SILVERSTEIN Properties has finally started construction of 600 W. 42nd St., and we’re now unveiling the design for its $700 million twin-towered plan at the corner of Eleventh Avenue.

The 60-story Silver Towers at River Place will be a sleek neighbor to its 40-story cousin next door, 1 River Place. Both were designed by Costas Kondylis & Partners for Silverstein.

In the north tower, 107 units on the second through seventh floors will have corporate housing and hotel-like services.

The entire rental project will have 1,359 units, with 317 of them, or 20 percent, classified as more affordable housing.

There is a mix of studio, one- and two-bedroom apartments connected through a six story base that will host a fitness center with an Olympic-size swimming pool, spa, lounge, library, parking for 194 cars, and more than 20,000 feet of retail. Many of the units have balconies.

As part of the Inclusionary Housing Program – a city-run program designed to promote affordable housing in new developments – an adjacent 12-story new building will have 83 units set aside for moderate-income tenants earning approximately 80 percent of the city’s median income.

A new 25,000-foot public park designed by Thomas Balsley & Associates will include an interactive mist fountain for children and a dog area.

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In a market-significant purchase, TIAA-CREF added 470 Park Avenue South to its portfolio for $157 million.

The all-cash deal, made at $604 per foot, was paid to seller SL Green Realty Corp.

The building was one of the originals in Stephen L. Green‘s portfolio before he took the company public.

It’s also significant because TIAA-CREF is a large institution making its first foray into a city property since they bought 780 Third Ave. years ago.

“It means they liked the market and building fundamentals,” said our spy, pointing to the gradually calming credit markets.

Darcy Stacom and Bill Shanahan at CB Richard Ellis acted as brokers for the 260,000-foot, 17-story plus penthouse office building that sits between 31st and 32nd streets.

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Developer/hotelier Morris Moinian‘s Fortuna Realty Group has a $40 million contract for 112-126 Ninth Ave. at the southeast corner of West 18th Street.

The deal will close early next month.

Eric Anton, Ron Solarz and David Johnson repped the seller, while Anton, Solarz and Azita Aghravi brought in Moinian.

At the confluence of Chelsea and the Meatpacking District, the six-story, pre-war property is just 100 feet north of the Maritime Hotel and has views of the Barry Diller/Frank Gehry designed IAC headquarters.

The 60,000-foot building has 64 residential units and eight retail units along 131 feet of frontage on Ninth Avenue.

Major renovations and new retail tenants are expected to reflect the trendy neighborhood.

Moinian recently purchased 129 W. 29th St. and the InterContinental Hotel Group’s Hotel Indigo at 127 W. 28th St. slated to open in January 2009.

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The American Cancer Society Building deal to sell its 25,000-foot small, mid-block office building at 19 W. 56th St. to United American Land was flipped to Sheldon Solow, who closed on it for $15 million, city records show.

United was in contract for a couple of years as the ACS was waiting to relocate to its new facility on West 31st Street.

As we previously reported, Solow is glomming buildings on West 57th and West 56th sreets with an eye on erecting a tony tower.

Yoev Olsner, Jon Epstein and Charles Kingsley of Cushman & Wakefield handled the original sale for the Cancer Soci ety.

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Beatles pal Peter Brown, who authored the band bio “The Love You Make,” is moving his public relations firm into 3,756 feet at 250 W. 57th St.

The firm, Brown Lloyd James (BLJ), will move in December from 41 W. 57th St.

Ruth Colp Haber of Wharton Property Advisors, represented the tenant in the negotiations, while Barry Zeller of Cushman & Wakefield, repped owners W&H Properties.

Brown’s firm has clients, including Andrew Lloyd Webber, the BBC, Polo Ralph Lauren, Christie’s and IBM.

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