Marriott Corp. said yesterday it has agreed to sell 29 of its fancy restaurants, including the Washington landmark Hogate's, to W. R. Grace & Co., for an undisclosed amount.

The sales is part of a Marriott plan to withdraw from the sit-down restaurant business and concentrate on fast food, spokesmen said.

Marriott will sell 13 Charley Brown's restaurants in the West, 10 Charley's Place restaurants in the East and six others: Hogate's and Barley Mow in the District, Carnegie's and H. I. Ribster in Virginia, Phineas in Rockville and Joshua Tree in Hartford, Conn.

In September, Marriott's Host International division agreed to sell its 24 Casa Maria Mexican restarants to El Torito Restaurants Inc., a majority-owned subsidiary of Grace.

"We feel the specialty restaurant business is not keeping with our long-term corporate strategy, which is to concentrate our restaurant expertise on Bob's Big Boy and Roy Rogers fast-foods establishments," said Marriott spokesman Robert T. Souers.

Last year, Marriott began the process of narrowing the focus of its restaurant division by disposing of its Gino's restaurants, by selling some and converting others into Roy Rogers.

The Marriott restaurant division contributed $679 million, or 22 percent, of the food and lodging company's $3 billion sales in 1983.

Grace, a New York-based conglomerate, said the acquisitions fit its strategy of increasing its restaurant market presence in both California and the Northeast, where it currently operates a variety of dinner houses.

Marriott and Grace said the proposed acquistion has been approved by the boards of both companies and is subject to execution of a final definitive purchase agreement.