Call it a visitor center, call it an educational center, call it "the 21st Century Gateway to Monticello," but the real purpose of the new building at Thomas Jefferson's plantation is crowd control.

Like the Mount Vernon visitor center that opened in 2006, the new Thomas Jefferson Visitor Center is basically a pressure valve, meant to prevent great floods of people from overwhelming the small and fragile 18th-century house it serves. The success of this new addition to "the mountain," the achingly beautiful Albemarle County hilltop on which Jefferson designed one of this country's most innovative and elegant homes, is how well it hides its essential purpose.

Like all new visitor centers, the addition to Monticello, designed by Ayers/Saint/Gross Architects + Planners of Baltimore, is a multipurpose structure. It educates, its entertains, it feeds the mind and the body (at a new cafe). And, of course, you can shop for Jeffersonian paraphernalia.

But the success of the structure, which had its grand opening on April 15, lies in its careful separation of functions - and in the freedom it gives the visitor to avoid any or all of them while waiting for a coveted, timed entry to the old house up the hill.

The new center is a collection of five distinct but connected rectangular buildings, erected around a pleasant courtyard that serves as a roof over the lower level of the whole complex. It is placed in the same spot as the old ticket booths and restrooms, built in 1972, well below and out of sight of the house.

After a decade of planning and more than two years of construction, the new facility consolidates services and amenities that previously were spread out, including a small exhibition space operated by Monticello since the early 1980s that was located even farther down the mountain, near Interstate 64.

The new facility is much more rational and convenient than the old, dispersed arrangement, and the free clustering of its parts - ticket booth, movie theater, gift shop, exhibition hall, cafe - is a welcome departure from the annoyingly linear regimentation of similar facilities, such as the one at Mount Vernon, where the building almost obliges visitors to pass through its spaces in designated order.

You can set your own agenda at Monticello. The classrooms and interactive children's room are set apart from the main courtyard, which is a sensible way to keep adult areas a little more calm and reflective. The cafe is off by itself at one end of the center, well lit by natural light, with outdoor seating that is very inviting in nice weather.

But if your primary interest is Jefferson's house, you can buy a ticket at one end of the facility and pass diagonally through the courtyard to the stairs that lead to the bus stop. Yes, it's a nuisance to have to take a shuttle bus, but unless demand to see Jefferson's house changes in better proportion to the relatively modest size and delicacy of the building, there's no avoiding some draconian crowd control.

The $43 million building is a modest structure compared with the $110 million center built at Mount Vernon. It is not necessarily beautiful to look at, in a purely architectural sense. The design firm isn't trying to make any bold statements. But it is scrupulous about not making any mistakes.

Consider all the possible pitfalls. A bold, contemporary building might overwhelm Jefferson's house. But a timid or apologetic effort inevitably would look and feel cheap. Something made of brick, in the style of Monticello, would instantly be labeled fake and derivative. But any sharp contrast of styles would be out of context.

And where do you put it? Placing visitor centers underground hasn't turned out to be a very successful strategy because they are never really invisible, and they leave visitors blinking like moles as they pass from contemporary, subterranean spaces to historical, naturally lit ones.

The architects have successfully sidestepped these fundamental problems and opted, in each case, for a "least bad" alternative. Much of the building is below ground, but, because it is integrated into the natural slope of the mountain, it doesn't feel dark or buried. The structure isn't slavishly historical - it has none of the red brick or white columns of Monticello itself - but by using wood and fieldstone, the designers have made a building that is materially connected to the time of Jefferson.

They have avoided the urge to create an "iconic" building, with an overpowering or memorable shape. But by designing an environmentally sensitive space with green roofs, geothermal heating and cooling, and intelligent use of natural light, they have stayed on the architectural and ethical high ground.

Of all the early presidents, Jefferson was the most committed to the only two ideals that really matter in domestic space: beauty and convenience. James Madison's mansion, Montpelier, is a hearty old pile of brick, but not particularly innovative. Washington's Mount Vernon is a bit of a fraud, a wooden house striving for false grandeur with fake stone cladding. John Adams' house in Quincy, Mass., is simple, honest, dour, and sprawling.

Creating something to sit near and amplify the power of Monticello, a space both intensely rational and beautiful, is an almost thankless task. The lesson learned, this time around, was to strive for the pragmatic side of the Jeffersonian ideal: Keep it simple, keep it honest, give people choices, and use your creativity to solve basic problems, rather than say something new.

If You Go

Monticello (931 Thomas Jefferson Pkwy., Charlottesville, Va.) is open daily except Christmas. March-October: 8 a.m.-5 p.m., admission $20 adults, $8 ages 6-11. November-February: 9 a.m.-4:30 p.m., admission $15 adults, $8 ages 6-11. For information, call 434-984-9822 or visit www.monticello.org.

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