It might be a challenge to get people to think carefully about a business term from real estate called “yield.”

But if public officials don’t take the time to really get it — and then maybe adopt a policy idea like making real estate developers set aside some units in their new apartment buildings that are considered affordable — odds seem high that the problem of not enough lower-cost housing in the Twin Cities only gets worse.

Here’s what could happen. The buildings we see going up right now were on the edge of making economic sense to build. If the expected performance of new projects gets a little worse, by insisting some units collect much lower rents, the new development won’t just slow down, it will stop.

Maybe that’s hard to imagine, with so much money flowing into new apartments in recent years and construction cranes all around. So the first part of grasping this problem is understanding how real estate development works.

Real estate developers are a little like Hollywood movie producers, relentless optimists who come to love an idea, hire the artists to execute it and then round up the money to make it into a movie. That’s the way I explained what my wife did for work to our kids when they were younger.

In both movies and apartments, unless the people with money see a way to get their investment back with a return, they’re not going to fund the project. So that means developers don’t decide what gets built after all. It’s the investors and bankers who do. Without their money, nothing happens, no matter how brilliant the vision.

My wife hasn’t worked on an apartment project for a while, but she volunteers for Project for Pride in Living, a nonprofit in Minneapolis. PPL is one of the housing developers that’s come together in a group to talk to Minneapolis policymakers about affordable housing policy ideas, including requiring new market-rate buildings to have some affordable units.

The firms involved seem to include many of the big multifamily housing developers, nonprofits to market-rate builders. The list includes Schafer Richardson, a firm cofounded by my brother.

And what this group seems to want to show policymakers is that a scarcity of affordable housing really comes down to this problem of not enough yield.

It’s one number, in apartment building, that reflects whether a deal makes financial sense. It’s calculated by taking the rent money left over after paying the operating expenses and dividing that number by the expected total project cost.

These days a medium size, market-rate building in the Twin Cities might cost $19 million or $20 million just in construction costs. More money is needed for the land, fees for architects, lawyers and other professionals, and so on. Let’s say the total cost comes to $26 million.

Once it’s leased up, the annual net operating income comes to more than $1.5 million, so the math works out to a yield on this project of about 6 percent. That’s a lower number than it was just a couple of years ago, because construction costs have continued to increase.

Maybe two-thirds of the required money comes from a bank in the form of a mortgage, and investors put in the rest as equity. The bank will charge less than 6 percent in interest, which boosts the expected return to the investors.

A project of roughly the same size built to be affordable to households at 50 or 60 percent or so of the median income would seem like it must be a lot cheaper to build, but not as much as you might think. It still takes money to buy land, pay the architects and lawyers, and so on, and the construction will cost almost as much.

With much lower rents and close to the same costs, the projected yield on this project only comes to maybe half that of the market-rate deal. That’s why nobody in the financial markets would touch this project without a big subsidy, like federal tax credits.

What happens if instead of subsidizing an affordable housing project, the city just requires just some of the units in a new, market-rate building to be affordable for lower-income families?

In that case, instead of all market-rate units in the new project, there will be some affordable-rate ones that collect a lot less in rent. But remember, there’s not that much of a difference in the total project cost between affordable and market-rate. So goodbye, 6 percent yield.

By keeping just 10 percent of the units affordable in a new building and shaving the construction costs a bit, on my own spreadsheet the yield drops closer to 5.5 percent. That doesn’t seem like much of a difference, but 6 percent already is considered “tight,” a project where it’s a close call on whether the money people would fund it.

At 5.5 percent, at least it’s no longer a tough decision. No one funds it.

With better opportunities elsewhere, investors will quietly take Minneapolis off the list of places they’ll look for deals.

The death of projects like this in the planning stage doesn’t just cost the city affordable units that need to be built, of course. It would mean lots of new market-rate apartments aren’t added to the supply. And rents seem likely to only go up.

Developers must be optimistic they can work with city leaders on this basic problem of economic viability, or they wouldn’t be planning to walk around City Hall to talk about ideas for how to do it.

Maybe publicly held land gets signed over, a form of subsidy that makes a project with some affordable units more competitive in the market for capital. Or the city could direct part of the incremental taxes generated from a new building back into the project.

What isn’t going to work is insisting that developers deliver a product they can’t find the money to build.