The stars have aligned for home affordability, according to a new analysis by Trulia which found that rising rents and low lending rates have conspired to make purchasing a home 45% less expensive than renting, on average, in the 100 largest U.S. metros.

That savings, which averages to $771 a month, does come with some conditions: It depends upon buyers qualifying for a 3.5% mortgage rate; it also is conditional upon buyers staying in their home for seven years, the average time that Americans have historically stayed in a home before moving; and it requires that buyers fall into the 25% tax bracket and itemize their tax deductions to take advantage of benefits afforded to homeowners. Trulia’s analysis also assumed that buyers put down a 20% down payment.

When those factors are in place, "homeownership is cheaper than renting in all of the 100 largest metros by a wide margin," wrote Jed Kolko, Trulia’s chief economist and head of analytics, in discussing the findings. "There is no market where the financial decision is even close."

Even the smallest difference between renting and buying, found in Honolulu, still produced a savings of 24%. The largest savings was logged by Detroit, where buying is a staggering 70% cheaper, even with all closing costs, maintenance, insurance, taxes, and other costs taken into account.

In dollar amounts, even savings on the lower end of the scale translate into large sums. Kolko points to San Francisco, which logged the second-smallest savings for buyers at 28%. Dollar savings in that metro would still come to $899 annually.

Even when conditions are less than ideal—if buyers stay for only five years, don’t itemize, and only qualify for a 4.5% mortgage rate—buying still remains cheaper than renting in 96 of the largest 100 metros, although savings shrink.

So, given the financial incentives, why are sales still dragging and the homeownership rate dropping? "The big obstacle holding back renters who want to buy is the down payment—even more than getting a mortgage," Kolko said, based on Trulia’s most recent survey of consumers. On top of the years it takes many buyers to save money for a down payment, unemployment has eroded the savings of many families during the recession. However, that problem is likely to be less of an obstacle among new-home buyers, he says, since those buyers tend to have higher incomes.

So how long will the housing market enjoy such a strong affordability advantage? "Owning will stay more affordable than renting" for some time, Kolko wrote in an email to Builder. "Right now, the gap is very wide. Even if mortgage rates rose significantly, buying would still be much cheaper than renting in most metros."

Claire Easley is a senior editor at Builder.

Learn more about markets featured in this article: Honolulu, HI, Detroit, MI, San Francisco, CA.