What You Need To Know About Co-Signing A Mortgage

Tim Lyon • May 16, 2024

So you’re thinking about co-signing on a mortgage? Great, let’s talk about what that looks like. Although it’s nice to be in a position to help someone qualify for a mortgage, it’s not a decision that you should make lightly. Co-signing a mortgage could have a significant impact on your financial future. Here are some things to consider.


You’re fully responsible for the mortgage.


Regardless if you’re the principal borrower, co-borrower, or co-signor, if your name is on the mortgage, you are 100% responsible for the debt of the mortgage. Although the term co-signor makes it sound like you’re somehow removed from the actual mortgage, you have all the same legal obligations as everyone else on the mortgage. When you co-sign for a mortgage, you guarantee that the mortgage payments will be made, even if you aren’t the one making them.


So, if the primary applicant cannot make the payments for whatever reason, you’ll be expected to make them on their behalf. If payments aren’t made, and the mortgage goes into default, the lender will take legal action. This could negatively impact your credit score. So it’s an excellent idea to make sure you trust the primary applicant or have a way to monitor that payments are, in fact, being made so that you don’t end up in a bad financial situation.

 

You’re on the mortgage until they can qualify to remove you.


Once the initial mortgage term has been completed, you won’t be automatically removed from the mortgage. The primary applicant will have to make a new application in their own name and qualify for the mortgage on their own merit. If they don’t qualify, you’ll be kept on the mortgage for the next term.


So before co-signing, it’s a good idea to discuss how long you can expect your name will be on the mortgage. Having a clear and open conversation with the primary applicant and your independent mortgage professional will help outline expectations.


Co-signing a mortgage impacts your debt service ratio.


When you co-sign for a mortgage, all of the debt of the co-signed mortgage is counted in your debt service ratios. This means that if you’re looking to qualify for another mortgage in the future, you’ll have to include the payments of the co-signed mortgage in those calculations, even though you aren’t the one making the payments directly.


As this could significantly impact the amount you could borrow in the future, before you co-sign a mortgage, you’ll want to assess your financial future and decide if co-signing makes sense.


Co-signing a mortgage means helping someone get ahead.


While there are certainly things to consider when agreeing to co-sign on a mortgage application, chances are, by being a co-signor, you'll be helping someone you care for get ahead in life. The key to co-signing well is to outline expectations and over-communicate through the mortgage process.


If you have any questions about co-signing on a mortgage or about the mortgage application process in general, please connect anytime. It would be a pleasure to work with you.


Tim Lyon

Mortgage Consultant

25 Year vs 30 Year Amortization: A Practical Guide

By Tim Lyon • January 25, 2026
Trying to choose between a 25 and 30 year mortgage amortization? Learn how each affects your payments, interest, and flexibility so you can decide with confidence.

How Rental Income Impacts Your Mortgage Approval (And Why Lender Choice Matters)

By Tim Lyon • January 20, 2026
If you are buying a home with a suite, keeping your current home as a rental, or already own a rental property, mortgage qualification can get confusing fast. The frustrating part is that you can do everything “right” and still get very different answers depending on which lender you talk to. Here’s a simple breakdown so you understand it and don’t miss out. What are Debt Service Ratios? In Canada, lenders qualify you using two main ratios: Gross Debt Service (GDS) This looks at housing costs only , typically: Mortgage payment Property taxes Heating 50% of strata fees (if applicable) GDS typically needs to be 39% or less of your gross income. Total Debt Service (TDS) This includes everything in GDS , plus other debts like: Car loans Credit cards Lines of credit Student loans TDS typically needs to be 44% or less of your gross income. These ratios are the foundation. If they do not work, the lender will not approve the mortgage, even with strong credit and a solid down payment. How Lenders Treat Rental Income Most people assume lenders look at rental properties based on simple cash flow (rent minus mortgage payment). In reality, most lenders use one of two methods: 1) Addback A percentage of the rental income is added to your gross income for qualification purposes. 2)Offset A percentage of the rental income is subtracted from the mortgage payment tied to the rental property. Different lenders use different percentages and different worksheets. That is why the same borrower can qualify with one lender and fail with another. Benefits of Understanding Lender Methods When you understand how rental income is calculated, you can: Avoid being under-qualified by a lender with conservative rules Get a more accurate picture of your real purchasing power Choose a lender that fits your situation (instead of forcing your situation to fit the lender) Important Considerations A few key points to keep in mind: Rental income is rarely counted at 100% , but some lenders are more generous than others. The method matters just as much as the percentage (addback vs offset). If you own multiple properties, lender worksheets can change the result dramatically. Your lender choice is a strategy decision , not just a rate decision. Real-World Example: Same Clients, Two Very Different Outcomes Here’s an example comparing lenders Scotiabank and Strive, using a fictitious couple: Scenario Household income: $160,000 Existing townhome: $800,000 value with a $525,000 mortgage ( $2,500/month payment) Market rent for the townhome: $3,400/month New purchase: property with a rental suite generating $1,800/month Down payment: 10% Other debts: student loan $165/month , car loan $500/month How Scotiabank viewed it For the townhome rental, they counted half the rent and subtracted the mortgage payment, leaving an $800/month shortfall that gets added into the debt ratios. For the new purchase, 50% of the suite income gets added to income. Max mortgage : $650,700 Max purchase price : $723,000 How Strive viewed it For the townhome rental, Strive used a rental worksheet and calculated $5.20/month of income that can be added to the application. For the new purchase, 100% of the suite income gets added to income, and they did not need to include taxes or heat. Max mortgage : $878,400 Max purchase price : $976,000 The result That’s a $253,000 difference in purchasing power , with the same clients, same income, same debts, and same properties. The difference was lender policy. Quick Summary GDS and TDS ratios are the backbone of mortgage qualification. Rental income is usually counted using Addback or Offset , and each lender handles this differently. Two lenders can produce wildly different results, even with the exact same file. In the example above, lender choice created a $253,000 swing in purchasing power. Next Steps If you are planning to: Buy a home with a suite Keep your current home and convert it to a rental Use rental income to qualify Reach out and I will run the numbers across multiple lenders so you see what you actually qualify for, not just what one lender will allow. Need help with your mortgage? Book a consultation or call 778-988-8409 . Glossary Addback : A method where a lender adds a percentage of rental income to your gross income for qualification. Gross Debt Service (GDS) : The ratio that measures housing costs as a percentage of gross income. Offset : A method where a lender subtracts a percentage of rental income from the rental property’s mortgage payment for qualification. Total Debt Service (TDS) : The ratio that measures housing costs plus other debts as a percentage of gross income.