In a specially created suite of offices in the southwest corner of the State Department is one of the newest outposts of American diplomacy: the Office of Foreign Missions, which has been established to do unto others what has been done unto U.S. diplomats abroad.

Set up in keeping with last year's Foreign Missions Act, the new office recently surveyed the nearly 300 U.S. embassies and other missions overseas to learn what problems they encounter in daily life as well as in their official functions.

The answers spoke of many travails, including poor living conditions, high taxes, restricted travel, the high cost of calling a plumber through a special bureau set up for foreigners, and incidents of harassment or worse in some parts of the world.

On Jan. 14 the new unit made its first move to put the U.S. government in position to respond in kind. A "circular diplomatic note" to the heads of all foreign diplomatic missions in Washington requires notice to the State Department any time they want to buy, sell or rent property in the United States. In deciding whether to approve, the State Department may "bring to the attention" of foreign mission officials any problems U.S. diplomats encounter in their country.

The department is considering a number of other actions. For example, it might require foreign embassies to go through the Office of Foreign Missions when they want to hire plumbers or other tradesmen to work on their property here.

This "tit-for-tat" policy will be directed by a small staff, now 10 people. But some members of Congress have questioned the choice for the office's director: James E. Nolan Jr., a retired FBI counterintelligence veteran.

The lawmakers have no quarrel with the way the new unit is pursuing its job. But there are doubts whether the selection of a former intelligence official will start the office on the right foot, either in the department or in the Washington diplomatic community.

STUNTED GROWTH? . . . The economist in the big office on the seventh floor, Secretary George P. Shultz, said he was "startled" to learn after taking office that the number of State Department employes for fiscal 1984 will be just about the same as 20 years ago, despite "the considerable growth of responsibilities around the world."

The actual numbers, according to department figures, are 24,400 full-time employes around the world for the next budget year, compared with 24,668 for fiscal 1964.

"The State Department hasn't been a growth industry," remarked Rep. Joel Pritchard (R-Wash.) at a recent House Foreign Affairs Committee hearing. In the past, Pritchard said, "there has been a certain amount of backing off" from the needs of American diplomacy.

DIPLOMATIC CROPS? . . . One way to grow bigger has been proposed by Sen. David H. Pryor (D-Ark.), who has introduced a bill to create the post of undersecretary of state for agricultural affairs (not to be confused with the undersecretary of agriculture for international affairs and commodity programs). Pryor's point is that foreign policy decisions affect farmers, without farmers having much effect on foreign policy decisions.

Shultz has expressed concern about the development of an agricultural trade war between the United States and its allies. But it isn't clear whether he is ready to expand his striped-pants aides to include a new undersecretary in overalls.