It all started so well. Back in precredit-crunch 2006, my boyfriend, Andrew, and I eagerly bought a two-bedroom flat in leafy Epsom, Surrey, for £220,000. Smug to be first-time buyers while only in our twenties, and desperate to escape awful landlords, we were too happy to be ascending the first rung of the property ladder really to consider what we were buying... and how we were buying it.

We weren’t alone. Many other unmarried couples — certainly most of our cohabiting friends — were equally as keen to buy into a long-term mortgage commitment before house prices rose even more steeply. Asking prices for flats increased by nearly £10,000 in the three months from when we made an offer to moving in February 2007.

As an actuary, Andrew, then two years older than me at 27, brought far more cash as a deposit — £20,000 — to the table than I could, as a newly qualified journalist. Indeed, I could only contribute furniture and the potential to amass a similar sum in savings. The purchase was always unequal.

Before we signed anything, Andrew emitted a few masculine noises about drawing up a contract to establish our shares, but I felt slighted to think that I couldn’t be trusted to be placed on the deeds. So we were romantic. We scrapped the contract and bought the property as joint tenants, with a verbal agreement in place that I would take out only what I had put in, should we part ways. Then we got down to more important matters, debating what shade of yellow we would paint the living room.

Needless to say, the paint could have waited. Three years and a failed relationship later, it turns out that, for the unwedded, the stress of a joint mortgage has heralded a total shutdown of even the smallest domestic niceties.

“When moving in together, people think that love is the most important thing to focus on,” says Merryn Somerset Webb, author of Love Is Not Enough: A Smart Woman’s Guide to Making (& Keeping) Money. “But cohabitations break down, on average, after three years, so you need to have a financial agreement in place. This is not unromantic, it’s essential.”

We weren’t the first couple to baulk at spending up to £1,000 on such a depressing task. Richard Collins, a partner at Charles Russell LLP solicitors, says: “When it comes to signing a contract covering what happens if the relationship ends, buyers would rather spend the money on Ikea furniture.”

It sounds all too familiar. We couldn’t wait to get to our nearest branch to blow our leftover cash on sofas and bookcases. I remember feeling pleased to have rescued the money from our solicitor’s clutches. We became the typical suburban couple, spending our weekends walking on the Surrey Downs or shopping at Borough market, cockily congratulating ourselves as our flat rose in value.

Then the credit crunch started to bite, and falling house prices mirrored the dips in our relationship. Money worries mounted, driven by anxieties on both sides about what we would each get from a potential sale as the value of our home plummeted to less than £210,000. Matters came to a head when I was made redundant in June this year. The evenings were soon lost to flaming rows. At the point of separating in September, we had reached a pathetic stage where we couldn’t even agree on which bin to use... so we each had our own.

Yet an inability to agree on the exact shares we should take out, and our failure to find a solicitor outside London — we couldn’t face paying £400 an hour for a City firm, probably the amount our flat has gone down in value every week since mid-2007 — kept us together. Again, we weren’t alone. Collins has seen the number of such disputes double in the past year. “The worry of paying a mortgage in this climate puts an enormous strain on the best relationship.

“Couples often don’t understand the financial contributions they’ve made. If they’d opted for a ‘tenants in common’ agreement instead of a joint tenancy on purchase, where you take out what you put in, their financial entitlement when separating would be clear. This amount will change, though, if you’ve relied on a partner to pay the mortgage and bills after redundancy.”

For us, it could have been a lot worse. Looking back, I know that we weren’t clueless. We thought our verbal agreement would be sufficient, but it wasn’t. In the end, we somehow agreed on an amount for Andrew to buy me out without getting (expensive) lawyers in. The prospect of spending between £10,000 and £20,000 debating the settlement, delving deep into the minutiae of household expenses, including holidays and travel expenses in court, was enough to make us sort it out.

I calculated my contribution — mortgage payments over three years — and factored in other costs and general expenses. Thankfully, Andrew agreed with the £12,500 total. At least I’m walking away with something. Others may not be so lucky.

Make a commitment

Get everything in writing. Ask your solicitor to explain the implications of buying your home as joint tenants (equal shares) or tenants in common (specifying the size of each share).

Cohabitation agreements can be expensive, but More Than Legal Services offers a deal where a legally binding document can be drawn up online for less than £20.

If you’re in stalemate, try mediation (see lawsociety.org for details). This should cost a third less than a solicitor's fees, and establishes the idea that you will be fair with each other from the off.

Familiarise yourself with legislation that may come in in 2010. It will give unmarried couples the same rights as married ones if they have children or have lived together for two to five years.

Consider making a will. Joint tenants automatically get the other tenant’s share, but for tenants in common, this will go to their next of kin. If you’re not on the deeds, you’ll get nothing.

Times recommends

• Second homes: credit crunch casualty or money-spinner?
• How I built my own sliding house
• Top 10 incentives for house buyers