Owners of the Mayflower Hotel announced plans yesterday for a $25 million expansion and renovation of the eastern portion of the 54-year-old Washington institution, creating 407 luxury hotel rooms and such related new facilities as a health club and coffee shop.

At the same time, Kingdon Gould Jr., one of the principal owners, indicated that the western half of the Nayflower, fronting on Connecticut Avenue NW, most likely will be converted into a combined retail and office complex by the mid-1980s.

The net result would be a smaller but more affluent-oriented Mayflower, with about half the total number of 800 rooms available in the hotel today (100 of which are part of suites), several levels of stores with an underground connection to the subway and about a half-dozen floors of offices.

Gould emphasized at a news conference, however, that the owners are considering several options for the Connecticut Avenur half of the hotel, where a $2.4 million refurbishing is nearly completed.

"The most promising at this point... involves changing the western portion of the property from 448 hotel rooms to a mixed office building /retail use. Such a change, if undertaken, would involve fundamental reconstruction... we are not moving (on that plan) at this time," Gould told reporters in the Mayflower's Chinese Room -- which, like other main function facilities, will remain open throughout the proposed renovation.

Also, in any event, the 448 hotel rooms in the western tower of the hotel will remain in use while the other half of the Mayflower is closed for the modernization announced yesterday. And, a Mayflower Hotel will remain in existence -- albiet different in ambiance and customer appeal.

Because the hote, opened in 1925, is listed as a D.C. historic site, any plans to change its facade or for demolishing the building would have to be approved by a Joint Committee on Landmarks, a D.C.-U.S. board. Permits seeking government approval for renovation of the eastern wing (three towers along 17th and DeSales Streets NW) were filed with the city yesterday.

According to Gould and architect Vlastimil Koubec, hired to design the renovation:

Four floors of new rooms would be added on top of the eastern section, bringing it up to the level of the Connecticut Avenue tower. There would be 407 new hotel rooms, larger than those now in use "in keeping with anticipated trends in the luxury hotel industry for more spacious rooms."

New meeting and private dining rooms will be provided on the mezzanine and second floor, food facilities will be expanded, plumbing and mechanical facilities will be replaced, central air conditioning will replace room units and new elevators will be built.

A health club wil contain a pool, sauna and racquet ball court.

New locker rooms and a dining area for employes will be built, as will a new laundry.

Since the hotel opened, no major renovations have been undertaken. The decision by Gould and his partners to move at this time toward a luxury-type hotel comes in the wake of recent decisions to construct a similar hotel inside the shell of the historic Willard and for a 1,000-room, convention hotel by the Marriott Corp., along Pennsylvania Avenue.

The Mayflower will continue to be managed by Western International Hotels, a subsidiary of UAL Inc., owner of United Air Lines. Gerald Wolsborn, the Mayflower manager, said rates in the new luxury wing would be higher than those of his hotel today but he declined to make a specific forecast.

Single rooms now range between $47 and $71 a night and suites go for as much as $250, with about 70 percent of the rooms occupied, on average. Wolsborn said he did not expect rates for the new rooms to reach $100 a night -- a level some lodging experts have said will be charged at the new Willard. At least one D.C. luxury hotel, the small Dolley Madison, now charges $90 a night.

If approved by the D.C. government, Mayflower construction will begin this summer and be completed by 1981, at which time the owners will make a decision about the Connecticut Avenue front half. The renovation planned to start this year will duplicate the outside facade and use bricks from the same West Virginia plant used for the 1920s building.

Gould, former ambassador to The Netherlands, and his son own about 35 percent of the May-Wash Associates, a partnership that owns the building. The estate of William Cohen owns 50 percent, restauranteur Ulysses (Blackie) Augur owns 10 percent and parking lot owner Dominic F. Antonelli owns about 1 percent.

Permanent residents in the eastern wing -- 28 of them -- have been notified by Mayflower management that they will have to move. Untouched by the renovation plan is one western wing permanent resident, professional stockholder and annual meeting gadfly Evelyn Y. Davis.