When the basement suite is legal, a lender may use part of the rent so the file carries more house than salary alone. You may qualify subject to lender and insurer approval. A suite that is not legal is not treated as income.
A first home in the GTA can be a house you live in with a self-contained basement suite. When the unit is legal, or the program accepts projected rent, lenders may use that rent so the file qualifies for more house than salary alone supports. Two methods show up again and again. A rental offset subtracts a percentage of the rent from the housing cost. The other method adds a percentage of the rent to your income. The percentage is commonly anywhere from 50% to 100%, depending on the lender and the program.
CMHC's published approach for an insured, owner-occupied two-unit home adds up to 100% of the gross rent to income, and it may leave property tax and heat out of the ratio. A three- or four-unit owner-occupied home is commonly capped lower, often at 50%. The appraiser's market rent schedule, or the lease if a tenant is already there, is what the lender reads. Many files use the lower of the two.
Ask the city whether the unit is a legal additional residential unit before you treat the rent as income. Ontario's Planning Act requires municipalities to allow up to three residential units on many fully serviced urban lots, but a permit and a final inspection are still what make a basement legal. Tell your insurer you will have a tenant, and plan on landlord obligations under the Residential Tenancies Act. Rent that arrives reduces what you pay out of pocket each month. It does not erase the mortgage payment in a vacant month.
The worked comparison — qualification with the rent and without it, plus the net monthly carry — is in Buying a GTA home with a separate-entrance basement. The figures there use an example rate for illustration only. If you will not live in the house, the rent rules change; that file starts on investment property.