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 trees and maybe even shipmunks and squirrels on top of its seven-story Washington headquarters.

The proposed rooftop refuge would be similiar 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, outside 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 the 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," said Hoke, "and the earth would protect the roof from thermal shock . . . the wind and weather that makes roof maintenance the largest expenses 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 swimming 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 envision that would be for wildlife only.

Interior's chief scientist, Theodore Sudia, says "it's a dynamite ecological idea 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 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 Kraiser Center in Oakland, Calif., which had a park and pool (but no fish or turtles) on top of its five-story garage, and the 23-story James Hudson, which opened last year in Lexington, Ky.

Hudson, who has been working with Hoke on the Interior rooftop proposal, 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 balustradeds.

"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 fit up to the eight story Dorcherster 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.

Hoke and Hudson first began working on the Interior rooftop proposal in 1972 with the blessing of bothe Interior officials and the General Services Administration, which designs, builds and maintains federal office buidings.

"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.