LARGO — In 1999, the Pinellas County Housing Authority decided to move its headquarters from downtown Clearwater to Greenhouse Shoppes, a mostly vacant Ulmerton Road shopping center the authority bought for $1.85 million.Nearly 13 years later, the housing authority is still there, but the rest of the strip mall is now entirely vacant. But not for long.Workers will demolish the old, drab shopping center, except for the housing authority's office, and Greenhouse Shoppes will become Pinellas Heights, a 153-unit affordable apartment complex for lower-income senior citizens."Affordable housing is what we do best," said Debra Johnson, the authority's executive director. "This is land that we owned, and we think this is the highest and best use of the property."The new four-story apartment complex will represent a collaboration between Pinellas County and Largo, with some financial help from a bevy of sources to reach the $24 million needed to build Pinellas Heights. The county is expected to issue $12 million in bonds for the project, Largo is sending $150,000 in state housing funds, and the city is discounting impact fees that developers normally have to pay.There are several other sources of funding, including a $1 million grant from the Federal Home Loan Bank of Atlanta, and $9 million the complex's developer expects to make selling tax credits issued by the Internal Revenue Service. Pinellas Heights, which will offer one- and two-bedroom apartments for income-qualified people 62 years and older, will help the county housing authority reduce its waiting list, said Johnson. More than 8,000 people are waiting for some type of rental assistance in Pinellas County, according to Johnson, many of them seniors.Norstar Development USA, a Buffalo, N.Y.-based developer, will build Pinellas Heights. Construction is scheduled to start in August and to finish by the end of 2013. The housing authority will start advertising for applicants late next year, Johnson said, and an on-site leasing office should be open in early 2014.Pinellas Heights residents will pay rent equal to 30 percent of their monthly incomes.The apartments will be near the Pinellas Trail and Largo Mall — an ideal location, Johnson said. And the added supply of affordable housing for seniors can't come soon enough, according to Paula Rhodes, a director of development for Norstar."We have a substantial senior population in Pinellas County, many of them are on fixed incomes, and it's a population that's growing," Rhodes said. "We're expecting an increase in demand. We're trying to address some of the need now before it becomes a crisis." Will Hobson can be reached at (727) 445-4167 or [email protected] To write a letter to the editor, go to tampabay.com/letters.