You can easily drive past Elm Brook and miss it. The 13-acre Concord, MA, housing project is tucked along a meandering cul-de-sac. A marshy area, dense with viny growth, encircles most of the development, blocking it from the views of neighbors to the east and west. To the south, Elm Brook abuts undeveloped land.

There are 12 houses: clapboard-clad capes, saltboxes, and two-story colonials, all with less than 180 square meters of living space. They are single-family units on small lots landscaped to conform to the natural environment. Each of these three-bedroom modular houses was crafted to meet exacting energy standards in central New York state and then shipped to Massachusetts for on-site assembly.

Completed just three years ago, Elm Brook offers no hints of the local resistance that Toby Kramer, SM '91, a Concord real-estate consultant, and Peter Roth, MAR '85, SM '86, developer of the project, encountered in bringing the project to reality. The Elm Brook homes seem, well, commonplace. But in a community where expensive historic properties abound, these houses are an increasingly rare commodity. They are "affordable."

Housing affordability is a complex issue. For many years, conventional wisdom held that a household should not spend more than 30 percent of its income on housing. But that view does little to help policymakers address the region's needs. Much more useful is a clear understanding of a community's proximity to job markets. Affordable housing that is 25 miles from a concentration of employers is not as valuable as housing within 10 miles. States and communities cannot develop appropriate housing policies without a tangible definition of the problem to be solved. And that means understanding just what affordability means. Through its new Housing Affordability Initiative (HAI), MIT's Center for Real Estate is helping the housing industry do that.