Where's housing headed? Follow rents

NEW YORK (Fortune) -- It may not be the most widespread measure of housing prices, but if you want to follow a powerful driver, look at rents.

Specifically, it's the rents Americans pay on condos, apartments or houses that are about the same size, and share the same neighborhood as your ranch or colonial, that in the end determine what your house is worth.

"If you look at the trend in rents to see where housing prices are headed, you're looking at the right measure," says Yale economist Robert Shiller.

In recent reports, Deutsche Bank demonstrates how steady or even falling rents have pulled down housing prices, to the point where in many markets it costs about the same amount to own as to lease. That's a golden mean that America hasn't seen in almost a decade. The DB research also offers convincing evidence that the wrenching adjustment in housing prices is finished for much of the nation, with a bit more pain to come in selected areas.

Before we get to the numbers, let's examine why rents exercise a kind of gravitational pull over home prices.

In normal times, people won't pay too much more to own a house than to lease it. After all, if you're paying rent instead of a mortgage and taxes, you still get to enjoy the same rec room, chef's kitchen, and casita for visiting grandparents. So the surest sign of a frenzy appears when owning becomes far more expensive than renting. That's precisely what happened during the last bubble.

And the surest sign that prices have fully adjusted arrives when the ratio of what people pay in rent versus what owners spend on the same property returns to its historic average.

That brings us to the Deutsche Bank studies. Its REIT research team first established a benchmark for a "normal" ratio of rents to ownership costs -- what it calls ATMP, or after-tax mortgage payment -- for 53 U.S. cities.

On average, DB found that families across America were spending about 87% as much to rent as to own in 1999. Hence, they were traditionally willing to pay a premium as homeowners, though not a big one.

But by mid-2006, with the craze in full swing, the figure fell below 60%. At that point, Americans were spending an incredible 66% more to own than to rent. It was far worse in the bubble markets: In Las Vegas, Phoenix and Miami, homeowners were paying twice as much as renters, and in San Francisco and Orange Country, owners' monthly payments were triple those of their neighbors with leases instead of mortgages.

So how did that happen? During the bubble, rents -- the real engine that drives values -- were inching along at more or less their usual pace. From 1999 to 2007, apartment rents increased only 32%. But home prices jumped more than three times as fast, around 105%.

DB reckoned that housing prices are more or less reasonable when the ratio returns to its 1999 level. Why 1999? Because the ratio was relatively stable throughout the 1990s, and it was the year the steep rise in prices began in earnest.. At the end of the third quarter of 2009, the overall number stood at 83%, meaning renting was just a tad more attractive than owning.

But the picture varies widely from city to city. In 15 of those 53 metro areas, including St Louis, Indianapolis, and remarkably, Phoenix and San Diego, it's now higher than in 1999, meaning that homeowners' costs actually dropped versus what renters pay, courtesy of the steep decline in prices. In California's San Bernadino and Riverside Counties, it now costs 10% less to own than to rent; in 2006, owners paid more than twice as much as renters.

In another 14 cities, a list encompassing Boston, San Jose, and Chicago, the cost of owning exceeds that of renting by 6% or less. In the remaining 24 markets, housing is still moderately to extremely overpriced. The biggest problem areas are Baltimore, Long Island, and Seattle, where the ratio is still between 24% and 32% above the 1999 benchmark.

What does that mean for future prices?

Given that analysis, it's likely that prices will fall another 5% or so nationwide. The drop could even be slightly greater. One reason: Rents, the force that govern housing prices, are still falling.

In 2009, apartment rents dropped 2.3%, and the fall continues. And enormous adjustments are needed in still-exorbitant markets such as New York and Baltimore. Thankfully, the improving economy and decline in the rate of job losses means that rents should soon stabilize and could even start increasing by the end of 2010.

But fortunately, for most of the U.S., the sudden, terrifying fall in prices worked its own black magic. The numbers are back in alignment, or close to it. It had to happen. That's what rents, housing's great master, were telling us all along.