Architecture firms change over the years--it is a natural response to new ideas and changes of the guard. No firm, however, has transformed itself with quite the radical, poetic precision of Torti Gallas and Partners/CHK, a Silver Spring group that is nearing its 50th anniversary.

Founded by Jack C. Cohen in 1953, the firm helped mightily to build Washington's suburbs. Under a succession of names--Cohen's alone, then Cohen & Haft, then CHK--it designed the astonishing total of some 300,000 residences in the metropolitan area. Colonials, ramblers, split-levels, Cape Cods, garden apartments, condo towers, gated communities--very nearly the entire post-World War II assortment.

Yet the difference a decade can make could hardly be greater. First came the real estate recession of the early '90s, a crippling blow. Then came the death of partner Jack Kerxton in 1993--he was the "K" in CHK. (The late Leonard Haft was the "H." Founder Cohen has long been retired.)

Architect John Torti, who joined CHK in 1973 and became its leading designer, was sure something radical had to be done. With Thomas Gallas, the firm's chief financial officer, he decided to change course "180 degrees" and turn CHK into a force for reform in the suburbs. In effect, the firm was dedicating itself to cleaning up the mess that it had helped to create.

Torti and Gallas will tell the tale of the turnaround at a slide lecture Thursday evening at the National Building Museum. Coordinated with the museum's excellent ongoing series of "Smart Growth" exhibitions, the lecture is titled "Building New Neighborhoods in D.C. and Beyond."

In conversation about his firm, Torti, 57, routinely cites a litany of problems associated with conventional suburban development--clogged arterial roads, limited-access subdivisions, residences separated from workplaces and services, schools you can only drive to, a lack of neighborhood-based community facilities, a lack of neighborliness in general--and on and on.

Torti's change of heart was gradual but thorough. He embraced the principles of the New Urbanist movement with an apostate's zeal--not only the judgment that we had been going seriously wrong with our suburbs, but also the conviction that by looking again at our traditional towns and cities, we can turn things around.

Torti remade the firm in the image of his beliefs--and he and Gallas changed the company name in 1998. Torti says the firm maintains something of the ethos of the old CHK--in particular, an understanding of what works in the real estate market. But Torti and his mostly new staff go about things differently.

For instance, Torti Gallas CHK has been commissioned to plan a total of 14 Hope VI projects--including two in Anacostia (the Frederick Douglass and Stanton Dwellings) and two in downtown Baltimore--for the federal Department of Housing and Urban Development. The idea behind the Hope VI program, simply stated, is to remake federal public-housing projects along New Urbanist lines--to make them less like projects and more like conventional neighborhoods.

In the Washington suburbs, Torti Gallas/CHK recently did the basic urban design work for two large, important "neo-traditional" communities--King Farm Town Center in Montgomery County, still in the planning stages, and South Riding in Loudoun County, a mixed-use residential community where about 1,000 of the planned 6,000 residential units (only a few designed by Torti Gallas/CHK) have been completed.

South Riding was an important milestone for the firm--its 1994 plan for the 2,000-acre community won the firm a prestigious national design award from Progressive Architecture magazine. This was a first, and it buoyed the architects' confidence that they were right in their new direction--and good at it.

Since then, winning prizes has become something of a habit. Several of the Hope VI plans have won important awards, and recently the firm learned it is to receive an Honor Award for urban design from the American Institute of Architects at the Philadelphia convention in May. This is for the design of Bahcesehir, a mixed-use community to be built on a hillside about 40 miles from Istanbul, Turkey.

Each of these plans reflects fundamental principles of the New Urbanism. Each bears a strong resemblance to traditional American towns and cities. In each, the whole is more important than the parts: Individual "object" buildings are few, and the ability of buildings to frame public parks and civic greens is emphasized. Streets are narrower than the suburban norm and are arranged in variations of the urban grid rather than the cul-de-sacs of the typical limited-access subdivision. Civic and commercial functions are integrated into the layout and situated for convenient access by foot.

On the other hand, each of the plans is different from the other--adaptation to local environmental and architectural conditions is another key New Urbanist standard. Sophisticated environmental analysis is employed, and so is what might be called aesthetic intuition. For one of the firm's Turkish projects, Torti recalls, the designers spent days tromping over the hilly site until they could "feel" its strengths and weaknesses.

Battles have been lost in the often perilous trip from drawing board to construction--at South Riding, for instance, builders rejected the idea of alley garages, preferring instead the suburban norm of driveways from the street. Still, as time goes by, these Silver Spring architects find themselves winning more and more of such critical little fights.

Discussing a site plan for a waterside development in Occoquan, architect Neal Payton, a chief of the firm's urban design studio, points out the traffic circles and arterial roads coming in at acute angles. "VDOT," he says with a smile, using the shorthand name of Virginia's powerful Department of Transportation, "usually doesn't allow things like that."

If Torti Gallas/CHK's credentials as urban designers of note now seem beyond question, the firm's recent architectural achievements are harder to gauge. In large part this is because not many of the actual designs have been built yet, due to the amount of time it takes to move from urban design plan to completed project.

But it also is a matter of attitude. These days, Torti Gallas/CHK designers compile studies of local and regional architectural styles and building types before actually settling down to design. At worst, this worthwhile endeavor assures a certain fittingness and popularity for new designs. At best, it can stimulate creative reinterpretations of the past and, occasionally, inventive responses to the present.

All the same, the practice can also be a basis for formulaic design--a bit of this, a bit of that, and consumers won't know the difference. Based on a preview of the lecture and a stroll around the Silver Spring studio, it is possible to conclude that Torti Gallas/CHK is in no danger of descending to that level. But whether the firm emerges with a strong, consistent design identity remains a question.

This uncertainty simply gives a bit of spice to an already remarkable story. Torti says he had two "epiphanies" on the way to remaking the firm. One was a realization that history had its uses. To most educated people, this would seem like common sense, but to architects of Torti's generation, taught in modernist academies, it came as a revelation.

The second inspiration, he says, was "Andres Duany--pure and simple." Duany is, of course, one of the charismatic founders of the Congress for the New Urbanism. Torti happened to attend a summer-school course at Catholic University in the mid-'80s, where Miami's Duany was teaching a course on traditional town planning, a k a the New Urbanism. All in all, you would have to say it was a fortuitous encounter.

John Torti and Thomas Gallas will lecture at 6:30 p.m. Thursday at the National Building Museum, 401 F St. NW. Admission is $10 for museum members, $14 for nonmembers. To register by phone, call 202-272-2448. The exhibition "Reimagining the Suburbs: Smart Growth and Choices for Change" continues through March 26. Regular museum hours are 10 a.m. to 4 p.m. Monday through Saturday; noon to 4 p.m. Sunday. Admission is free.

CAPTION: Tom Gallas, above left, and John Torti's Silver Spring firm designs "neo-traditional" communities, such as King Farm, left.