Renovations are set to begin on the façade at 100 Church St. in the next two weeks, a face-lift the landlord aims to use to attract tenants to a 21-story office building that has sat almost half-empty since the terrorist attacks of September 11, 2001.

After losing a widely publicized lease 10 days ago when Newsweek magazine balked, choosing to move to SoHo instead, the landlord, the Sapir Organization, is sitting on some 400,000 square feet of space that has been vacant for almost seven years.

Situated between Park Place and Barclay Street near ground zero, 100 Church is being transformed with a new black granite exterior, mechanical upgrades, and double-paned windows.

The renovations will bring the 50-year-old building “to 2008 levels,” the president of the Sapir Organization, Alex Sapir, said in an interview.

Designed by architects Emory Roth & Sons in 1958, 100 Church previously housed investment firms such as the Bank of New York and Merrill Lynch, both of which moved following the September 11 attacks. The New York City Law Department was also displaced after the attacks, but returned in 2002 and recently re-signed its lease.

The Sapir Organization has been unable to find a tenant to fill the remainder of the space, with several deals falling through. Three years ago, the Toy Industry Association pulled out of an agreement to lease 300,000 square feet at the Class B building, and in November, Omnicom Group, which had looked seriously at 100 Church, signed a lease at 195 Broadway instead.

In February, Newsweek announced it had leased nearly 200,000 square feet, a deal that was touted as a turnaround for the building. The real turnaround was yet to come: In a decision that took many in the industry by surprise, the news giant instead leased 165,000 square feet at 375 Hudson St.

The Newsweek deal’s unraveling “is a setback” for 100 Church, a principal of Brentler Realty, Paul Bostick, said. “The big-tenant market is slowing,” and it could be several more months before a tenant of Newsweek’s quality is found, he said.

Part of the building’s problem, Mr. Bostick said, is its location. “It’s a little north of what people consider the downtown market, but it’s not where you get the oomph of TriBeCa. The area is very difficult to get excited about — it lacks an identity,” he said.

Also making the marketing of the space difficult, experts say, is the reputation of the landlord. The Sapir Organization has been plagued by legal difficulties, settling a cluster of suits and countersuits with the Metropolitan Transportation Authority in 2003. The company also has been sued by Newmark Knight Frank and other real estate firms over allegedly unpaid commissions.

Mr. Sapir downplayed the allegations. “We have a very good reputation with everybody that’s done right by us,” he said.

Now, the Sapir Organization is counting on its strategy of revamping the building to make it a success. The renovations, slated for completion this fall, include a new security system, an elevator upgrade, a fire system, and a refurbished diesel generator, as well as a messenger center and concierge services.

An earlier renovation added flat-panel LCD televisions and granite floors to the lobby, which displays a collection of Swarovski crystal chandeliers and a fountain specially designed by Mr. Sapir’s father, Tamir, a former taxi driver who is a self-made billionaire.

“A lot of what is dated about the building will go away,” the leasing agent for the property, Stephen Siegel, a chairman at CB Richard Ellis, said.

The asking rents at 100 Church St. range between $45 and $55 a square foot, depending on the floor. This is about on par with the $50 a square foot average asking rent for downtown in the first quarter, according to a Cushman & Wakefield market report.

Since taking over as broker for the building a little more than a year ago, CB Richard Ellis has signed one new tenant. In September, Hamptons Magazine publisher Niche Media signed a lease for 45,000 square feet.

Mr. Sapir, who took over the reins of the company from his father in 2006, said he is attempting to breathe new life into the building as the area around it develops.

“The neighborhood was damaged and bruised” following the terrorist attacks, he said. “Now is really the best timing. We believe that we’re in a prime location in a great building.”