Consideration is what is given or promised in exchange for a contractual promise. Promissory estoppel can restrain a party from insisting on strict legal rights after a promise or assurance has been relied on, where going back on it would be inequitable.
They do different work. In English law, promissory estoppel is not a general replacement for consideration or a way to enforce every promise that feels unfair to break. Start with the contract and the proposed exchange, then consider whether estoppel affects enforcement.
What does consideration require?
Identify what each party supplies in return for the other’s promise. Consideration must have legal value, but the court does not generally require an economically equal bargain. Chappell & Co v Nestlé [1960] AC 87 illustrates that something of modest economic value can form part of the stipulated exchange.
Example: Asha agrees to sell an old desk to Ben for £20, although it might fetch £80 elsewhere. The disparity does not itself mean consideration is absent. Other vitiating factors would be separate questions.
Contrast a bare promise: “I will give you £20 next week.” If no return is requested and no deed or other basis is involved, gratitude or a desire to help does not supply a bargain by itself.
Can something already done be consideration?
An act completed before the promise is ordinarily past consideration. But do not stop at the dates. Pao On v Lau Yiu Long [1980] AC 614 explains an important qualification: an earlier act may support a later promise where it was requested, remuneration or another return was understood, and that return would have been enforceable if promised in advance.
Example: Ben voluntarily cleans Asha’s driveway as a surprise. Afterwards, Asha promises him £30 as thanks. On those facts, the earlier voluntary act is not an exchange for the later promise.
Changed facts: Asha had asked Ben, who runs a cleaning business, to do the work in circumstances where both understood he would be paid; they fixed the amount afterwards. Analyse that requested, remunerated work separately. It is not simply the first scenario with a different date on the invoice.
What if someone promises to pay more for existing work?
Performance of an existing contractual obligation to the same party raises a consideration problem, but it is not the end of the analysis. In Williams v Roffey Bros [1991] 1 QB 1, practical benefit could support a promise of extra payment in the circumstances, with fraud and economic duress excluded.
Example: A main contractor offers a subcontractor extra payment to complete agreed work on schedule, obtaining a practical benefit in avoiding disruption. Identify that benefit and examine the circumstances of the promise. Do not conclude that every demand for extra money is enforceable merely because completion would be useful.
Check any formal requirements for variations too. A consideration argument does not automatically override an effective clause requiring modifications to be made in writing.
Is accepting less money the same thing?
No. The rule associated with Foakes v Beer (1884) 9 App Cas 605 is that part-payment of an existing debt is not, by itself, consideration for a promise to give up the balance.
Example: £1,000 is undisputedly due today. The creditor agrees to accept £800 today in full settlement, and the debtor supplies nothing different in return. Identify the lack of fresh consideration before considering any separate estoppel argument. Different facts, such as an agreed different performance or a genuine compromise, need their own analysis.
Do not say the Supreme Court abolished this distinction in Rock Advertising. It decided that case on the no-oral-modification clause and did not resolve the consideration issue. Rock Advertising v MWB [2018] UKSC 24.
When might promissory estoppel matter?
Look for an existing legal relationship, a clear assurance about enforcing rights, reliance and circumstances making retraction inequitable. The precise assurance and its context determine the protection sought.
Example: A landlord clearly promises to accept reduced rent for a defined period of disruption. The tenant relies on that promise and pays the reduced sum. A later demand for the waived balance raises an estoppel question: what was promised, for which period, how was it relied on and would retraction be inequitable?
The classic authority is Central London Property Trust v High Trees House [1947] KB 130. It does not mean every rent reduction lasts forever. Rights for later periods and amounts affected by the assurance must be considered separately.
Why is estoppel described as a shield?
The traditional promissory-estoppel doctrine restrains enforcement of existing rights; it does not independently create a new cause of action to enforce a gratuitous promise. Combe v Combe [1951] 2 KB 215 is the leading illustration. The distinction is reiterated at paragraph 35 of COPA v Wright [2024] EWHC 3315 (Ch).
Do not confuse promissory estoppel with proprietary estoppel, which has a different role and requirements. Nor should “shield” become a substitute for identifying the actual promise and reliance.
A revision checklist
Ask: What promise is being enforced? What was requested in exchange? Was the act past, an existing duty or part-payment? Is there a relevant exception, deed or variation clause? If consideration is absent, what existing right is estoppel said to restrain, and why?
MCQ Master helps you practise these distinctions through contract questions. For the complete revision library alongside that practice, Law Answered+ gives you access to all our guides and casebooks covering the full syllabus, with MCQ Master included. Work through this topic and the rest of your subjects in one membership. Compare similar scenarios and identify the one fact that changes the analysis.
If the uncertainty comes earlier in the negotiations, use our offer and invitation-to-treat examples to establish what was promised.