“You promise, honest and true?” - When will a promise be legally binding?

Promises are a common way of assuring others that we will, or won’t, do something. While we generally expect people to keep them, not all promises are as reliable as they first appear, with some not being legally enforceable, as shown in the recent High Court decision in Haynes v Hamersley Investment Anstalt [2025] EWHC 432 (Ch) (Haynes).  

This article looks at why the promise in this case wasn’t legally binding and compares it with other scenarios when it may in fact be possible to enforce a promise. The estate, trust and will dispute team at Mills & Reeve comes across situations described in this article regularly.  

Non-binding promises

Haynes concerned a couple, Ms Haynes and Mr Austin, who were together for 25 years, and had two children together, but never married. After their relationship ended, Ms Haynes claimed she was promised half the value of their £18 million London house by Mr Austin if they separated, and that Mr Austin had said as such in a letter of wishes to the trustees of the trust which owned the property. She cited a trip to Liechtenstein in 2014, where Mr Austin's trust manager assured her that she would be financially looked after, and that Mr Austin's promise would be honoured. This assurance was repeated in a later letter from another trust manager.  

The Judge found that while it was reasonable for Ms Haynes to rely on these assurances, they did not constitute a legally binding promise. While the judgment hasn’t been released yet, some parts of it are available which help explain the reasoning behind this decision, namely:

• The trust manager did not clearly promise Haynes an interest in the property, only that he would use the letter of wishes as a basis for administering the discretionary trust.  A letter of wishes is not binding on a trustee
• The assurances related to a discretionary trust, meaning Ms Haynes' claim was subject to the trustees' discretion.  As an aside, it is not clear whether Ms Haynes was a beneficiary of the discretionary trust; and
• The property was owned by the trust, not Mr Austin, and the promise originated from Mr Austin’s letter of wishes, not the trustees.  

To compare Haynes with another kind of unbinding promise which individuals may try to enforce, it might be the case that a testator makes a verbal promise to give an asset to someone, usually a family member, but then doesn’t include reference to that promise in their Will. While each case will turn on its specific facts, generally the terms of the Will take precedence, and the promise is not generally enforceable. Much like in Haynes there is a mechanism (the Will, or a Trust in Haynes) which overrides the promise. However, there are certain exceptions to this general rule which we explore below. 

Binding promises

One exception can be found in the doctrine of proprietary estoppel, which can offer promisees a way to enforce verbal promises where there is little or often no written evidence of the promise. A common example of this in practice is often found in the farming world, where a child may work with their parents on the family farm for many years, expecting to inherit the farm when their parents retire or die, only to then not receive the farm in their parent’s will. Proprietary estoppel offers an avenue for the claimant to enforce the promises made to them where it would be unfair for them not to be enforced, provided certain requirements are met:

• First, the promisor needs to have given a sufficiently clear assurance to the promisee. There has been some debate as to what a clear assurance is in these cases, and often it will depend on the facts, albeit one case saw a nod of the head by the promisor deemed a sufficiently clear assurance.
• The promisee then needs to rely on the clear assurance to their detriment, which can include time spent working and missed opportunities, not just financial loss.
• It must also be unconscionable (ie unfair or unjust) for the promisor to renege on the assurances given to the promisee.  

In cases similar to Haynes involving unmarried couples who own or live in a property together, Courts have sometimes enforced promises of an interest in a property based on a "common intention constructive trust”.

Again, these cases rely on the promisee relying on a promise to their detriment. In one such example, a woman successfully enforced her partner’s promise to her that she would have a home for life. The Court found that:

• The claimant relied on the promised to her detriment by leaving her secure tenanted property, job, and moving her children to live with her partner, spending additional money on their new house which was in his sole name. This made the promise irrevocable, as it was unconscionable for the partner not to keep his promise of an interest in the value of the property.
• The partner made such reassuring promises as were necessary to persuade her to move in with him, giving up her independence and security, with the intention she would rely on them and knowledge that she did. These included promises of secure rights of occupation which would be recognised if the relationship broke down.

Conclusion – why some promises are legally enforceable while others are not

The examples above highlight the necessary elements for a claimant to enforce a promise and help explain why Ms. Haynes was unsuccessful in her case. Key barriers in her case appear to have included:

• Ownership: The property must be owned or in the control of the promisor. In Haynes, the house was owned by a trust, not Mr. Austin, making the promises set out in his letter of wishes subject to the trustees exercising their discretion to give effect to those promises.
• Clear Promise: The promise must clearly entitle the promisee to an interest in the property. In Haynes, the trust manager's assurances were not binding as they were subject to trustee discretion and so were not necessarily meant to be binding. 
• Detrimental Reliance: The promisee must show reliance on the promise to their detriment. It's unclear if Haynes' actions were due to reliance on assurances made during the 2014 trip to Liechtenstein, and whether she was entitled to rely on the assurances to begin with.
• Unconscionability: Failure to enforce the promise must be unconscionable, preventing claimants from imposing undue burdens on the promisor. While it was noted by the Judge in Haynes that it was reasonable for Ms Haynes to have relied on the promises made to her by Mr Austin and the trust managers, the facts of that case meant it was not possible for her to claim it was unconscionable for the promisors to renege on their promise so as to make it enforceable.  

Our content explained