Co-signing a family mortgage could sink your own homebuying power
When you co-sign, the whole payment lands on your file — and it can cut what you qualify for by more than half. Here's the math nobody runs before signing.
It usually starts as a favour. A parent can't qualify on their own anymore, or a sibling needs a stronger file to get approved, and you have good income and clean credit. You sign. Nothing changes day to day — they make the payments, you never see a bill.
Except something big did change: as far as any lender is concerned, that entire mortgage payment is now yours. Co-signing doesn't split the obligation in half. You're on the hook for 100% of it, and 100% of it counts against you the day you apply for your own mortgage.
The math before and after
Take someone with a $100,000 household income and $50,000 saved for a down payment, at a 4.5% contract rate.
- Before co-signing: they qualify for a home around $456,000 — right in reach of a typical Toronto condo.
- After co-signing a mortgage with a ~$2,500 monthly payment: their maximum drops to about $179,000.
That's a 60% cut to their buying power — from a real condo to essentially nothing on the Toronto market — without borrowing a dollar for themselves. The co-signed payment eats the room in their debt-service ratios that their own mortgage was going to use. You can run your own before-and-after with the affordability calculator: just add the co-signed payment as a monthly debt and watch the number move.
Lenders stress-test you at roughly two points above your contract rate, so the qualifying payment is even bigger than the one your family member actually sends the bank each month.
For scale: a $100,000 household income sits near the 55th percentile in Ontario — a middle-class family, not someone with slack to absorb this. Check where any income lands with the income percentile tool.
"But I'm not the one paying it"
Doesn't matter. Qualification math doesn't care who writes the cheque — it cares who's liable. And if your family member misses payments, you don't just inherit the debt on paper; the missed payments hit your credit report too.
There's a stress-test question worth asking before you sign: could I carry this payment myself if I had to? A $2,500 payment against a $100,000 income is a 30% housing burden — already in "stretched" territory by the standard bands, and that's before your own rent or mortgage. Try your own numbers in the house-poor calculator.
What to do instead (or at least first)
- Run the numbers before the family conversation. Know exactly what the co-signed payment does to your own qualification, so "yes" is an informed yes.
- Ask about the exit. Lenders don't remove co-signers easily. The realistic paths are a refinance in the borrower's name alone or a sale — if they couldn't qualify solo today, ask what changes that.
- Consider a smaller role. A gifted down payment, help with a few payments, or a formal loan may solve the problem without tying your entire borrowing power to someone else's house for 25 years.
- Time it around your own plans. If you expect to buy within five years, understand you may be choosing between their approval and yours.
The takeaway
Co-signing is often framed as a signature, not a sacrifice. The math says otherwise: a typical co-signed payment can take a $456,000 budget down to $179,000 — the difference between owning in this market and being locked out of it. Help family if you want to. Just run the before-and-after first, so everyone at the table knows what the favour actually costs.
Figures delivered by Metrestick. Underlying data: Canadian mortgage qualification rules (GDS/TDS stress test) + 2026 CRA tax parameters (Open Government Licence – Canada), 2026; income percentiles from Statistics Canada Table 11-10-0192-01 (Canadian Income Survey, 2024).