When the first U.S. ambassador accredited to wintry and remote Mongolia goes to his or her post later this year, he will not need an airplane, just an elevator.

In an unusual arrangement, the new envoy, not yet selected by President Reagan, will be posted to a desk at the State Department instead of residence in the Mongolian capital of Ulan Bator, department officials said.

"There's no recent case that I know of, of an American ambassador assigned to a country who operated out of a State Department office on a regular basis," said one of several department officials working on plans for Mongolia.

"That's the plan in this case, for some special reasons, mostly financial, and . . . common sense," the official said.

Planning for the new embassy in Ulan Bator became necessary last January when the United States established diplomatic ties with the Mongolian People's Republic, a communist nation with limited contacts outside the Soviet orbit.

For the first 63 years of Mongolia's independence, the United States saw no pressing need to establish ties with the nation of 1.9 million people. Relations finally were forged under a policy of reaching out to communist countries in hopes of interesting them in voices other than those from Moscow.

The decision to establish an ambassadorless embassy grew from the State Department's desire to avoid the costs of a high-level mission to Ulan Bator at a time when it is closing other missions to save money.

By diplomatic tradition, ambassadors are given expensive trappings -- a big car, servants and a house. With only a charge d'affaires in Ulan Bator, the U.S. establishment will consist initially of two apartment-offices leased from the Foreign Ministry and costing about $20,000 a year.

The ambassador, who would make periodic trips to Ulan Bator and be accredited to that government, most likely will be a high-level State Department official who would continue doing his or her current job and carry the additional title of ambassador to Mongolia, the officials said.

Life for Americans who go there will be tough, according to an official who spent several weeks in the Mongolian capital.

There is not much for foreigners to do in Ulan Bator, although it has a population of 500,000. Foreign contacts with ordinary citizens are discouraged by the government; relations with envoys from other communist lands are formal, and the remaining diplomatic corps is tiny, comprising only Indians, Japanese and British.

Except when special permission is granted, Americans will be restricted to the capital and its environs, a radius of about 45 miles.

The weather is wintry for all but two or three months of the year and, at the coldest times, the wind from Siberia whips temperatures to far below zero. But snow is rare.

For the few U.S. diplomats in Ulan Bator, privacy will be a luxury. It would be too expensive to make the apartments surveillance-free, and, because all international telephone and telex lines are routed through Moscow, eavesdropping is expected.

Any secrets bound for Washington would be taken as messages during rest-and-recuperation trips or by regular couriers bringing food and amenities from Beijing.

The food would be necessary because the State Department does not expect its emissaries to subsist on what one official called "the typical Mongolian diet of mutton, mutton and more mutton of varying ages, and different kinds of fermented milk products."

"I suppose if you like yogurt you could manage," one official said. Fruit and vegetables are not part of the traditional Mongolian diet and are available only in late summer.

British diplomats who subsist on food packages from Beijing installed a greenhouse to grow fruit and vegetables, and the Americans are likely to build one, too, the official said.

"I don't blame the State Department for not sending an ambassador," said Denis Sinor, president of the Mongolia Society based at Indiana University. "It is not a dreamland. There is no reason to send a U.S. ambassador to Mongolia to go berserk with nothing to do."

U.S. diplomats who go there would provide consular services, if necessary, to about 500 Americans who travel to Mongolia annually. Some visitors are hunters seeking rare bighorn sheep, but most travel on package tours from Moscow.