ST. PETERSBURG — Unable to afford life in California any longer, Susan Olsen moved to St. Petersburg and found an apartment in a building downtown for $750 a month.

She might not be there much longer.

On Wednesday, the city's Development Review Commission unanimously approved a $69-million, 20-story tower that would replace several quaint old apartment buildings that are home to Olsen and dozens of others. The unanimous vote came despite complaints that St. Petersburg's stock of affordable housing is rapidly disappearing.

"I'm probably going to have to completely move out of the area,'' said Olsen, a retiree who lives on her Social Security income.

Illinois-based Inland National Development plans to demolish seven apartment buildings, some dating to the 1920s, on Fifth Street N and Third Avenue near Mirror Lake. They would be replaced by a U-shaped tower with nearly 10,000 square feet of ground-floor retail space.

City staffers recommended approval of the tower, which would add another 270 units to the more than 2,000 recently built or under construction in and near downtown St. Petersburg.

The newest project would be "displacing a whole block of people,'' said Lynn Merhige, co-owner of a small commercial building in the area. "I wonder if the city should maybe slow down, take a pause and see what happens with all this rush to development in the Edge District, the (Grand) Central District. You should see all the empty, dark apartments.''

Merhige also raised concerns that affordable apartments would be knocked down for a project that might never get built.

"We saw what happened in 2005, 2008,'' she said. "It's gonna happen again, there's boom now but there's going to be a bust.''

City officials said the developer would be required to submit "full and complete'' construction plans but no proof of financing before demolition permits were issued.

The 20-story, contemporary style tower would be by far the tallest structure in a neighborhood of mostly older, low-rise buildings, including the Beaux arts library built in 1915.

"This is an incredible change for this part of town,'' Peter Belmont of Preserve the 'Burg told commissioners. "This city really needs to think what it's going to look like in the future.''

Attorney R. Donald Mastry, representing the developer, noted that the city had approved the site in 2005 for an even bigger tower -— 25 stories. He also noted that condo towers on Beach Drive had replaced older, shorter buildings.

"That's how the redevelopment of downtown occurs,'' he said.

Commissioners listened to the objections but made almost no comment themselves before their unanimous vote of approval. Opponents could appeal the decision to the City Council.

After the hearing, John C. Brown of Inland National Development said the company originally planned to put a Marriott hotel on the site but didn't think it was "the right time or right place.''

"But having spent a year in town, we did educate ourselves on the apartment market and became bullish,'' he said. "This is a dynamic market and we feel like we're going to activate the Mirror Lake side of town.''

He wouldn't disclose the price range for rents in the proposed tower. Asked if they would be as low as the $750 that current area residents pay, he quickly said: "No.''

Contact Susan Taylor Martin at smartin@tampabay.com or (727) 893-8642. Follow @susanskate.