Three definitions of "housing cost," and the household pays all three. The lender's ratio counts the mortgage payment, property taxes, insurance and association dues, and it counts nothing else about running a home. HUD's cost burden measure, as applied by the Census Bureau, is broader on one dimension and includes utilities alongside mortgage principal and interest, real estate taxes, homeowner's insurance and condominium fees. Neither includes maintenance and repairs, which on an owned home are not optional and do not arrive on a schedule. So a household can pass the lender's test, fail HUD's, and still be describing neither number when it says it cannot afford anything.
Gross income is the quiet distortion, and it runs through both formal measures. A lender's debt-to-income ratio uses income before tax and withholding, and HUD's cost burden measure uses income as well rather than take-home pay. A household with meaningful tax withholding, retirement contributions and payroll deductions can be spending a third of gross income on the house and half of what actually reaches its account. Neither measure is wrong for its purpose; both are describing a different quantity from the one the household experiences. The one number a household can compute for itself, and the one that matches its lived position, is the housing payment divided by take-home pay.
The cost of ownership does not stop at the payment, and the extras arrive unevenly. Maintenance, repairs and eventual replacement of major systems are real annual costs that a renter does not carry and that no underwriting ratio counts. Property taxes and insurance premiums also rise on their own schedule, and because most borrowers pay them through an escrow account, the increase arrives as a change to the monthly payment rather than as a bill, sometimes with a shortage to make up on top. A household that stretched to the maximum at closing has no room for that adjustment, and the adjustment is not a rare event.
The damage is usually invisible rather than dramatic. Foreclosure is the outcome people picture, and it is the least common one. What normally happens is that the payment gets made every month and everything else stops: retirement contributions are reduced or suspended, the emergency fund is never built or is spent and not replaced, ordinary maintenance is deferred until it becomes an expensive repair, and any surprise goes onto a credit card. The household looks stable from outside and has no capacity to absorb a shock, and the opportunity cost compounds quietly for as long as the situation lasts.
Getting out is harder than getting in, which is the argument for taking the question seriously at the start. Selling a house has transaction costs on both sides and takes months. Refinancing helps only if rates or the balance cooperate, and lowering a payment by extending the term raises the total interest paid. Renting a room, appealing a property tax assessment, shopping the homeowners insurance, and dropping mortgage insurance once the loan-to-value threshold is reached are the levers that exist without moving, and they are all small relative to the payment itself. The asymmetry between how easily a household can commit and how slowly it can retreat is the reason the number to test before signing is the one measured against take-home pay.