Nowhere else in the country has home affordability gotten so out of whack than along the northern Front Range, according to a housing report released Thursday.

Of the 12 counties in the U.S. where relative home affordability is at its lowest level, seven are in northern Colorado, according to an affordability index that ATTOM Data Solutions compiles every quarter. The counties topping the list of unaffordable homes: Adams, Arapahoe, Denver and Weld.

Of the 379 counties examined, Jefferson County ranked seventh, Larimer County 10th and Boulder County 12th.

“Denver is experiencing the growing pains of transitioning from a typical Midwestern, meat-and-potatoes housing market to a West Coast-style trophy market attracting buyers across the country and the world,” said Daren Blomquist, a senior vice president at ATTOM.

Every quarter, ATTOM looks at the median prices of homes sold in hundreds of counties and compares that to average annual wages to determine how much income a mortgage and related payments would consume, given current interest rates.

It then compares the current housing burden to the historical burden through an affordability index. Nationally, the home affordability index is at 103, near the historical average of 100. A typical buyer needs to devote about a third of their average income toward a home payment, given current prices and interest rates.

But in about a quarter of the counties surveyed, home payments are now a heavier burden on incomes than is usually the case. In those counties, buyers must allocate 43 percent of their incomes to housing, at the upper limit of what banks are willing to lend (assuming no large debts such as student loans or auto loans are in the mix).

And then there is the northern Front Range, where housing affordability is diverging from the norm in a way not seen since the housing boom a decade ago.

What the ATTOM index tries to discern is how affordability changes over time, not absolute affordability. Buying the median-priced home in Manhattan, San Francisco or Maui will require every penny of the average annual wage and then some. Only the well-off need apply.

But an expensive market staying expensive or a cheap market staying cheap isn’t as disruptive as a once-affordable market becoming much more expensive.

Some of the most affordable housing markets in the region — Adams, Arapahoe and Weld counties — are diverging the most from their historic norms for affordability, even though they still look like bargains to someone moving from Boulder or Aspen.

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Historically, buyers earning the average wage in Adams County had to commit 35.9 percent of their paychecks to afford payments on a median-priced home. Now they must commit 45.4 percent. In Arapahoe County, the share of income burden is 38.9 percent, up from the average of 31.7 percent, while in Denver it has hit 42.5 percent, versus the historical 34.7 percent.

Burdens grow heavier to the north. In Weld County, the share of average annual wages that must be spent for a median-price home has risen from 39.6 percent to 48.3 percent. In Larimer County, it has moved from 46 percent to 54.4 percent.

“What we are seeing is home prices rising far beyond the pace of wage growth in Colorado. Many of these counties were already deemed unaffordable, historically,” said Emily Nilsen, a spokeswoman for Housing Colorado, an advocate for affordable housing.

With the exception of Denver, wages across the region have risen a respectable 5 to 7 percent over the past year. The disconnect is that home prices the past five years have risen 11 percent a year on average.

For example, the median price of a home sold in Adams County dropped from $184,000 in the summer of 2006, the last market peak, to $145,000 in late 2011. But in the first quarter of this year, the median was up to $290,000, a doubling in a little over five years, according to ATTOM’s numbers.

Does that suggest home prices are in a bubble and a damaging price correction is ahead? Blomquist notes that two key ingredients from the last housing bust are missing: loose lending standards and excessive new-home construction.

“I don’t see any signs of an imminent correction in prices or spike in foreclosures mostly because lending standards are still quite rigid,” he said. But those living in stretched markets can expect more volatility in home prices, a pattern seen in California.

One emerging trend that supports a transition to a “trophy” market is an increase in “sharing buyers,” where multiple parties show up on the deed, something more common in the most expensive markets.

“The latest data we have on this, from last year, shows that 21 percent of buyers in the city of Denver were these ‘sharing buyers,’ above the national average of 16 percent,” Blomquist said.

While Denver has a long way to go to reach the 44 percent rate of sharing buyers seen in San Jose, Calif., or the 34 percent in San Francisco, its 152 percent increase in sharing buyers was off the charts, Blomquist said.

Some of those shared buyers are parents helping their adult children. Others are millennials — a generation known for a propensity to share — who are banding together with friends to acquire homes they otherwise couldn’t afford.