Correction: A previous version of this story incorrectly stated that the area’s median income for a family of four is $53,000. According to U.S. Department of Housing and Urban Development 2013 estimates, it is actually $107,300 . This version has been updated.

David Cristeal, who has been acting director of Arlington County’s housing programs for the past two months, won the permanent job Friday and now has to address one of the biggest problems facing the region: the shortage of affordable housing in an area ever more dense and expensive.

“Yeah, congratulations, and now get to work,” Cristeal joked as he talked about the task facing him.

Arlington helps renters primarily through its Affordable Housing Investment Fund, a low-interest revolving fund that provides incentives to developers to build affordable housing. Cristeal, 55, who has worked for the county for nine years, called that a “pretty successful” model, at least for those who can afford near-market-rate rentals.

The challenge of finding housing for people who earn less than the area’s median income — roughly $107,300 for a family of four — is only going to get tougher in coming years. Nearly 40 percent of Arlington renters spend more than 30 percent on housing costs, according to the George Mason Center for Regional Analysis.

School Board member Emma Violand-Sanchez has pointed out that one-third of school employees make less than $50,000.

“There’s a fairly significant demand out there,” Cristeal said, citing a George Mason study that found that by 2040, Arlington will need 30,000 to 35,000 new housing units, based on expected job growth of about 50,000. A more detailed county study of housing is underway and is expected to be completed in the fall.

Cristeal is not the only one who’s worried. Virginians Organized for Interfaith Community Engagement, or VOICE, attracted more than 500 people to an Arlington church Sunday afternoon. Speakers called for the county to start using publicly owned land to build housing for families who make less than $50,000.

The County Board has agreed to spend $12.5 million in the coming fiscal year on the housing investment fund, and a year ago it approved a Columbia Pike Neighborhoods Area Plan, intended to preserve affordable housing in South Arlington as the area redevelops. Cristeal was leader of a team that worked on that issue, and he also led efforts to improve housing financing and planning, and to leverage state and federal funding. Between 2004 and last year, those efforts helped Arlington preserve or build more than 2,300 affordable housing units.

Cristeal said he regards VOICE as a partner; he started his career as a community organizer in Chicago. He said the county has also built good relationships with for-profit and nonprofit developers that he wants to continue.

“From my perspective, the more people we educate around housing issues, the better,” he said. “The challenge is that the demand is acute and the prices are going up.”