Among the innovations championed by the New Urbanist or neo-traditional movement, and by many other architects and planners, are "form-based" zoning codes.

The primary goal of form-based codes is to guide the configuration and architectural quality of urban and suburban environments. That contrasts with conventional zoning, which often concentrates on the use of buildings, such as whether a block is residential or commercial.

Judging from the aesthetic dysfunction in much of what we have built, form-based codes are long overdue.

Actually, we should eliminate the term zoning. It implies separation, exclusion and disconnection, and it suggests nothing positive about how neighborhoods and buildings should look or relate to each other.

In Houston, a city without zoning, the term is taboo. In a recent report recommending new housing strategies for the city, I wrote that, in America, "planning is not zoning, and zoning is not planning. Conventional zoning generally has failed as an effective planning tool for creating balanced growth, good urban design, beautiful cityscapes, or affordable housing. In many jurisdictions, the effect of zoning has been to exclude the less affluent."

To further convince Houstonians of the merits of planning and land-use regulation, the report went on to state that "unconstrained by conventional zoning regulations, Houston has a unique opportunity that no other American city has: it can undertake effective planning not trumped or compromised by existing zoning."

Conventional zoning ordinances divide a municipality or county into zones, define and designate the land use for each zone and stipulate for each zone and zoning category maximum densities and building heights, maximum lot coverage and minimum setback, yard and lot dimensions. Zoning regulations also often require minimum parking accommodations.

Historically, the purpose of zoning has been less as an urban design tool and more as a way to protect public health, safety and welfare, and private property values. Zoning presumably maps the future. Yet for many property owners, zoning's primary benefit is ensuring that potentially harmful, incompatible uses of neighboring properties will not threaten their properties and their legitimate uses.

But zoning codes, frequently drafted by lawyers rather than designers, tend to be too free and flexible where more guidance is needed and too limiting where flexibility is appropriate. Typically, the most constraining inflexibility concerns zone boundaries and use limitations, especially prohibitions against mixed-use development. The most problematic over-flexibility is the lack of clear criteria to guide site planning, streetscape design, building massing and architectural form.

Consequently, conventional zoning has produced patchwork quilts of single-use districts and private enclaves, often with minimal vehicular, pedestrian or visual connections between neighboring zones. It guarantees automobile dependency and, within neighborhoods, reinforces socioeconomic homogeneity and isolation from other neighborhoods.

In contrast, form-based codes, while allowing great freedom in determining uses, establish definitive criteria for essential urban and architectural design elements: street network and block patterns; layout and configuration of public open space for parks and plazas; disposition of buildings, drives and parking; and height, volume and even facade design requirements for both general building types and special buildings -- civic and institutional structures, schools, retail shopping, entertainment and sports venues. Unlike conventional zoning, they seek to erase rather than reinforce boundaries.

Form-based codes also regulate density, but they specify minimum as well as maximum densities and include reasonable provisions for increasing density where justifiable, such as near public transit stops.

Regrettably, implementing form-based zoning is not easy.

First, because long-standing zoning ordinances create vested property rights and values, any changes must take those rights and values into account and, to a reasonable extent, preserve them. Otherwise, changes can be confiscatory and unconstitutional. Yet preservation of the status quo, based on zoning maps drawn many decades ago, can seriously conflict with current planning and development aspirations.

Second, because land-use planning precedes land-use regulation, form-based codes must be based on well-researched, comprehensive master plans. Without such plans, there is no rational way to establish and legally justify form-based urban design. However, many jurisdictions have neither the resources nor the planning staff necessary.

Third, because some state charters and statutes -- for example, in Virginia, Pennsylvania and Texas -- limit local government's ability to tinker with land-use regulation and thus infringe on property rights, imposing requirements beyond those already in place can require state legislative approval.

Finally, because the concept of form-based codes is unfamiliar, a city or county cannot enact such a measure without first helping residents understand the concept. That requires diligent explanation, illustration and demonstration, including comparing alternative development scenarios that show what conventional zoning yields with what results from form-based zoning.

Probably the most feasible strategy is to make new plans for particular sectors within a jurisdiction where intense growth is desired or is likely to occur as a result of development trends, zoning or both. Form-based design and development criteria can then be applied as a zoning overlay, without rescinding existing ordinances or drawing totally new zoning maps.

A good example of that approach is the Carlyle neighborhood in Alexandria, a multi-block development south of Duke Street and west of Old Town. Now nearing completion after nearly 15 years of construction, Carlyle's overall urban pattern and architectural character are governed by a detailed master plan and form-based design criteria. The Alexandria City Council approved them as a special-purpose zoning overlay before a single spade of dirt was turned.

Form-based design regulations, rather than old-fashioned zoning, are shaping the redevelopment of the centers of Silver Spring and Rockville.

Fortunately, city and county planning officials throughout the United States, along with urban designers and architects, are increasingly acquainted with form-based codes. At the same time, there are more examples of new development shaped by form-based regulations instead of conventional zoning.

Thus, despite impediments, persuading constituents to embrace form-based codes should get easier.

Roger K. Lewis is a practicing architect and a professor of architecture at the University of Maryland.