What is a transferable development right? Is it something that might be found under a Christmas tree, something to fight for and defend, or could it be something to guard against?

According to whom you ask, it could be any of the three. A transferable development right (or TDR) is what you get when you detach the right to develop a piece of land from the land itself, and trade it as a commodity. The development right then can be sold to anyone developing land where increased densities are allowed, while land stripped of its development rights can be preserved.

The concept came into vogue as an approach to land-use planning sometime after World War II and has been tossed around in planning circles ever since. But there are problems with the concept, not the least of which is figuring out what it means, and most jurisdictions toss it out once they start looking at its complexities.

"TDR can mean many different things for many different people," said Edward Finnegan, attorney for Loudoun County, where farmers and planners are looking at the possibility of establishing a TDR program to protect farmland. "There's a whole range of options."

If the theme doesn't sound revolutionary, then consider the options: TDR program can be voluntary or mandatory. They can be used to preserve historic properties, farmland, open space, or anything else a community might want to preserve. They can be supported by local government to encourage people to use it, or they can be entwined in regulation to discourage overuse.

In the District a TDR program has been set up that is purposely restrictive in nature. Under the ordinance, development rights can be transfered only to another property in a city block, and only with the consent of all the property owners on the block. The TDR must move from one parcel to a parcel contiguous with it, and the overall density for the block can not be increased.

The first group that tried the TDR process spent seven years fighting in the courts for the right to sell their rights.

"It was pretty grim for a long time," said Perry Fisher, executive director for the Columbia Historical Society, a preservation group that wanted to sell the development right to its headquarters building in an effort to preserve it and raise revenue to maintain it.

The historical society finally won, despite repeat attacks from the Dupont Circle Citizens Association, which did not want the development rights used to increase density elsewhere on the block. The TDRs sold for $550,000 to the developers of an office building going in next to the society's historic mansion, and they used the TDRs to increase their building from 90 feet to 147 feet.

The District's TDR option has been used rarely, largely because it is restrictive. Other historical societies in the city say the program is too cumbersome for preserving individual buildings, and developers, who have tried to use it on commercial properties, agree.

The city council briefly considered a bill last year that would have allowed TDRs to be traded between parcels that were not contiguous, but the bill died a quiet death when city residents saw it would allow great numbers of TDRs to be assembled from areas where low-rise buildings such as town houses already were protected in historic districts.

The only other local jurisdiction actively using TDRs as a zoning tool is Montgomery County, where the second-largest TDR program in the United States is entering its second year of operation. The program is designed to protect farmland from the fragmentation of subdivision development and is purposely unrestrictive. The TDRs can be bought and sold on the open market and be used in any area that the county has designated for intense development.

Montgomery County also has a TDR program designed to help preserve historic properties, but it has not been as successful as the farm program because of restrictions that limit it to parcels that are contiguous, similar to the District's program. Dennis Canavan, a planning and zoning analyst with the county, said the county staff is considering opening up the TDR historic preservation program by extending it to commercial properties and allowing the TDRs to float freely between noncontiguous parcels as they do in the farmland preservation program.

The Montgomery farm TDR program has attracted the interest of farmers in Loudoun County, where rapid development has drawn thousands of acres out of farming over the past decade. But Finnegan, the county attorney, believes enabling legislation will be necessary before the county can start a mandatory program, and he said there is little guarantee that it would be easy to get such legislation out of the Virginia General Assembly.

Fairfax County staff considered, briefly, using a TDR program to protect land in the Occoquan watershed from development they feared would increase pollution in the county's resevoir. They abandoned the idea after talking with their representatives in Richmond, who told them such a revolutionary change in zoning law would be difficult to get through the legislature.

The Loudoun County Board of Supervisors recently adopted a voluntary TDR program in their management plan for the Leesburg area, but Finnegan said no one had taken advantage of it yet.

A group of citizens assembled by the county planning commission to suggest ways of preserving farmland has discussed the possibility of using a TDR program in Loudoun, but opinions among group members on the details of such a program are so far ranging it apepars unlikely the group will recommend a mandatory TDR program when they report to the planning commission next month. If the county does decide to pursue the idea, Finnegan said they probably would put together a specific program before approaching the state legislature.