The estimated monthly house payment for a median-priced three-bedroom home purchased in the fourth quarter of 2013 — and that includes mortgage, insurance, taxes, maintenance, and subtracting the estimated income tax benefit — increased an average of 21% from a year ago in the 325 U.S. counties included in an analysis by RealtyTrac.

The rise in monthly housing payments was the result of an average 10% rise in median prices across the 325 counties, combined with a 33% increase in the average interest rate for a 30-year fixed rate mortgage as reported by Freddie Mac in its Primary Mortgage Market Survey.

“A potent combination of rapidly rising home prices and the often-overlooked but significant uptick in interest rates in the second half of 2013 caused the monthly cost of owning a home using traditional financing to jump substantially in many markets over the last year,” said Daren Blomquist, vice president at RealtyTrac. “The monthly cost of owning a home is still less than renting in the majority of markets, but the cost of financed homeownership is becoming dangerously disconnected with still-stagnant median incomes, driven not by shoddy underwriting practices this time around but by investors and other cash buyers who are not tethered to the typical affordability constraints.”

The average house payment was based on a 30-year fixed rate mortgage with an interest rate of 4.46% and a 20% down payment. With that calculation, the average monthly house payment across all counties for three-bedroom homes bought in the fourth quarter was $865, up from $714 a year ago, when the interest rate was calculated at 3.35%.

Even with the increase, the average house payment was lower than the average fair market rent for about 80% of the population in the analyzed counties. Not surprisingly, the 20% of the population who were paying less in house payments than they would in fair market rent were in some of the country's most heated rental markets around major metros, including San Francisco County, Orange County, Suffolk County and Westchester County.

Among the 15 most-populated counties analyzed, the estimated monthly house payment increased an average of 34% from a year ago, making the house payments higher than the average fair market rent for a three-bedroom home in six of those 15 largest counties. A year ago only one of those 15 counties — Santa Clara County in the San Jose area of Northern California — had an estimated monthly house payment above the average fair market rent.

Another finding of the analysis was the increase in the minimum qualifying income needed to buy the homes at the increased price. Across all 325 counties, the average minimum household income needed to qualify for a median priced home jumped from $34,262 in the fourth quarter of 2012 to $41,544 in 2013. This qualifying income was based on no more than 25% of household income going to the monthly house payment. 

In some counties, of course, the minimum income necessary was much higher, including $228,569 in San Francisco County and $177,992 in San Mateo County, both in California.

"One simply needs to look at the minimum income needed to qualify for a median-priced home in some markets to realize the extent of the disconnect between prices and incomes," Blomquist said. "For example, in Los Angeles County, the minimum qualifying income needed to purchase a median-priced home is at more than $95,000, up from about $68,000 just a year ago."