The remaining piece of the Foundry Square puzzle is starting to fall into place.

With Equity Office Properties well under construction for the Barclays Global Investors building, Wilson Meany Sullivan this week started marketing the fourth and final building at the South of Market development.

Company principal Tom Sullivan said WMS has been waiting for the market to strengthen to the point where it was robust enough for tenants to start considering build-to-suit projects. With rents for the most desirable space hitting $60 a square foot, he said the time has arrived to step up the marketing campaign in hopes of landing the large tenant, or tenants, needed for construction to begin.

Webcor will build the nine-story building, to be called Foundry III, and could deliver it in 16 months. The 200,000-square-foot building will have an earthy yellow-gold stone veneer.

Phil Tippett and Bill Walsh of CB Richard Ellis have the listing.

In addition to Barclays, the other Foundry Square buildings are anchored by Gymboree and Orrick Herrington & Sutcliffe. WMS acquired the Foundry III site as part of the settlement when it split from EOP in 2002.

Sullivan said the large floor plates and campus-style architecture is most appealing to non-traditional users, mainly technology companies. While a single user would be preferable, construction could start with half of the building leased.

Sullivan said he is projecting the economy to continue picking up steam with a modest expansion of traditional office users and more rapid job creation in the tech sector, which he sees as particularly strong in the city.

"San Francisco is now effectively part of Silicon Valley and some of that will come into the city," he said.