(iv)  Conclusion on estoppel and pre-contractual expenditure

It is clear that either promissory or proprietary estoppel is theoretically capable of dealing with certain claims relating to wasted pre-contractual expenditure. In fact, in a successful claim of proprietary estoppel, it is not simply that wasted expenditure is recovered, but rather that the right in land is itself upheld (in Thorner, the claimant was awarded the property itself, not simply wasted expenditure he had undertaken in the belief he was owner). That provides protection not merely of the restoration but the performance interest. By contrast, the utility of promissory estoppel varies jurisdictionally so far as wasted pre-contractual expenditure is concerned: in English law, it is unavailable, given the need for a pre-existing legal relationship and the further restriction that promissory estoppel claims are a defence rather than a foundation of rights; in US law, promissory estoppel may found a claim for wasted pre-contractual expenditure, as the Red Owl case shows; in Australia, promissory estoppel may not only found a claim but may be used to enforce an exchange of promises which lack only the legally required form.

Promissory estoppel is clearly one way to remedy the problem of informally made promises. It is only a partial solution, however, and the underlying question remains as to why only promises which are marked by reciprocal consideration or which give rise to reliance should be considered worthy of enforcement. Or to put it another way, if proper respect is given to personal autonomy and freedom of the will, why ought not all promises, if seriously uttered, be enforced according to their terms? One might also question whether, despite the views of their Lordships in Thorner, there is any overriding need to continue to insist that promissory and proprietary estoppel remain independent doctrines, given the similar functions they play in English law in preventing unconscionable denial of a prior promise by the promisor. If enforcement of both unilateral and gratuitous promises were permitted in English law, then estoppel would have to do a lot less work and would no longer have to be tied to the idea of promise. Indeed, in Australia, the traditional promissory nature of promissory and proprietary estoppel is arguably changing in just such a way,38 through development into a doctrine characterised less by promise and more by the prevention of inconsistent conduct causing loss to another. Such a development would give it a very similar characteristic to that of personal bar in Scots law.

38  See Robertson, ‘Estoppels and Right-Creating Events’.