A positive redevelopment trend is gaining momentum in downtown Washington. A number of dense, mixed-use “ensemble” projects are underway or being planned.

They promise shopping galore, millions of square feet of office space, countless new dining opportunities and thousands of new residential units, some of which will be affordable.

Furthermore, being adjacent to or within easy walking distance of transit, ensemble residents can choose not to own a car.

The term “ensemble” characterizes a compositionally unified group of urban buildings and civic spaces. An ensemble has a collective identity, eliciting a palpable sense of place and sense of community. Ensembles are greater than the sum of their architectural parts. Visible and accessible, they become public destinations as well as desirable residential neighborhoods.

An ensemble is not created piecemeal. Rather, a form-based, three-dimensional master plan governs site layout and deployment of functions; determines placement and massing of new buildings; shapes civic spaces and streetscapes; and denotes existing structures — and infrastructure — worth preserving. Ensemble master plans, while somewhat prescriptive about design, nevertheless enable diverse architectural expression.

(Roger K. Lewis)

CityCenter is designed as an ensemble, much of which has been constructed and is now being marketed. Along with a pair of office buildings, four residential buildings — two condo and two rental — containing 674 apartments occupy reconstituted city blocks where the Washington Convention Center once stood.

Fronting 9th, 10th, 11th, H and I streets NW, CityCenter’s six 11-story buildings frame a mid-block public plaza and pedestrian alleyway stretching from 9th to 11th Street. Dozens of chic stores and restaurants, filling building ground floors, line streets and pedestrian spaces. Thus, CityCenter is destined to become an activated public place as well as a spirited place to live in the heart of the city.

In the Shaw neighborhood a few blocks north of CityCenter, construction of the more modestly sized but relatively dense CityMarket at O ensemble is being completed. Bounded by O, P, Seventh and Ninth Streets NW, CityMarket is anchored commercially by a 78,000-square-foot Giant supermarket and architecturally by the historic, red-brick O Street Market building.

New, multi-story residential buildings on the site contain 645 rental apartments. A 182-room hotel, plus about 8,000 square feet of neighborhood retail and eatery space, completes the functional menu, which also incorporates several outdoor community activity spaces. Clearly this ensemble is more domestic in its aspirations and spirit.

The Wharf ensemble, the long-awaited redevelopment of the Southwest Waterfront, will be a linear array of individual mid-rise buildings parallel to Maine Avenue. With views of the Washington Channel, East Potomac Park and the Potomac River, the Wharf’s buildings will abut and overlook a publicly accessible waterfront promenade, marina and new recreation pier.

The Wharf master plan envisions more than 1,300 apartments, almost a million square feet of office space and more than 350,000 square feet of retail space. Hotels and a conference/performance venue round out the redevelopment program.

Like CityCenter, the Wharf will be a vibrant urban destination, a physically identifiable, waterfront ensemble where Washingtonians will want to live, work and play. It’s also a place tourists will want to visit.

But few redevelopment ensembles in Washington match the scope and technical challenge of Burnham Place at Union Station. Entailing air-rights development, Burnham Place will be a dense, mixed-use ensemble perched above Union Station’s completely reconstructed railroad tracks and platforms. From behind the iconic, neoclassical edifice designed by Chicago architect Daniel Burnham in 1903, Burnham Place will stretch northward past H Street to K Street NE.

A large plaza will straddle H Street, where a soaring, glass-clad, light-filled train station structure — covering new concourses, tracks and platforms — will provide a north entrance to the station from H Street.

Office, hotel and apartment buildings flanking public courtyards and promenades will rise above track level. The current, award-winning master plan incorporates more than 1,300 apartments, 500 hotel rooms, 1.5 million square feet of office space and 100,000 square feet of restaurants and shops facing Burnham Place’s public spaces.

Burnham Place is part of a complex, costly makeover and expansion intended to make Union Station a world-class, multi-modal transportation hub. This requires demolition of the existing parking garage and track-platform network; extensive excavation to build new, lower-level parking and service spaces; and reconstruction of new tracks, platforms and concourses. Construction complexity is compounded by institutional complexity attributable to the ensemble of disparate development entities planning, financing and coordinating the project.

These four urban redevelopments — CityCenter, CityMarket at O, the Wharf and Burnham Place — will yield about 4,000 residential units. More apartments likewise will be built in other mixed-use ensembles being planned, such as redevelopment of the Walter Reed campus and the McMillan Reservoir site.

With their sizable housing components, such projects bode well for the city. They are emblematic of population growth as people increasingly opt to move to the nation’s capital. And they produce jobs and augment tax revenue, contributing to the city’s economic health.

But ensembles accomplish one other thing worth noting, a qualitative rather than quantitative payoff. They enhance Washington’s urbanity, making it an ever more cosmopolitan place to live, work and visit.

Roger K. Lewis is a practicing architect and a professor emeritus of architecture at the University of Maryland. To view his cartoon, go to www.washingtonpost.com/realestate.