideration has changed in recent years, and the implications of the change are still rather unclear. In the case of Stilk v Myrick (1809), the court held that the captain’s promise to pay extra money to the eight crew members (in the absence of 2 crew members) could not be enforceable as it was considered there was no consideration as the sailors had already contracted to sail to the destination and back and that no extra work was done out of their required duty. But in Hartley v Ponsonby (1857) the court decided differently as the captain was ordered to pay the crew members who stayed on board the ship as half of the members had deserted the ship. The court was of the opinion that the crew members were quite few to do the job and merit to be paid an extra fee in conformity with the captain’s promise.

  1. Contractual duties to supply goods and services: In the case of Williams v Roffey (1990), the court held that if one party’s promise to perform an existing contractual duty confers an additional practical benefit on the other party, then providing that no duress is involved, it will be sufficient consideration to make a promise given in return binding, even though in legal terms they are only agreeing to carry out their existing contractual duty.
  2. Contractual duties to pay debts: Special rules apply to contractual duties regarding debts. Where someone owes another money and cannot pay the full amount, they will sometimes offer to pay a smaller sum, on condition that the creditor promises to accept it as a full settlement for the debt. In other words, agree not to sue later for the full amount. Even if such an agreement is made, it is only binding if the debtor offers some consideration for it by adding some extra element. Pinnel’s Case (1602) is illustrative of this position.
    Existing contractual duty to a third party
    In some cases, two parties make a contract to provide a benefit to a third party. If one of the parties makes a further promise to that third party, to provide the benefit they have already contracted to provide, that further promise can be a good consideration for a promise made by the third party in return even though nothing more than the contractual duty is being promised by that party.
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