Couples who live together in Ontario often assume that a cohabitation agreement is a lesser version of a prenup — a placeholder that expires at the altar. The opposite is closer to the truth.

Section 53(2) of the Family Law Act provides that if the parties to a cohabitation agreement marry each other, the agreement is deemed to be a marriage contract. The document does not lapse. It does not need to be re-signed. It converts automatically and carries on governing your affairs.

That is convenient. It is also the source of most of the trouble, because the legal landscape the agreement now operates in is meaningfully different from the one it was drafted for.

The Two Regimes Are Not the Same

Common-law partners in Ontario do not have property rights against each other by virtue of the relationship. There is no equalization of net family property for unmarried couples, however long they have lived together. If your name is not on the asset, your claim depends on trust principles — resulting trust, or an unjust enrichment claim seeking a constructive trust or monetary award — and those claims are fact-heavy, expensive, and uncertain.

Married spouses are in an entirely different position. On separation, Part I of the Act triggers an equalization of net family property: each spouse calculates the growth in their net worth over the course of the marriage, and the spouse with the greater growth pays half the difference to the other. It is a formula, not a discretion.

Married spouses also get the matrimonial home rules in Part II, which have no equivalent for unmarried couples. The home each spouse ordinarily occupied at separation receives special treatment: its full value on the separation date goes into the equalization calculation with no deduction for what it was worth on the wedding day, and the date-of-marriage deduction and most exclusions do not apply to it. Both spouses have an equal right to possession regardless of title, and neither can sell or encumber it without the other's consent or a court order.

Support is the one area that looks similar on the surface. Unmarried partners who have cohabited continuously for at least three years, or who are in a relationship of some permanence and have a child together, can claim spousal support under Part III. Married spouses can claim from the date of marriage. But the entitlement analysis on a fifteen-year marriage looks different from the analysis on a three-year cohabitation, and an agreement that made sense for one may sit awkwardly on the other.

Why the Conversion Matters in Practice

A cohabitation agreement drafted for two people with modest assets, no children, and no shared home is often a straightforward document. It confirms that each partner keeps what they own, addresses joint expenses, and perhaps deals with support.

Convert that same document to a marriage contract five years later, after the couple has bought a house together and one partner has left the workforce, and its silences start to matter:

  • Does it address the matrimonial home? Almost certainly not, because there was no matrimonial home when it was written — and the concept did not apply to the parties in any event.
  • Does it address the date-of-marriage deduction, which now becomes the anchor for the entire equalization calculation? Probably not.
  • Does the support term reflect a relationship in which one partner's earning capacity has been affected by choices the couple made together? Often not.
"The agreement remains valid and enforceable. It simply may not do what the parties would now want it to do."

The Disclosure Problem Is Quieter but Real

Section 56(4) allows a court to set aside a domestic contract where a party failed to disclose significant assets, debts, or liabilities existing when the contract was made. The relevant date is when the contract was made — so for a converted cohabitation agreement, that is the original signing date, not the wedding.

This cuts both ways. It means disclosure that was adequate at the time is not retroactively deficient because circumstances changed. But it also means the parties are now married under an agreement built on a snapshot of finances that may be years out of date, and neither of them has had a fresh opportunity to assess whether they would still make the same bargain.

What to Do About It

The straightforward answer is to review the agreement before you marry, and to amend or replace it rather than letting it convert by default.

An amending agreement can address the marriage-specific gaps directly: how the matrimonial home will be treated, what date-of-marriage values apply, whether existing exclusions such as gifts and inheritances are preserved, and whether the support provisions still reflect the parties' intentions. It should confirm updated financial disclosure and be supported by fresh independent legal advice for both parties. This is a shorter and cheaper exercise than drafting from scratch, because the architecture already exists.

Where the original agreement is thin, badly drafted, or was signed without independent legal advice, replacement is usually the better route. A new marriage contract signed well before the wedding, with proper disclosure and advice on both sides, removes the enforceability question rather than layering a good document on top of a shaky one.

If You Are Starting from Nothing

Couples who are living together and expect to marry eventually sometimes ask whether to do a cohabitation agreement now or wait for a marriage contract later.

There is a reasonable case for doing it now. The conversation is easier when there is no wedding date creating pressure, the disclosure exercise is simpler while the finances are less entangled, and section 53(2) means the work is not wasted. The agreement should be drafted with the conversion in mind — written to function properly as a marriage contract from the day the parties marry, rather than as a document that will need rescuing later.

That is a drafting choice, and it costs nothing extra to make it at the outset.

This post is general information about Ontario law and is not legal advice. Every situation is different. If you have a cohabitation agreement and are planning to marry, speak with a lawyer about reviewing it.