A national analysis of rents and wages shows the New York City region as more affordable than the statistical area including Santa Rosa, part of a broader analysis that provides new perspective on the way national and North Bay rents are impacting the average household.

When compared to all areas across the U.S., Santa Rosa ranked ninth among areas where median rental prices take the largest share of median income. Six California cities required an even larger portion of average household wages for rent, but the share of income required for many Santa Rosa renters has seen greater increases over the past three decades than even San Francisco -- the sixth least affordable region in the U.S.

Los Angeles was ranked No. 1 on the list of least affordable.

While increasing rents have been widely documented across the North Bay, the analysis by Seattle-based online real estate market developer Zillow provides a new picture for how those expenses have tracked with average wages.

"Rents have gone up steadily. It's almost like the player in the shadows -- it has been chugging along quietly," said Svenja Gudell, director of economic research at Zillow. "A key component has been incomes. They haven't really increased for 10, sometimes 20 years."

The analysis differs from more commonly discussed trends involving average cost of rent, measures that make Santa Rosa a relative bargain to cities like San Francisco. Zillow began incorporating median income as part of its regular analysis this year, a measurement that illustrates how rents have tracked with the average household compensation, Ms. Gudell said.

Median-income households in the Santa Rosa metropolitan statistical area, which includes Petaluma, spent an average 39.8 percent of their monthly income on rent at the end of 2013. That rate represented a 49.1 percent increase compared to the Zillow benchmark for average share of income used for rent in the period between 1985 and 2000, beating the rate of increase for the high-rent stalwart of San Francisco over the same period by 3.7 percent.

Median-income renters in the Napa region spent an average 38.5 percent of their household income on rent, up 42.6 percent from the average share used for rent between 1985 and 2000. Similar data was not available for elsewhere in the North Bay.

In the broader New York City area, which also includes some other parts of the state and of New Jersey, for comparison, median-income households spent an average 39.5 percent of their monthly income on rent. In San Francisco, median-income households spent an average 40.7 percent.

The trend is not limited to California and other high-cost pockets around the country: the average proportion of payments was 29.6 percent across the United States, representing a 24.9 percent increase over the same period.

While Ms. Gudell noted that the individual economic trends within some statistical areas and the varying nature of their borders made more minute analysis difficult, the overall picture on a national and regional scale clearly points toward increasing expenses.

"Everyone kind of goes for that '30 percent rule,'" said Ms. Gudell, noting a widely cited benchmark for sustainable expenditures on rent or mortgage costs. "Looking at our data, it might be a good time for people to rethink that."

Sonoma County rents for all types of units have steadily increased to an average $1,421 per month as of March 31, up 15.2 percent since the first quarter of 2012, according to Novato-based rental data tracking firm RealFacts. Rents averaged $1,523 per month in Napa County, up 13.1 percent over the same period.

Demand has continued amid those price increases: Sonoma County rentals were 96.6 percent occupied as of March 31, with some recently-completed apartment construction in Napa lowering occupancy to 92.3 percent, according RealFacts. Napa's occupancy was 95.9 percent in the prior quarter.

Part of that demand could be attributable to a shortage of entry-level housing across the region, with first-time buyers competing heavily with all-cash investors seeking rental income or to "flip" a home for profit, said Pat Provost, broker and owner at Century 21 NorthBay Alliance.

"An average person can purchase about 10, 15 percent of the houses on the market in Sonoma County," said Ms. Provost, noting that a first-time buyer with a minimum down payment is potentially eligible for around $370,000 in financing. "The difference I see in this market is there is a lot of cash out there. There is a huge disadvantage for the first-time buyer."

For those who are able to purchase a home, affordability has been largely unchanged for mortgage payments over the period of the Zillow analysis. Though expenses in the Santa Rosa and Napa regions were more than twice the national average, median-income homeowners in the Santa Rosa MSA spent an average 35.2 percent of household income on mortgage payments at the end of 2013. Expenses were an average 32.5 percent in Napa.

That relative affordability -- largely attributed to the income requirements for mortgage lenders -- is likely to keep interest in home ownership high in the North Bay and support a demand for home ownership if new stock becomes available, Ms. Provost said.

"I think it's all supply and demand. They're not really building anything, but people have to live somewhere," she said.

Rental costs have traditionally "been lock-step with income" in rural communities like Santa Rosa and Napa, said Dr. Robert Eyler, professor of economics at Sonoma State University and director of its Center for Regional Economic Analysis. The Zillow data shows affordability essentially unchanged until the early 2000s for the Santa Rosa and Napa areas.

Yet affordability has declined along with the inflation of the housing bubble, with Santa Rosa rental expenses breaking 30 percent in the first quarter of 2004 and for Napa in the third quarter of 2007.

As rental expenses remain high in the North Bay, Dr. Eyler noted likely implications such as a higher percentage of households shared by multiple families and less income available for long-term savings. An improving economy is likely to drive rental demand -- and potentially, the percentage of income devoted to rent -- even further, he said.

"As employment rises you'll see immediate rental demand, but not immediate wage increases," he said.Rents rise as a share of income

The analysis by Seattle-based Zillow shows median monthly rent as share of median household income rising throughout the country, with some of the greatest expenses seen in the North Bay. Below are areas provided for comparison, ranked by average percentage of median income devoted to median rent at year-end 2013. 

1985--2000, average2013, year-endMedian income by county, 2012San Francisco MSA28.0%

40.7% $73,802Santa Rosa MSA26.7%39.8% $63,565New York MSA23.6%39.5% $63,841*Napa MSA27.0%38.5% $69,571Sacramento MSA30.2%31.8% $55,846United States24.9%29.6% $51,371

Sources: Zillow, U.S. Census Bureau. Note: *New York metro area includes several counties and parts of New Jersey.