For a growing number of high-net-worth couples, moving in together has become as much a financial decision as a romantic one. Marriage, once the default framework for shared life and shared wealth, is no longer a given. Cohabitation is now the fastest-growing family type in the UK and, according to the Office for National Statistics, the number of cohabiting couples rose from 1.5 million in 1996 to more than 3.6 million in 2021. Among the affluent, this living arrangement often also comes with a multi-layered financial set-up: family money, offshore trusts, business income and intergenerational gifting.

Against that backdrop, a deceptively simple question emerges. If you move into a £10m townhouse with your partner and you are not married, what do you actually walk away with if you split up? The answer, as family lawyers will tell you, is rarely what people expect.

What you really own when you’re not married

The starting point for unmarried couples is stark: there is no automatic right to a share in a partner’s property. Instead, ownership depends on legal title, contributions and evidence of intention, not on the duration or emotional substance of the relationship.

“Much depends on how the property is owned (for example, whether it is held jointly or in one person’s sole name) and whether there is a Declaration of Trust setting out the parties’ respective interests,” says Kate Moran, family and divorce partner at Collyer Bristow.

She adds: “If you are not married, disputes about property are determined by property and trust law, rather than by family law principles. This means the court does not apply concepts of needs or overall fairness in the way it does for divorce. Instead, an unmarried partner must rely on complex property law remedies, such as constructive trust or proprietary estoppel claims, which are heavily dependent on evidence. As a consequence, if you are unmarried, you may, in some circumstances, walk away with no interest in even a £10 million home, [regardless of whether] you may have been in a 25-year relationship and given up your career to raise your now adult children.”

Rebecca Christie, family and divorce partner at Wedlake Bell, says the legal position often comes as a shock. “It is a common misconception that if couples have a child and/or live together they have acquired the same legal rights as those who are married. Unfortunately, unmarried couples have no claims on each other’s property save under general contract and property law, or on death if they qualify under certain statutory provisions.”

Neil Russell, partner and head of family at Seddons GSC, is equally clear. “Unmarried partners have no automatic right to a share in their partner’s property. To establish an interest, you will ordinarily need to demonstrate a financial contribution or a clear common intention that an interest has been granted.”

Where unmarried couples do attempt to assert a claim, the route is far narrower than many expect. As Christie explains, these disputes are treated as property matters rather than relationship breakdowns. “Property claims for unmarried couples are determined under the Trusts of Land and Appointment of Trustees Act 1996 (TOLATA). This is like any other dispute between the co-owners of property. The Act deals with agreements about the ownership of property.”

The myth of common law marriage

Few legal misconceptions are as persistent or as expensive as the idea of the ‘common law marriage’. It sounds reassuringly official, a shorthand for commitment that feels equivalent to marriage without the ceremony. In reality, it offers no protection at all.

“In England and Wales, the idea of common law marriage is a myth,” says Moran. “Yet surveys consistently suggest many people still believe that living together creates the same legal rights as marriage.”

That gap between perception and reality is where financial risk takes root. Among affluent couples, where one partner’s income or family wealth may underpin a shared lifestyle, assumptions about joint ownership and future security often go unchallenged.

“Misplaced belief in common law marriage frequently informs the choices couples make about work, money and family life,” Moran explains. “When an unmarried relationship ends, however, there is no obligation to divide assets fairly or meet a former partner’s financial needs.”

In practical terms, that can mean years of financial interdependence unravelled overnight, with no legal mechanism to rebalance the outcome. The partner who compromised on career progression, pension contributions or independent assets may find that those decisions carry lasting consequences. Russell puts it plainly: “The biggest myth is that there is a ‘common law spouse’ and there is no such thing. You are either married or you are not.”

The distinction is not semantic, it defines the entire legal framework a couple must navigate when they split. As Christie points out, the rights that many assume already exist are, in fact, triggered only by marriage. “It is only when a couple marry that they assume financial obligations to each other,” she says. “Most significantly, it is only marriage that empowers a court to make orders redistributing their assets and for the payment of maintenance in the event that they subsequently separate.”

Children introduce a limited safety net, but not one that protects the former partner directly. Financial provision can be made for a child’s benefit, including housing in some cases, but it is not a substitute for the broader claims available on divorce. “Importantly, these are not payments in favour of a former unmarried partner,” Christie adds. “Any decision is at the court's discretion.”

For high-net-worth couples, the misconception is particularly costly because the stakes are higher. In the absence of legal structure, even the most established relationships can leave one party financially exposed.

When to have the conversation

Timing, perhaps unsurprisingly, is where many couples hesitate. The early stages of moving in together might not exactly feel like the moment to talk about the legal consequences of breaking up. “In an ideal world, a cohabitation agreement should be put in place before a couple moves in together and reviewed as the relationship evolves, particularly following major life events,” says Moran. “It should be treated with the same seriousness as the other practical and financial arrangements that accompany moving home.”

That may sound overly cautious, but in practice, early clarity can prevent far more difficult conversations later. As relationships become financially intertwined, informal understandings can quickly give way to assumptions.

Christie highlights more formal mechanisms that can be put in place from the outset. “In order to avoid future disputes about the extent of each party's interest in a property, unmarried partners can enter into an express declaration of trust setting out each of their interests before moving together or when they or one of them is buying a property.”

Where that structure is missing, even well-intentioned arrangements can unravel under scrutiny. Without documentation, questions around contributions, ownership and expectations are left open to interpretation. Russell emphasises that delay is often the real risk: “The sooner the better. The problem is sometimes not knowing when the cohabitation actually began, it can be an elusive date to trace back to if the time spent together has been incrementally increased.”

Social dynamics play their part too. Conversations about money, particularly at this level, are rarely straightforward. They can feel at odds with the emotional optimism that typically defines a new shared home. “Most people do not want to spell out to their partner that they do not have and will never have an interest in their £10 million property,” Russell adds.

For Moran, the value of these agreements lies as much in communication as in legal protection. “Cohabitation agreements encourage open and constructive conversations about money and financial expectations at an early stage,” she says.

Beyond breakups

The risks for unmarried couples are not just limited to separation. In many cases, the more significant financial shock comes later, when assumptions about inheritance and security are tested. “The risks can also arise if one partner dies,” says Moran. “Many unmarried couples do not make wills because they believe their partner will automatically inherit. That is not the case.”

For high-net-worth couples, where property and wider assets may sit within complex structures, that misunderstanding can have serious consequences. Without a will, there is no automatic right to inherit, no guaranteed role in administering an estate and no legal priority in decision-making.

There are also tax implications that are often overlooked. “Unlike spouses, unmarried partners do not benefit from the inheritance tax exemption,” Moran explains. “Even where a jointly owned property passes automatically to the surviving partner as a joint tenant (where a property is held as joint tenants), that survivor may still face a substantial inheritance tax bill.”

The effect is not just administrative but financial. In high-value estates, the absence of spousal protections can translate into substantial and immediate liabilities.

Act now or wait for reform?

With a government review of cohabitation laws expected this year, it is tempting to assume that change is imminent. In practice, reform has been discussed for years, with little concrete progress.

“Couples should act now,” says Moran. “While the government is expected to review cohabitation laws, there is no certainty as to what any reform will look like, or when it might take effect.”

The gap between policy discussion and legal change can be significant. Even widely supported reforms can take years to move from consultation to implementation, leaving couples exposed in the meantime. Russell, for one, is sceptical about the pace of progress: “This has been on the agenda for a long time and the last time a proper review was scheduled in 2022 it was rejected and the issue was kicked into the long grass. There is a real risk this will happen again.”

Christie’s view is similarly pragmatic: “Regardless of the government's impending review of cohabitation laws, at present the effect of a Cohabitation Agreement is a matter to be decided by the court exercising its discretion.”

In other words, waiting for any meaningful reform is a bit of a gamble. For those stepping into a £10m townhouse, the real question is whether they have put the right protections in place before they really need them.

Read more: What infidelity really costs the super-rich