CLEVELAND, Ohio -- A downtown Cleveland building constructed by the nation's oldest labor union for the country's first labor bank could become 287 apartments, under plans being considered by local developer Weston, Inc.

Weston expects to buy the Standard Building, near Public Square, in a deal set to close today. The sale will be the first in the history of the 90-year old office building, which was built by the Brotherhood of Locomotive Engineers and Trainmen and has housed the union's offices since 1989.

The 21-story building, at 1370 Ontario St., will add to a chain of office-to-apartment conversions that are reshaping Cleveland's central business district. In the last six years, residential developers have renovated or staked their claim on empty and obsolete offices across downtown, replacing workspaces with bedrooms, bathrooms and kitchens.

"Last year, downtown Cleveland absorbed 800 units of housing without any reduction in occupancy," said Joe Marinucci, president and chief executive officer of the Downtown Cleveland Alliance. "We're still at about 95 percent. This year, we have about 1,100 units under construction that will come online within the next 18 months. Early pre-leasing information from those developers indicates that there's continuous demand."

The alliance's recent research indicates that the next wave of conversions could add 2,000 to 2,500 more apartments downtown. Strong demand for rental housing, combined with a relatively soft office market and preservation tax credits that support renovations, has investors including Warrensville Heights-based Weston scouting for projects.

T.J. Asher, the company's chief executive officer, said the Standard Building makeover will be a $60 million project. He wouldn't disclose what Weston is paying for the building, which hit the market in early 2012 at $8.8 million. The Cuyahoga County Fiscal Office places the market value of the property just short of $6.1 million.

Working with Sandvick Architects, Weston is considering two redevelopment plans. The first would fill the building with 287 apartments, with retail on the ground floor. The second plan calls for roughly 240 apartments, with ground-floor retail and offices on the second through fifth floors.

"With the onset of urban living and the urban excitement of downtown Cleveland, we think that it's the right time," Asher said. "With the anticipated redevelopment of Public Square, we just feel that this property really makes a lot of sense for us."

The apartments would range from 600 to 1,100 square feet - one to two bedrooms. Asher wouldn't pinpoint what Weston expects to charge, beyond saying the rents will be "competitive with the market." Downtown Cleveland landlords were asking an average of $1.25 per square foot - $1,250 for a 1,000-square-foot apartment, say - early this year, based on the alliance's most recent quarterly report. But new projects are fetching higher rates.

Construction won't start immediately. The 400,000-square-foot building is 45 percent occupied, and tenants still have time on their leases. The railroad union, whose bylaws require it to maintain headquarters in the Cleveland area, reserved the right to lease its offices back from Weston for at least two and up to three years, as a condition of the sale agreement.

Asher said renovations might begin a year to 18 months from now. The apartments would open a year later - in the second half of 2016.

That timeline dovetails well with the most recent scheme for Public Square, where a $30 million overhaul might start in the fall and be finished in spring 2016. Weston's bet on the Standard Building has a lot to do with that public-space plan, which calls for closing Ontario Street and narrowing Superior Avenue to create a more inviting central park.

Asher said Weston is working through some details of the Standard Building plan. The company is considering ways to spruce up the building's blank southern face, a windowless wall that rises behind the Old Stone Church facing Public Square. And Weston doesn't have a firm parking plan yet, though the Asher family's nearby Warehouse District parking lots are a logical place for apartment-dwellers to park.

As for financing, Weston expects to seek state tax credits for historic preservation in September. The state expects to announce its next round of awards, which are highly competitive, in December. Developers pair those state credits with federal preservation tax credits, which cut the cost of challenging rehab projects.

Built in 1924 as the Brotherhood of Locomotive Engineers Cooperative National Bank Building, the Standard Building is named after Standard Trust, a labor bank that was forced to liquidate during the Great Depression. In the 1930s, Eliot Ness - the famous prohibition agent and Cleveland safety director - had an office in the building. The railroad union moved in after its previous home, the Engineers Building, was demolished to make way for the Marriott hotel at Key Center.

"The Standard Building is a significant part of the brotherhood's heritage, but it has reached a point in time where considerable assets are necessary to repurpose the building," Dennis Pierce, the union's national president, said in a written statement. "It is very gratifying that the Asher brothers and Weston have committed those assets and will maintain this treasure as a cornerstone of downtown Cleveland life."

In 2010, Cleveland developer John Ferchill considered the building for a hotel project, but the deal never came together. The union officially put the property up for sale in early 2012.

At one point, investor duo Stuart Lichter and Chris Semarjian had an option to buy the building, which they pitched as a potential location for Cuyahoga County's new government headquarters. The county decided to build new offices at East Ninth Street and Euclid Avenue, and the potential sale to Lichter and Semarjian fizzled.

David Wagner of the Hanna Chartwell real estate brokerage, which has been marketing the building, was traveling and unavailable by phone last week. In a brief email, he said the apartment project will be a "brilliant" development.

"It hits the sweet spot where we're seeing the most demand," Marinucci said, "which is taking some of these older, less competitive office buildings and allowing them to transition into adaptive reuse, in particular for housing."