It may not be the Hanging Gardens of Babylon, one of the Seven Wonders of the ancient World, but the Department of Interior is considering a rooftop garden and wildlife habitat with a trees and maybe even chipmunks and squirrels on top of its seven-story Washington headquarters.

The proposed rooftop refuge would be similar to the popular marsh ponds that were created out of some of the dozens of stagnant reflecting pools around the nation's capital during the past eight years. They are filled with flowering water hyacinths and lilies, wild rice, fish and turtles and occasionally sea gulls and wild ducks.

A wild mallard hen this summer raised six ducklings on the small former reflecting pool beside the Simon Bolivar statue at the busy intersection of 18th and C streets NW, ourside the entrance to Interior and Constitution Hall. The ducklings were hatched across the street in the bushes of the Pan American Union, with the mother mallard and her brood frequently stopping traffic to cross to the Bolivar pond.

Turning barren city pools into rural ponds teeming with plant and animal life has been one of the pet projects of National Park Service urban parks specialist John Hoke. His proposal to do the same thing eight stories up on top of the huge Interior building, and if it's successful on top of other government buildings here and around the country, is not only enthusiastically endorsed by government wildlife experts but could save thousands of dollars in annual roof repair and heating bills, says Hoke.

The "skin of earth would act as insulation, just as the earth roof of a yurt keeps it warm in winter and cool in summer," says Hoke, "and the earth would protect the roof from thermal shock . . . the wind and weather that makes roof maintenance the largest expense in most government buildings."

Although the earth would be 6 to 8 feet deep in hills where trees would be planted, Interior and most large buildings here could easily support the weight, Hoke said. The pond needn't be deep for fish or fowl, as the turtles, fish and ducks in the Bolivar pond get along swimmingly in less than 22 inches of water.

Hoke estimates it would cost less than $150,000 to create a rooftop park and pond on one of the dozen wings of the Interior building. While it would have paths and benches for employees, Hoke also envisions rooftop refuges on the other wings or buildings which would be for wildlife only.

Aelred Geis, urban wildlife specialist for the U.S. Fish and Wildlife Service, says "I'd love to see either kind of rooftop refuge. There are a lot of migrating birds over Washington, and some will end up on top of Interior, at least we think they would. It's an interesting question whether ducks and geese and other birds would land up there. Perhaps some of my enthusiasm is prompted by my curiosity. But it's a unique opportunity to create a wildlife habitat in the city, not just finding and preserving existing habitats."

Interior's chief scientist, Theodore Sudia, says "it's a dynamite ecological idea and fantastic opportunity . . . on rooftops there are thousands of unutilized acres of land. Such refuges could bring all sorts of birds into Washington even bring back peregrine falcons. One could get misty-eyed over the whole thing. And the rewarding feature is that it would reduce roof maintenance and cut the costs of heating and air-conditioning . . . not much but some."

While many buildings in the U.S. have heavy roof-top swimming pools and small trees and shrubs planted in boxes, few have the tons of dirt and grass and trees, and muddy fish-filled ponds that Hoke is proposing. Among the best known are the Kaiser Center in Oakland, Calif., which has a park and pool (but no fish or turtles) on top of its five-story garage, and the 23-story apartment building of Virginia architect James Hudson which opened last year in Lexington, Ky.

Hudson, who has been working with Hoke on the Interior rooftop proposal garden not only created a huge roof garden with more than 35 tons of dirt, groves of aspen and dogwood and large grassy areas on top of the Lexington building, but had dirt gardens included in the balconies of every apartment - where tenants have vegetable gardens and flowering plants cascade over the balustrades.

"And there's no question about birds landing on a high-rise roof garden. We've got hundreds up there, many of them nesting in the trees and bushes."

Birds also frequently flit up to the eight-story Dorchester Gardens apartment house on Columbia Pike in Arlington, the only Washington-area building with grassy lawns, flower beds, trees and a full-time gardener on its roof. Built in 1960 the small roof garden has had no problems in 17 years outside of one or two easily-fixed leaks at the edge of the flagstone terrace, said building manager R. R. Tschantz. "The tenants love it. They're up there all the time sunbathing on the grass or sitting in lawn chairs." Although the building's "lawn" is planted on one-foot of dirt , the earth is light because it is half vermiculite, says Tschantz.

Hoke and Hudson first began working on the Interior rooftop proposal in 1972 with the blessing of both Interior officials and the General Services Administration, which designs, builds and maintans federal office buildings. "But that was when Watergate occurred," said Hudson, "and the agencies had too many problems to think about rooftop gardens."

The Carter Administration and new officials at Interior are again looking up to the roof of the huge 1937 building that originally had patios and a snack bar for its employees and was designed for such use. The patios were removed in World War II, when anti-aircraft guns were mounted in their place. The 50-caliber guns made history on Sept. 3, 1942, when one went off as a soldier allegedly was cleaning it, and three bullets hit and chipped the top of the Lincoln Memorial, leaving the only military bullet scars in the nation's capital since the British sacked the city in 1814.

"The roofs of Washington are a vast undiscovered country," says Hoke, "and here we have an opportunity to put back, eight, 10, 15 stories up in the air the natural environment we destroyed and stripped bare on the ground."