As Phoenix and other cities across the country face a housing shortage, one segment is especially tough to find.

It’s called the "missing middle" — properties made up of two to four units. 

The National Association of Homebuilders said 12,000 townhouse construction projects began last year. That’s flat from the year before while most segments grew. 

Two- to four-unit developments make up 3.2% of all multifamily production. Between 2000 and 2010, it made up just under 11%. 

The NAHB said missing middle construction has lagged since the Great Recession and will continue to do so without zoning reform focused on increasing that segment.