It may be the most expensive building ever built in Charleston that is not, technically speaking, a building.

But the two-story mock-up that recently sprouted at Anson and Calhoun streets does give the city, the architects and others a good idea of what the remodeled Gaillard Auditorium will look like when completed in 2015.

The $142 million renovation is being paid for using a mix of public and private funds.

“It’s a mini version of the whole construction sequence,” says Craig Williams, project manager with David M. Schwartz Architects.

“The idea was to show them as many materials as possible, succinctly,” he adds. “We are very pleased with how this has come out. There were no surprises.”

These sorts of sample panels are commonly required by the city on big projects, but the Gaillard’s is believed to be the most ambitious and costly to date.

It includes 60-80 pieces of stone, as well as windows and stucco and a granite base — and a series of roofing materials on the other side.

While Williams had no specific price tag for what it cost to build as it’s part of the whole construction contract, he described it this way: A school official complained that the sample panel for Buist Academy just across Anson Street cost as much as a BMW.

With the Gaillard, Williams says, “I think we have a car lot.”

Mayor Joe Riley is familiar with most all of these sorts of samples and says of the Gaillard’s, “I would think this would be the most costly and arguably the most important.”

The city’s Board of Architectural Review required the sample, but Williams says it’s something he would recommend even if that weren’t the case.

For instance, Williams says the mock-up gave the contractor, Skanska, and its subcontractors an appreciation for getting the concrete block right before applying the stucco.

“It’s a complicated puzzle as to how it all goes together, and the model enabled everybody to understand how complicated it was,” he says.

The model also helped affirm some of the decisions already made, such as using variegated limestone rather than buff limestone. And it underscored other design decisions, such as using three pieces to form the triangular pediments over the windows, rather than two stone pieces joined at the peak, right where water is most likely to intrude.

“The ghost of Alberti would be rolling in his grave if we did that,” Williams jokes about Leon Alberti, the 15th-century Italian architect and principal initiator of Renaissance art theory.

The Board of Architectural Review got a close look at the sample panel recently and then weighed in on it.

Board member and architect Jay White questioned why the joints between the stones were filled with a sealant, not mortar.

Williams said 95 percent of new limestone buildings use sealant, which will lose its sheen over time.

Board chair Craig Bennett asked if there would be sand imbedded in the sealant, but Williams says, “That really makes a difference for such a short period of time, it doesn’t seem to be worth doing.”

City architect and chief preservation officer Dennis Dowd says the subtle contrast between the Gaillard’s stone and stucco makes for a very rich effect. While he liked the panel, he made several detailed recommendations that the board agreed to require as part of its final approval.

Some of these include: The sealant should never be wider than 3/8 of an inch; the sealant color should match the stone; the windows’ mullions’ joints should be flush; and every effort should be made to avoid color contrasts.

There also was a lively debate over whether the city should allow a four-inch line at the peak of its gray metal roof almost 100 feet above the ground, but the city is allowing it. Board members figured few would notice. And while it’s not a historic detail, the Gaillard isn’t a historic building.

This sort of attention to the building’s minutiae might seem boring, even unnecessary, but it’s better to work them out on a panel that will be torn down in several months than on a building many hope will stand for centuries.

Reach Robert Behre at 937-5771.