A number of factors disqualified the ‘deserted wife’s equity’ from recognition as a property right in National Provincial Bank Ltd v Ainsworth  AC 1175:
I suggest you look at the developing concepts of fairness, because this is why the MHA 1967 was developed. Proprietary Estoppel for cohabitees is becoming less prevalent due to the decisions in family home trust. Thus, fairness is at the centre of the approach, except the clear provision of a proprietary interest is necessary and not merely the provision of a roof over the other’s head. This is common to Ainsworth, proprietary estoppel and the family home constructive trust. The rationale is that it would not be fair to impose a proprietary right without a proprietary intention.
The case of National Provincial Bank Ltd v Ainsworth  AC 1175 holds a limited approach to understanding non-occupier’s rights in property.
National Provincial Bank Ltd v Ainsworth held that the common law right for the husband to provide a roof over the head of the deserted wife was merely in personam. This means selling the property to a third party will allow the husband to avoid his obligation to his deserted wife
It is important to note that it predates the Matrimonial Homes Act 1967 (MHA 1967).
The MHA 1967was developed to remedy the flaw in National Provincial Bank Ltd v Ainsworth, which indicates that the legislature recognised that the existing law with respect to deserted wife’s equity and its enforceability against third parties was manifestly unfair.
The law on proprietary estoppel provides that the third party find their rights will be interfered with.
The elements of proprietary estoppel can result in an in personam right defeating an in rem right if the following element is fulfilled:
Reasonable belief that the person will have interest in property
Acts reasonably in reliance
Gillet v Holt
This is illustrated in a number of cases that have expressed that the main factor is that there is a clear expression of a proprietary right in the property (Thorner v Major  UKHL 18). The case of Walsh v Singh  1 FLR 1658 held that conduct plus detriment is not enough is not enough to allow a claim for proprietary estoppel. In addition, the case of Negus v Bahouse  1 FCR 768 held that statement to provide a roof over the individual’s head or a determination to move in is not enough to allow a claim for proprietary estoppel.
The Negus v Bahouse Case is, in part, applies the same formulaic approach, as The implication is that there has to be a clear expression of a proprietary right, in order for proprietary estoppel to be used.
There are a series of cases on the constructive family home trust, which may change the goal posts on what an expression of a proprietary right when it comes to a spousal/partner interest. These cases are Oxley v Hiscock  EWCA Civ 546, which identified that in family relationship there is an obligation to ensure that there is fairness in the rights of a non-property owning spouse/partner.
In these cases the use of the constructive trust would be better for the family member who has relied on a property right inferred by the property owning spouse/partner (
The “deserted wife” (partner) has to show that she “has any interest in it [the property] at all” (Stack v Dowden at 56). This means the intention is imputed through the relationship (i.e. relationship plus contribution = share in the property). Thus, both proprietary estoppel and the family home constructive trust has move away from the in personam right not trumping an in rem right. However, for this to work there has to be a clear expression of a proprietary interest and not merely providing a roof over the individual’s head (Negus v Bahouse cf. National Provincial Bank Ltd v Ainsworth for similarity).
The impact of the fairness rulings in Oxley v Hiscock. Stock v Dowden and Jones v Kernott may change the mere expression argument if the nature of the relationship imputes an assumption of a proprietary right. Thus, potentially the obligation to provide a roof over the head of the other party is sufficient.
Additional References to Consider on top of Proprietary Estoppel:
Baroness Deech, ‘Cohabitation’  Family Law 39
Fretwell, K “Fairness is what justice really is: Kernott v Jones in the Supreme Court” (2011) Family Law 41(7)
Hayward, AP “Family Property and the Process of Familialization of Property Law” (2012) Child and Family Law Quarterly 24(3)
McGhee, M “Shifting the Scales of Social Justice in the Cohabitation Context: The Juridical Basis for the Varying of interests in Residential Property” (2012) Oxford University Law Journal 1(19)
Mee, J “Burns v Burns: The Villain of the Piece?” in Probert, R, Herring, J and Gilmore, S Landmark Cases in Family Law (Hart, 2011)
Mee, J “Ambulation, Severance and the Common Intention Constructive Trust” (2012) Law Quarterly Review 128(500)
Miles, J “Charman v Charman (No 4)  EWCA Civ 503 – making sense of need compensation and equal sharing after Millar: MacFarlane” (2008) Child and Family Law Quarterly 20(376)
Pawlowski, M “Joint ownership and the family home” (2011) Property Law Review, 1(68)
Probert, R “Cohabitation: Current Legal Solutions” (2009) Current Legal Problems 62(1)
Probert, R “Cohabitation in Twentieth Century England and Wales” (2004) Law and Policy 26(1)
Smithdale, J “Inference, Imputation, or BothConfusion Persists over Beneficial Interests in the Family Home” (2011) CSLR 74, p 79