This is the single most important mechanic in investment lending and almost nobody explains it, so here it is with numbers.
Take a rental renting for $2,000 a month, with a mortgage payment of $1,800, property tax of $250 a month and heat of $100. Total costs $2,150.
Method one: rental offset
The lender takes a percentage of the gross rent, commonly fifty to eighty percent, and subtracts it directly from the property's costs. Whatever remains is added to your debts.
At an eighty percent offset: eighty percent of $2,000 is $1,600. Costs of $2,150 minus $1,600 leaves $550. That $550 is added to your monthly obligations.
At a fifty percent offset: $1,000 against $2,150 leaves $1,150 added to your debts. More than double the drag from the same property.
The percentage used is the whole game, and it varies by lender from fifty to as much as one hundred percent.
Method two: add to income
The lender takes a percentage of the rent, adds it to your gross income, and then counts the full property costs among your debts.
At fifty percent: $1,000 added to income, and the full $2,150 added to debts.
This sounds similar and behaves quite differently, because your income sits in the denominator of the ratio calculation while debts sit in the numerator. Adding to income inflates both sides of the equation, which dilutes the benefit. On most files the offset method produces a materially better outcome.
Why it matters so much
In the example above, an eighty percent offset adds $550 to your monthly obligations. A fifty percent add-to-income treatment effectively adds far more once the ratio arithmetic runs through.
At the qualifying rate, several hundred dollars a month of difference in how one property is treated is worth roughly eighty to ninety thousand dollars of borrowing capacity. On a portfolio of three properties, the choice of lender is not a detail. It is whether you can buy a fourth.
This is the strongest single argument for using a broker on investment files. Nobody at a branch is going to tell you that the lender down the road uses a more favourable method, because they do not know and it is not their job.
Which rent figure gets used
Not necessarily the one on your lease.
Most lenders require an appraisal that includes a market rent assessment, and many will use the lesser of your actual rent and the appraiser's market figure. So a property rented above market to a good long-term tenant may be counted at the lower market rate.
Conversely, a vacant property being purchased can often be qualified on the appraiser's market rent estimate, which means you do not need a tenant in place to buy.
Some lenders apply a vacancy allowance on top, reducing the usable figure further. Ask which figure a lender uses before assuming your lease governs.
What documents prove the income
For a property you are buying: the appraisal with market rent, and any existing lease.
For properties you already own, lenders want the T776 Statement of Real Estate Rentals from your tax return, usually for two years. That is the figure they trust, and it creates a familiar tension: expenses claimed to reduce tax also reduce the income a lender will recognise. Capital cost allowance can typically be added back, since it is depreciation rather than cash spent.
Leases and bank statements support the picture but the tax return is what carries weight.
Suites, permitted and otherwise
Income from a secondary suite is only counted where the suite is legal and permitted. An unpermitted basement suite generally contributes nothing, however reliably it rents, and some lenders view the property less favourably for having one.
Where you are buying a home with a suite that you will occupy, some lenders will count the suite income toward qualifying, which can materially improve affordability. That is a different and often better position than buying a pure rental. See Edmonton rental financing for how this plays locally.
Vacancy, and what lenders assume
Most lenders build a vacancy assumption into their treatment, either by using a lower percentage of rent or by applying an explicit allowance. That is why an eighty percent offset is common rather than a hundred percent one: the twenty percent covers vacancy, management and maintenance.
It is worth applying the same discipline to your own numbers. A property modelled at twelve months of rent every year will disappoint. For a fuller picture of Alberta vacancy risk, see investing in Alberta.
The practical consequence
Before you make an offer on a rental, establish which lenders use a generous offset and whether your file suits them. That sequencing matters, because a property that works at an eighty percent offset may be declined outright at fifty.
Once you own more than one, the arithmetic compounds and the lender choice becomes the binding constraint on growth. See financing multiple rentals.