Cash Flow 101: What First-Time GTHA Investors Should Underwrite Before Making an Offer

Most first-time investors underwrite a rental purchase using one number: does the rent cover the mortgage payment? That’s a start, but it misses most of what actually determines whether a property is a good investment. Here is the fuller picture, built on real comparable data — not a listing agent’s optimistic rent estimate.

Start With Real Comparable Rents, Not the Listing Sheet

The single biggest source of bad underwriting is an inflated rent estimate. A listing that assumes top-of-market rent with zero turnover and zero vacancy is not a forecast — it’s a best case. Before you underwrite anything, pull actual comparable rentals in the immediate area (same building type, similar condition, similar unit size) and use a realistic, defensible number, not the highest figure you can find.

Cap Rate vs. Cash-on-Cash: Two Different Questions

These two numbers get used interchangeably and shouldn’t be. Cap rate (net operating income ÷ purchase price) tells you the return as if you paid all cash — it ignores your mortgage entirely. Cash-on-cash return tells you the actual return on the money you put down, after mortgage payments: annual pre-tax cash flow ÷ total cash invested (down payment plus closing costs). On a leveraged residential purchase, cash-on-cash is almost always the more relevant number, because it reflects how the deal actually performs for you.

Build the Operating Expense Ratio Honestly

New investors consistently underestimate operating expenses. A realistic budget for a small residential income property typically includes:

• Property tax — confirm the actual current assessment, not an estimate
• Insurance — landlord/rental-specific policy, not a standard homeowner policy
• Vacancy allowance — typically 4–8% of gross rent even in a strong market, to cover turnover gaps
• Repairs and maintenance — a realistic reserve, not just what came up last year
• Property management — even if you plan to self-manage, model it at 8–10% so you know the number if your circumstances change
• Utilities — whatever the landlord is responsible for under the specific unit’s arrangement
• Condo/HOA fees — if applicable, and confirm whether a special assessment is pending

A common rule of thumb is that operating expenses (excluding mortgage) run 35–50% of gross rental income on a typical residential income property — higher for older buildings, lower for newer, more efficient ones. If your spreadsheet has expenses under 25%, something is being missed.

A Simple Worked Example

That property cash flows — but only modestly, and only if the vacancy and maintenance assumptions hold. This is exactly the kind of property where a single unexpected repair or a longer-than-expected vacancy between tenants can turn a “cash-flowing” property into a break-even or negative one for several months.

Don’t Forget the Stress Test

Canadian mortgage rules require lenders to qualify borrowers at a stress-test rate above their actual contract rate. This affects how much you can borrow, not your actual monthly payment — but it’s worth modelling your cash flow at a modestly higher rate than your current contract rate too, so a renewal in a higher-rate environment doesn’t come as a surprise.

Red Flags Worth Slowing Down For

• A rent roll that only shows one or two months of history
• A seller unwilling to share utility bills or recent repair invoices
• An asking price based on “what similar units sold for” with no reference to actual achievable rent
• An older building with no visibility into roof, furnace, or plumbing age