A prenup and a cohabitation agreement are both ways to set out a couple’s finances in writing — but they apply to different situations, and confusing the two can leave you unprotected. The short version: a cohabitation agreement is for unmarried couples who live together, while a prenup is for couples who are about to marry. Which one you need depends simply on whether you are marrying or not.
This guide explains what each document does, why the distinction matters so much in England & Wales, and why many couples end up using both — one after the other.
The key difference: married vs unmarried
A cohabitation agreement (sometimes called a “living together agreement”) is a contract between partners who share a home but are not married and have no plans to marry imminently. It records who owns what, who pays for what, and how things would be split if they separated. A prenuptial agreement does a similar job, but for couples who are getting married — and marriage changes the legal picture completely.
Prenup vs cohabitation agreement at a glance
| Feature | Cohabitation agreement | Prenup |
|---|---|---|
| Who it is for | Unmarried couples living together | Couples about to marry |
| Legal claims it addresses | Very limited (no marriage rights) | Broad marital claims |
| Governed by | Ordinary contract & property law | Radmacher / Matrimonial Causes Act |
| Often paired with | A declaration of trust | A will and disclosure schedule |
| Trigger event | Separation while unmarried | Divorce |
Why it matters: “common law marriage” is a myth
Many people believe that living together for long enough gives you the same rights as a married couple. It does not. There is no such thing as common law marriage in England & Wales, however many years you have been together and whether or not you have children. Unmarried partners have very limited financial claims on each other if they split up — which is exactly why a cohabitation agreement (very often alongside a declaration of trust over any shared property) matters so much for couples who live together but have not married.
Without either marriage or an agreement, an unmarried partner can find they walk away with little, even after a long relationship. The agreement is what fills that gap.
What changes when you marry
Marriage gives partners significant financial claims on one another that cohabitation simply does not. On divorce, a court can share assets, order maintenance and look at pensions — powers that do not exist for unmarried couples. So the moment a cohabiting couple decides to marry, the prenup becomes the relevant document, picking up where a cohabitation agreement leaves off. If you are marrying, see do you need a prenup? and what to include.
What each document typically covers
A cohabitation agreement
- Who owns the home, in what shares, and what happens to it on separation.
- How the rent or mortgage, bills and household costs are split.
- How jointly bought items and savings are divided.
- Arrangements for any joint bank accounts or shared debts.
A prenup
- Property each partner owns, and how the family home is treated.
- Savings, investments, pensions and business interests.
- Debts brought into the marriage, and inheritances to be ring-fenced.
- How wealth built up during the marriage is shared.
Do you need both?
Often, yes — but in sequence. A couple who buy a home together and then marry a year or two later commonly use a cohabitation agreement (and a declaration of trust) while unmarried, then a prenup as the wedding approaches, so their contributions are protected at both stages. See buying a house together before marriage and, if you already live together, getting a prenup when you already cohabit.
How the law treats each document
This is where the two really part company. A cohabitation agreement is an ordinary contract: if it is properly drafted, signed and supported by each partner giving something up or taking something on, it is generally binding and enforced like any other contract. A prenup is not automatically binding. Since Radmacher v Granatino (2010) a court in England & Wales gives a fair, properly made prenup significant weight, but it keeps a discretion under the Matrimonial Causes Act 1973 that no agreement can remove — because marriage brings those wide financial powers into play. So paradoxically the agreement for unmarried couples can be the more directly enforceable of the two, precisely because marriage adds a layer of judicial oversight that cohabitation does not. See are prenups legally binding? for the full picture.
A worked example
For example, imagine Sam and Jess buy a flat together while unmarried. Sam puts in a £40,000 deposit from savings; Jess puts in nothing but pays a larger share of the mortgage. A cohabitation agreement, alongside a declaration of trust, records that Sam’s £40,000 comes back first on a sale, with the rest split to reflect their contributions. Two years later they get engaged. Marriage will bring the flat into the matrimonial pot, so as the wedding approaches they make a prenup that carries the same intention forward — Sam’s deposit is treated as his separate contribution if the marriage ends. Same couple, two documents, each doing its job at the right stage. Without the second document, the careful protection they built while unmarried could quietly dissolve on the wedding day.
What a cohabitation agreement cannot do
It is important to be realistic about the limits. A cohabitation agreement cannot create marriage-style rights out of thin air: it cannot give an unmarried partner a claim on the other’s pension, and it cannot bind the court on arrangements or support for children, whose welfare is always decided separately and comes first. What it can do is set out property, savings and household arrangements clearly, so that if the relationship ends there is a written record rather than a painful, expensive argument about who intended what. For couples who never marry, that written record is often the single most valuable piece of financial protection they have. It also spares the survivors of a break-up from having to reconstruct, years later, who paid for what — a task that is stressful, uncertain and, if it ends up in court, expensive.
Timing: when to move from one to the other
A practical rule of thumb: while you are living together with no firm wedding plans, a cohabitation agreement (plus a declaration of trust over any shared property) is your tool. Once you decide to marry, start thinking about a prenup, and give yourselves time — ideally several months, and at least the 28 days before the wedding suggested by the Law Commission in 2014. Do not simply assume your cohabitation agreement will keep working after the ceremony; it was not designed for married life, and marriage changes the legal test that applies. See combining finances before marriage and a financial checklist for engaged couples for the wider planning.
Prenup vs cohabitation agreement: which do you need?
In the prenup vs cohabitation agreement question, the answer is purely about status: a cohabitation agreement is for unmarried couples living together, while a prenup is for couples who are marrying. The distinction matters because — contrary to the “common law marriage” myth — unmarried partners have very limited claims on each other, so each document protects a different stage. Couples who cohabit and then marry often use both, in sequence.
Prenup vs cohabitation agreement: FAQs
Does a cohabitation agreement work after marriage?
Once you marry, marital law takes over and a prenup becomes the relevant document; a cohabitation agreement is not designed for married couples (see prenups when you already live together).
Is common law marriage real in the UK?
No — there is no such thing in England & Wales, however long you live together, so an agreement is the only way for unmarried partners to set out their rights.
Is a cohabitation agreement legally binding?
A properly drafted cohabitation agreement is generally treated as a binding contract, unlike a prenup, which is persuasive rather than automatically binding (see are prenups legally binding?).
We are engaged — do we need a cohabitation agreement or a prenup?
If you will marry, a prenup is the right document; a cohabitation agreement is for couples staying unmarried (see do you need a prenup?).
Can we upgrade our cohabitation agreement into a prenup?
You would make a fresh prenup rather than convert the old document, carrying over the intentions you agreed while cohabiting (see what to include).
Does having children change whether we need an agreement?
Having children does not create common law marriage or give unmarried partners new claims on each other, so an agreement is, if anything, more important; but it can never bind the court on child arrangements, which are decided on the children’s welfare (see prenups and children).
Should a cohabitation agreement include a declaration of trust?
They work well together: the declaration of trust fixes the ownership shares in your property, while the cohabitation agreement covers the wider household finances (see prenup vs declaration of trust).
Create your prenuptial agreement online
UK Prenup lets couples in England & Wales create a clear, fair prenuptial agreement online from £199, with your document generated instantly as a PDF. See how it works or get started.
UK Prenup is not a law firm and does not provide legal advice. A prenuptial agreement in England & Wales is not automatically binding, and both partners should take independent legal advice before signing.