Third in an occasional series on the future of housing.

Here’s an old real estate joke you maybe haven’t heard before:

Know how to create more affordable housing? Just build a new market-rate apartment building.

Then wait 40 years.

A well-maintained, 1960s- or 1970s-era building is likely to charge so much less than a new building in rent that it’s affordable to households at the low end of the income scale. And without any subsidy.

There’s a term for it: naturally occurring affordable housing. It’s called NOAH, like the name of the biblical character who built a big boat.

That old joke about waiting 40 years isn’t so funny to advocates for more affordable housing, of course. How and where to build new housing seems to get all the attention, while hundreds of old and affordable apartments are renovated back to upscale or simply knocked down every year. If we are serious about working on our problem of not enough lower-cost places to live, preserving the places built when grandpa and grandma were young should be a priority.

The reason NOAH is affordable is not that hard to understand. It’s sort of like how a well-maintained 15-year-old Chevy TrailBlazer that still runs like a top could be affordable to somebody with $3,500 to spend while a Chevy Tahoe off the new car lot is way out of reach.

There are pockets of naturally occurring affordable housing all over the metropolitan area, maybe 160,000 units altogether. One place with a lot them is the south metro community of Bloomington.

The median household income in Bloomington is not as high as the Twin Cities as a whole, yet Bloomington has a much lower poverty rate and its residents are more likely to have at least a bachelor’s degree from college, according to census data.

Maybe the best way to think of Bloomington is as a community with a wide slice of the middle class. That’s important when understanding what’s meant by the term affordable housing.

It doesn’t necessarily mean a place for poor people to live. It can also be a decent place for people who might have full-time work in early childhood education, retail, health care or any number of other fields with no better than middle-class pay.

What’s affordable for rent is based on the area median income for a household. The Twin Cities is a relatively affluent place, and a recent housing study for Bloomington by the firm Maxfield Research and Consulting of Golden Valley assumed an area median income for a four-person household of $85,800.

Bloomington generally developed from east to west, with multifamily projects mostly clustered around intersections of arterial streets. There were more than 8,800 market-rate apartments in the city as of Maxfield’s count, in buildings of at least two dozen units.

To be affordable at 80 percent of the area household income, the rent can’t be any more than maybe $1,350 for a one-bedroom apartment. Only about a quarter of the market-rate apartments in Bloomington can be considered affordable at that level, very few of them built after 2000.

In older buildings, though, thousands of units might be considered affordable to households at just 60 percent of area household income.

The pool of available properties in Bloomington isn’t very deep, though. A search for one-bedroom units at around $850, the top end of what’s affordable for households at around $43,000 in income, turned up just a handful of vacancies. One is at the Concord Green property on Nicollet Avenue S. at 92nd Street.

The landlord did not return a call, but based on the callback rate with other private landlords in Bloomington, no one last week seemed all that eager to discuss their properties as affordable.

Concord Green has a pool and fitness room. A walk-around confirmed the buildings and landscaping are well-maintained. It’s also close to a freeway interchange and with a bus to downtown Minneapolis that stops just outside the front door.

The biggest knock on Concord Green is that it was built in 1962, according to Maxfield, and it looks it.

The city of Bloomington has come to care a lot about preserving affordable housing of this type, recently settling on three ideas to try right away, as described by program manager Bryan Hartman of the Bloomington’s housing and redevelopment authority.

One is a tenant transition law, giving a NOAH tenant a 90-day break on rent increases or a nonrenewal of the lease when an apartment building changes hands. Another is adopting a citywide affordable-housing strategy that might include ways to make sure new or redeveloped properties have lower-rent units.

Keeping the housing already there more or less as is remains the best idea, but that’s no easy trick. A new buyer of an older suburban place almost certainly hopes to raise the rent after putting in some improvements. That’s one reason the Twin Cities loses around 1,300 units of rental NOAHs every year, mostly in the Twin Cities suburbs, according to the state’s latest estimate.

A lot of affordable units are going into the hands of nonprofits, but a new for-profit owner can make the finances work out to leave the buildings affordable if they have a patient source of low-cost capital, said Rachel Robinson, fund manager for NOAH projects with the Greater Minnesota Housing Fund, a nonprofit in St. Paul.

Maybe one reason people don’t quite grasp that the affordable housing shortage is only getting worse, Robinson said, comes from the NOAH acronym. The term “naturally occurring” suggests that the market will just always provide it.

The idea of waiting 40 years for new affordable housing may not quite work anymore, she added. The new buildings entering the market aren’t much like the middle-class apartments built across Bloomington in the 1960s and 1970s. The new ones are mostly for an upscale tenant and will likely be maintained as upscale for decades.

“It isn’t always going to be there, it isn’t naturally going to be part of our housing stock,” she said. “We have to appreciate it, and we have to decide what we’re going to do to maintain it.”