Where it comes from
US mortgage underwriting standardised two ratios decades ago: housing costs at or under 28% of gross monthly income (the front-end ratio) and all debt service under 36% (the back-end). The numbers were chosen as default-risk thresholds from lending data — levels above which borrowers started missing payments in meaningful numbers. As lender arithmetic, they work: banks that follow them lose less money.
The gross-income trick
Notice whose risk the ratio prices: the bank's. A default is a bank problem; a household that makes every payment and cannot afford a holiday, a repair or a resignation is not. Reading 28% against gross income means a household at a 30% effective tax rate is actually committing 40% of the money that exists — which is why bank pre-approvals so often exceed what feels liveable by a wide margin.
The honest version keeps the ratio and fixes the base: 28% of net income, with the full carry counted — payment, property tax, insurance, upkeep — not the payment alone. That single substitution turns a lender's guardrail into a household's.
The stress clause
A ratio met at today's rate is a photograph, not a plan. Rates reset — at renewal, on variable products, or simply when you next move — and a purchase that only clears 28% at signing has borrowed its affordability. The house convention stresses the rate two points before believing any answer; several regulators (Canada's B-20 stress test, historically the UK's affordability rules) institutionalised the same instinct.
The house position
The Home Affordability instrument runs the honest version: 28% against net, full carry, stressed two points, worked backwards to a purchase price. Pair it with Cash or Mortgage once the price is known, and treat the gap between the bank's pre-approval and this page's figure as what it is — the room your life was going to live in.