During and after the 1967 World's Fair in Montreal, architect Moshe Safdie's Habitat generated a lot of excitement. People were impressed more by Habitat's dramatic and complex form than by its innovative, but costly, method of prefabricated concrete construction.

Habitat was intended to suggest new directions for the design of multifamily housing, directions that promised more than conventional housing types -- garden apartments, high- or mid-rise slabs and row housing -- could deliver.

But Habitat was not without its precedents. It evoked images of settlements on hillsides in Greece, Italy or North Africa. Many of these settlements are collages of densely packed, cubic forms homogenous in scale, material, color and detail. Picturesque pedestrian passageways, narrow streets, plazas and courtyards seem to be carved out of the assemblage. Viewed as a whole, an entire town can appear to be a single, richly textured building wedded to the land.

Of course, something often is missing: the automobile. Cars may be excluded from entire sections of such towns. At Habitat, Safdie piled up cubes to create a kind of manufactured hillside; under the "hill" were voids for parking cars.

Well into the 1970s, architects continued admiring these innovative, romanticized housing models with their modular, visual complexity and relatively high density. Derivative apartment projects were built in cities such as London and Paris. Georgetown University built a dormitory, designed by John Carl Warnecke, that looks like a brick-faced, hillside habitat stepping down toward, and overlooking, the Potomac River.

The romantic, Mediterranean associations of Safdie's Habitat, along with its technological and economic implications, eventually were questioned. Not only did it seem impractical as a way of building, but it offered an architectural vision stylistically unrelated to most urban or suburban contexts.

Long before Habitat, architects had invented housing configurations that departed from the tried and true. In Renaissance and baroque cities of Europe, designers rejected the accidental picturesque quality of medieval cityscapes and collage building. Instead they sought to introduce rational spatial order into city streets and plazas. Nonmonumental buildings, especially housing, would constitute the in-fill required to shape and line public spaces.

This strategy resulted in villas and "condominiums" being configured on the exterior by the urban circumstances of their sites, while their interiors assumed their own spatial ordering. exterior massing and facades often told little about interior room arrangements and shapes. One could not look at a building and automatically deduce that it was an apartment building or the palazzo of an aristocrat.

Architects occasionally took familiar housing unit types and created new building types. One of the most notable examples is the terrace housing of John Nash, built in London in the early 1800s. Like his Renaissance and baroque predecessors, Nash recognized the importance of using buildings to shape urban spaces. Thus he didn't hesitate to assemble dozens of town-house units into much larger buildings, hundreds of yards in length.

Cumberland Terrace, edging the east side of a London park and fronting on a grand street, is more than 700 feet long. Classically detailed and finished with stucco, Nash's terrace housing has great presence. Yet each house within the row has its own identity, commencing with a small entry garden bordered by a low, unobtrusive cast iron fence. An approach walk from the public sidewalk leads through the garden to an elevated, elegant porch and front door. Carriage houses, the garages of yesteryear, are built at the rear with access from an alley roadway.

Historically, mixed-use development that includes housing also has produced special building types. Traditional European and Asian city streets often are lined by shops with apartments above, perhaps owned or occupied by the shopkeepers below. Periodically between shops are doorways, sometimes with little or no identification, leading to stairs to the residential floors.

Sometimes the second floor, directly over the shop, is used as office space, producing a more hospitable stratification of uses. With potentially noisy, less salubrious, intensely public retail activity at ground level, semi-public, more subdued office activity at intermediate levels serves as a transition to private, hopefully tranquil residential occupancy at the top.

The traditional housing-over-shops model has been stretched considerably in recent times. Today, where zoning permits, developers have erected sizable projects with many floors of apartments, either flats or duplexes, over floors of leasable office space, in turn perched over one or more levels of retail shopping, eateries and entertainment establishments. Like many urban buildings of pre-modern Europe, some contemporary projects may not reveal immediately where and how uses occur, or what the uses are.

Recently, developers and architects have recycled older buildings such as surplus schools, defunct factories and outmoded warehouses to create housing. Leaving the old shell intact, designers can insert a variety of apartment unit types, sometimes exploiting high ceilings or carving out interior courtyards and multilevel atrium spaces. With growing interest in historic preservation, plus the desirability of maintaining existing urban fabric, the number of such transformations is likely to increase.

Another kind of atypical housing blend occurs when projects interweave conventional housing types unconventionally. Harvard University's Peabody Terrace, graduate student housing on the Charles River in Cambridge, was designed by Jose Luis Sert in the 1960s.

Sert's plan, although hardly contextual, with its exposed concrete frame and in-fill panels of red, green, brown and white (Cambridge is mostly red brick and wood clapboard.), seemed at the time to break aesthetic ground.

It juxtaposed and visually integrated apartment building masses that ranged from low and horizontal to high and tower-like. Thin, tall structures seemed to grow innately out of low- and mid-rise structures surrounding grass-covered or paved courtyards. It was urban yet suburban all at once.

Tiber Island, part of D.C.'s Southwest urban renewal effort of the 1960s, was designed by Keyes, Lethbridge and Condon in a somewhat analagous way. Although more traditional in its use of building types, it, too, employed a common design vocabulary throughout to unify and enclose a variety of exterior spaces flanked by dwellings, from town houses to high-rise flats, all on one site. Like a Mediterranean village, Tiber Island can appear to be a single building of many layers and levels when seen from a distance.

Possibilities for new multifamily-housing types increase in the absence of restrictive zoning. Indeed, typical single-use zoning may discourage architectural innovation. Segregating land uses by housing type seems questionable given today's diverse lifestyles, mobility, technological amenities and design options. In fact, why zone by type at all? Parameters of density, not building type, should prevail.

A reader's letter recently posed an interesting question: Why not develop housing as part of -- perhaps on top of -- shopping centers? Why not explore new ways to weave more housing, and new types of housing, into downtown districts that become lifeless after working hours? Why not use more underutilized land or structures for housing, even if it is in nonresidential zones?

All that's needed are some changes in attitudes, some enlightened public policy and a few amended laws.

NEXT: Special streets.