The Washington Post plans to begin printing part of its daily press run at a new "satellite" plant -- probably in Fairfax County -- starting in the fall of 1980.

An agreement to buy three offset newspaper presses for the new plant from American and Japanese companies -- at an estimated cost of $20 million, including related equipment -- was announced yesterday.

Washington Post Co. President Mark Meagher said the firm has no solid estimate yet of the project's total cost. In addition, Fairfax County authorities must approve plans for construction of the plant in a strip of land zoned for industrial use near the Springfield area.

Currently, the Washington newspaper has nine press lines of eight units each at its downtown headquarters in the District, but these presses are not capable of turning out enough papers to meet increasing circulation and advertising volume.

According to Donald Graham, general manager of The Post, a maximum of 540,000 newspapers can be produced now between the hours of 12:30 a.m. and 4:30 a.m., when The Post's late-news edition is on the presses.

Daily circulation of the newspaper in the six months ended last Sept. 30 averaged 559,000, a gain of 18,000 over the previous comparable period. New printing capacity is necessary to print more newspapers and more papers with later news, Graham emphasized in an interview.

Each of the three, eight-unit presses ordered yesterday can produce 128-page newspapers at a rate of up to 75,000 copies an hour. Current press capacity is limited to 112 pages.

Graham said The Post expects to print up to 200,000 late-edition copies a day at the Fairfax plant, once trial runs are completed.

"I hope we're going to take advantage of the improved technology to bring much later news than now to our readers," he added, explaining that many of the papers for circulation in Northern Virginia would come from the Fairfax plant while papers for D.C. and the Maryland suburbs would continue to be printed downtown, with all subscribers benefiting from The Post's new ability to print more papers at a later hour.

Two of the presses have been ordered from the Goss division of Rockwell International, a leading U.S. press equipment manufacturer, while the third will be built by Tokyo Kikai Seisakusho, a Japanese firm that has built presses for Far East newspapers during much of this century.

The Japanese firm is a relatively new entrant in the U.S. newspaper production market and its press for The Post will be the largest the company has manufactured for an American paper.

Delivery of the three new presses is expected early in 1980 and they will be operational that fall.

A Washington Post Co. subsidiary, Robinson Terminal Warehouse Corp. of Alexandria, already owns 27 acres on Wimsatt Road, near the intersection of the Capital Beltway and Interstate 395, which is being considered for the new printing plant. The Post would occupy a portion of this land, for which the county must approve subdivision.

In addition, the Post Co. owns land in Montgomery County near the Beltway, which could be the site for a future suburban press plant. Graham said he could not forecast when there would be a need for a second suburban plant but he did say that once the Fairfax presses are rolling, The Post will shut down some of its downtown press facilities on a temporary basis to modernize existing facilities and install technology to improve the quality of newspaper printing.

Unlike the existing presses, which use a direct letterpress technology, the three new press lines will produce papers by a lithographic process known as offset -- with substantially improved printing quality in both black and white and full color.

Graham said no decision has yet been made on what technology will be used to transmit finished newspaper pages from the downtown plant to the Fairfax facility. One newspaper that prints editions at many locations around the nation, The Wall Street Journal, uses several methods -- including satellite transmission. Other papers with suburban plants to print some of their press runs include The New York Times and The New York Daily News.

The Post's new plant in Fairfax would include a pressroom, mailroom and warehouse facility but no business or editorial offices. Graham emphasized that "The Post is dedicated to staying here" at its 1150 15th St. NW plant into which the newspaper expanded earlier this decade for its sixth home in a century of daily publication.

He described the Fairfax facility as a "small addition to our manufacturing plant" and noted that its location includes on-site trucking and rail connections for delivery of newsprint -- some of which will be produced in future years from a Virginia factory in which The Post and Wall Street Journal have an interest. A site plan for the project will be filed with Fairfax County by early March, with construction scheduled to start in the spring, if approved by the county.

All current Post presses were manufactured by Goss but only two are less than 18 years old.Because of heavy demands for new offset presses, Goss could sell The Post only two for delivery in 1980, Graham said. As a result, the Japanese firm's presses were studied extensively over the past year.

Tokyo Kikai Seisakusho will first build the press in its own factory next fall, so Post Co. personnel can run it there prior to shipping.

David Edwards, executive director of the Fairfax County Economic Development Authority, said yesterday he knows of no potential roadblocks that would prevent construction of the plant. The Post Co. and Fairfax officials have been talking about potential sites for months, and Edwards said, "The Washington Post is the kind of firm we like to have."

Graham said the newspaper "probably will hire some new people... I don't know how many," for the Fairfax facility, and Meagher said it is too early to know how many workers will be needed.

A Boston firm, Charles T. Main, is working on a preliminary design for the satellite printing plant.