The protectors of the great outdoors and the builders of the great indoors, after years of arch-rivalry, have found some common ground.

The Sierra Club and the National Association of Home Builders have issued a joint statement on land use, an eight-paragraph document that took slightly more than a year to negotiate.

It calls for building needed housing as infill on "appropriate vacant land within urban and adjacent suburban areas at densities sufficient to encourage cost-effective transit service," as well as energy-efficient building designs and water conservation measures.

Basically, this means they want to see developers build on lots that have been skipped over or have deteriorating structures on them in and around urban areas.

Asked if such a joint statement would have been possible five years ago, when builders were taking a different direction, Larry McBennett of the homebuilders' group said, "I doubt it."

Five years ago, builders were focusing on large, sprawling suburban communities--just the kind of development the Sierra Club opposed.

The homebuilders hope the statement will help convince local zoning officials to approve higher densities and infill developments in and around cities, a spokesman for the homebuilders' group said.

Officials of the two groups credit a new conciliatory attitude on both of their parts for their ability to find areas of agreement.

But the accord is just as much a sign of a changing housing market.

Builders have downsized homes and added energy-saving features to cut down on costs and to satisfy the demands of smaller families with active life stylesthat do not lend themselves to caring for large, rambling homes.

"The market is changing, no doubt about it, and builders are adapting to that," said McBennett, adding that the Sierra Club project had become a pet project of his. "The cost factor is driving builders to other things, and these happen to coincide with what the Sierra Club believes."

NAHB spokesman Jay Shackford agreed: "Old development patterns are obsolete. People want more town houses, more duplexes, things like that. These take less land, so they preserve open spaces."

Joe Fontaine, president of the Sierra Club, also said developers have made a significant conversion to an attitude that coincides more with his group's views. But attributes it more to builders' increased awareness of environmental issues than buyers' lowered expectations.

"Over the period of years in the 1970s, more of our opponents started listening to what we had to say about environmental concerns," Fontaine said from his home in San Francisco. "They have begun to realize this not just because their industry is depressed, but also because of a new social consciousness about the environment."

At the same time, the practicality of building smaller homes may have spurred developers' new partiality to environmental concerns, he added.

"Things have changed an awful lot in just one year. . . . The cost of energy has gone up so much, it may have been a stimulus" to the joint statement, Fontaine said.

The goal of the statement received the approval of another environmental group, the Friends of the Earth.

"There is a tremendous amount of potential for rebuilding inner cities," said Friends of the Earth official Connie Parrish. Infill "would help meet people's needs for housing but also would improve the quality of life in the cities" by replacing deteriorating structures with new or rehabilitated buildings, she said.

The conversations between the two groups were first started between Fontaine and Merrill Butler, NAHB president at the end of 1980 when the discussions began. Officials of the two groups say both presidents brought to their offices a philosophy of negotiation rather than confrontation with organizations with which they had not been traditionally aligned, and the goals of both began to converge somewhat.

The statement's reference to "urban and adjacent suburban areas," on which the two groups are endorsing infill development, was a key compromise, McBennett said. The homebuilders insisted that suburban areas be included, while the Sierra Club wanted to make the point that it could not support "urban sprawl."

In addition to the infill recommendations, the groups call for "compatible mixes" of housing, commerce and industry; development of cost-effective transit service and government acquisition of parks and open spaces.

"These development patterns would conserve energy, water, land and building materials. Such patterns would improve air quality and make better use of existing urban infrastructure. Additionally, these patterns would reduce development in forest lands, agricultural lands, wetlands and other natural areas," the statement says.

The groups urge local governments to reduce delays in approving this kind of development and to adopt incentives for higher densities and more efficient site design for infill areas.

The statement will be used as a selling point to local leaders considering zoning changes to accommodate this type of development, McBennett said.