Entering the 78-year-old home of the New Playwrights' Theater, one catches that instant sense of anticipation which is an essence of theatergoing.

The building at 1742 Church St. NW began as a gymnasium for the posh Holton-Arms School and later was the Evelyn Davis Dance Studio. Church Street has an individual air. At the 18th Street end is St. Thomas' Church, where F.D.R. and Eleanor Roosevelt once worshiped.Only its arches and aisles remain after a fire set by arsonists. There are shade trees, private homes and the throb of nearby Dupont Circle.

What an individual, raffish entry NPT presents. It's in the middle of the block with worn wood, world-weary bricks and, just beyond the entry, an arena where seating may be on planks or on chairs arranged in circles, rectangles, ovals or squares. One never can be certain what form the auditorium will take. The anticipatory question always arises: What will the evening be like this time?

Not only is the building unique, so is its premise. NPT stages only new plays in what it guardedly calls either "full productions" or simply, on Mondays and Tuesdays when the season is in full swing, "readings." And the tastes of producing director Harry Bagdasian and artistic director Paul Hildebrand Jr. are heartily catholic. Anything goes - raw drama, historical addenda, farce or musical reviews, so long as the honchos have been intrigued.

Atmospheres and audiences are peculiarly Washington, a city to which most of its residents have propelled themselves from all over for any number of reasons. This is, after all, every American's city, and those who come to stay, for a few years or a lifetime, have the zest to search out new facets and interests within themselves. One of the places passivists and activists have been discovering is NPT.

However, both Bagdasian and Hildebrand are locals from way back who know the territory and think of their theater as serving a unique role in the area.

"It's dizzying to realize how this place has ballooned from a simple idea," says Bagdasian. "We opened with 24 seats in that basement room on 20th Street and now we're into our fourth year here. Six times as many seats. Originally we had two subscribers. Last season we had 600, and once fall starts I hope we'll widen our audiences. There's no way for a theater to stand still. It expands or it fades.

"Playwrights are what we're about. The initial aim seemed so simple. We graduated from the University of Maryland and wanted to grow as creators of plays. But we've had to do so many other things, seemingly unrelated.

"That first night, Nov. 16, 1972, we began with one-act plays and since have done over 45 staged productions, readings and, of course, our Dramathons. We've found some solid playwrights. T.J. Camp, one of our first, is still writing in California, in fact just tossed over a good-paying movie job because he thought he was slaving on junk. And there's Ernest Joselovitz, author of 'Hagar's Children' and our resident playwright through next April.

"But sometimes it seems as though instead of putting every moment and atom of energy into our playwrights, we've had to do other things. I've learned about real estate and banking. Thanks to very good friends and understanding financial people, we made a down payment and have a mortgage here.

"There's the world of grants and how to get them. Certain things are expected but they have to be set forth, made clear. Rising costs of everything, energy above all, can make mockery of budgets.

"We've developed a good level of acting, I think, though that wasn't our major goal. Still, you can't really help develop playwrights if you can't get fine actors for their plays. It's not fair to either. As others have said, actors support our theater. They work at anything to support themselves or to build up a stake so they have time for acting.

"Those practical things are so often at the top of our minds around here.

"May be it does make getting to the scripts themselves more rewarding. Anyway, that's the exciting time, the rewarding time creatively, when we're all occupied with a new play.

"Our development here began with dialogue. So many of our early plays were strong on dialogue. What we failed to get at was structure. It's a failing of most modern plays, all dialogue and character, almost no structure.

"That's what we're bearing down on now. We're just back from a three-week playwrights' workshop in Staunton, Va. We work with Theater Wagon, the Shenandoah company. This year David Young, of the American College Theater Festival, worked with us, bringing grants from McDonald's, Norman Lear and the David Library to bring three winning ACTF playwrights for our Staunton Retreat. We worked over all their plays, to advantage, I think.

"The Virginia Foundation for the Humanities, Public Policy and Theater Wagon sponsored a colloquium on 'Freedom and Community,' during which we presented the new work which will start our season here Oct. 4.

"This is called 'A Whitman Sonata ,' which Paul and Thom Wagner have drawn from the life and works of Walt Whitman. The play's in the style of a country get-together, with music, tall tales, stories and country dancing. Wagner, our resident music director, has scored his work for a 10-piece orchestra.

"In November we'll do the new one by Joselovitz, which we introduced in our reading series last year. This is 'Splendid Rebels,' about Emma Goldman's fight for conscientious objectors of World War I against the young J. Edgar Hoover. For the Christmas holidays we'll have my return to playwriting, a new adaptation of Dickens' 'A Christmas Carol.'

"Mark Stein's 'Pinnacle' was respected last season and now he has a new, related play, 'Breaking the Sweet Glass,' to be followed by what we hope will be Tim Grundmann's 'Eddie's Matchy Tunes.' He's been working on that for two years but his 'Sirocco,' 'Bride of Sirocco.' 'Nightmare!' and 'Out to Lunch' got in the way.

"There are other new ones for this year and the dates of our Dramathon are set, March 30, 31 and April 1. Having started that madness, we can't easily drop it."

Bagdasian sighs. NPT's Dramathon is three non-stop days and two nights of plays and readings, all drawn from current and past works. It's the year's major fund-raiser. Getting the performers, staff and public together is a logistical puzzle of chaotic potentials. But that's the way it is at NPT. It's in a class by itself, the least predictable theater for miles around.