Rental income offset methods, compared: add-back, net-income, and offset, side by side.

For an insured mortgage where the rental unit is part of the property being financed, CMHC's published guidance allows up to 50% of the gross rental income to be added to the borrower's gross annual income, with property taxes and heat then excluded from the GDS/TDS calculation for that unit. For a two-unit, owner-occupied property with a secondary suite specifically, CMHC allows up to 100% of the secondary suite's gross rental income to be added instead of the standard 50%.

The mechanics matter: this is an addition to income, calculated on the gross rent, not a netting of the rental unit's own carrying costs against the rent it generates. A borrower with a legal basement suite renting for $1,800 a month sees up to $900 added to their monthly qualifying income under the 50% rule — or up to the full $1,800 if it's a secondary suite on a two-unit owner-occupied property.

When the rental property is a separate asset — not the one securing the mortgage insurance application — CMHC's guidance shifts to a net rental income approach: net rental income from that other property can form part of the borrower's gross annual income, rather than the gross add-back used for a subject-property suite.

That distinction — subject property vs. a separate investment property — is the single most important fork in this whole topic, because it determines which of the two methods even applies before any percentage gets discussed. A file with both a subject-property suite and a separate rental property can see both methods used side by side on the same application.

Uninsured lenders — 20% or more down, or any file above the $1.5 million insured price cap — aren't bound by CMHC's published add-back method and commonly set their own policy. The most common alternative in the Canadian market is an offset approach: the rental property's own carrying costs (principal, interest, taxes, heat — sometimes condo fees) are measured against the rental income first, and only the net shortfall or surplus flows into the borrower's ratios, rather than a flat percentage of gross rent being added to income.

Some uninsured lenders instead run a variation closer to CMHC's add-back model, applying their own percentage — which may be higher or lower than 50% — to gross rent. Because this varies by lender and by program rather than following one published national standard, the specific percentage or offset formula a given uninsured lender applies should be confirmed directly with that lender before it's built into a client conversation, not assumed from a prior file.

• →Add-back vs. offset changes the math structurally, not just the number — an add-back increases income directly, while an offset can reduce or increase the property's net contribution to the ratio depending on whether the rent covers the carrying costs.
• →The percentage applied to gross rent varies even among lenders using a similar add-back structure to CMHC's.
• →Documentation standards differ — a signed lease, an appraiser's fair-market-rent opinion, and a prior year's rental income reported on a tax return can each be weighted differently depending on the lender and program.

This is exactly the kind of file-specific variation covered in our broader guide to underwriting rental and investment property files properly — worth a full read before quoting a client a number based on last file's method.

Whichever method applies, the income still has to be evidenced the same way any other income is: a signed lease agreement, an appraiser's opinion of fair market rent where no lease exists yet, or a prior tax return showing the rental income already being declared. A rental figure that isn't backed by one of these tends to come back as a condition regardless of which offset method the lender uses.

Getting the method and the documentation right before submission — rather than after a condition comes back — is exactly the discipline our GDS/TDS reference on the ratio side of a rental file works through with three complete worked files.