OSFI is setting a new minimum qualifying rate, or “stress test,” for uninsured mortgages (mortgage consumers with down payments 20% or greater than their home price).

The rules now require the minimum qualifying rate for uninsured mortgages to be the greater of the five-year benchmark rate published by the Bank of Canada (presently 4.89%) or 200 basis points above the mortgage holder’s contractual mortgage rate. “The main effect will be felt by first-time buyers,” says James Laird, co-founder of Ratehub.ca. “No matter how much money they put down as a down payment, they will have to pass the stress test.” The effect of the changes will be huge, resulting in a 20% decrease in affordability, meaning a first-time homebuyer will be able to buy 20% less house, explains Laird.

MoneySense asked Ratehub.ca to run the numbers on two likely scenarios and find out what it would mean for a family’s bottom line. Here’s what they found:

SCENARIO 1: Bank of Canada five-year benchmark qualifying rate

In this case, the family’s mortgage rate, plus 200 basis points, is less than the Bank of Canada five-year benchmark of 4.89%.

According to Ratehub.ca’s mortgage affordability calculator, a family with an annual income of $100,000 with a 20% down payment at a five-year fixed mortgage rate of 2.83% amortized over 25 years can currently afford a home worth $726,939.

Under new rules, they need to qualify at 4.89%

They can now afford $570,970

A difference of $155,969 (less 21.45%)

SCENARIO 2: 200 basis points above contractual rate

In this case, the family’s mortgage rate, plus 200 basis points, is greater than the Bank of Canada five-year benchmark of 4.89%.