WITH $750 INVESTED, we thought it might just be a 90-night fling," says John Kinnamon of Burn Brae Dinner Theater, which is about to celebrate its 10th anniversary.

"Why not a first class chef?" thought Frank Matthews as he prepared to open Hayloft Dinner Theater six years ago.

Both men are at the top of the Washington area's two most successful dinner theaters. The dinner theater is an enterprise that nationwide now employs more performers than any other branch of live theater. Even Broadway, considering crassly commercial, sometimes enjoys non-profit status.

The dinner theater is the only growing area of American commercial theater. It gets no tax advantages, no foundation grants, no city, state or federal funds. It exists for whatever public it can develop, just as the companies of Shakespeare and Moliere did. The aim is to please audiences.

The bait is the convenience of one stop for drinks, food and entertainment and a night out at a comparatively modest cost. Most patrons do not regularly attend live theater, but good dinner theaters can push them toward more traditional theater-going.

On Wednesday, partners Kinnamon and Bernard Levin, the producers and owners of Burn Brae, will celebrate the theater's anniversary in what before it entered show business was the faltering Burn Brae's Swim Club in Burtonsville, Md.

As a mark of his success, Matthews has just added Starloft Cabaret to the Hayloft in Manassas, Va., which is currently offering a splendidly presented Harold Arlen review, "Get Happy," with more modest fare at lower prices than the dinner and "Same Time, Next Year" offered downstairs.

Both theaters have been developing audiences who return for every change of bill. Gaithersburg's Harlequin is the third in the area's major trio for chow-'n'-show.

Though Columbia's Garland with its fare of sex farces was earlier on the scene, it was Burn Brae that developed the Washington market, an act of faith following the demise of in-town's ALOC, the American Light Opera Company, of which Kinnamon had been a major performer.

A handsome fellow in his 20s, Kinnamon earned his living as a director for NBC-WRC-TV, but using his smooth baritone on state became his heady avocation. He was convinced that the solid voices and performers developed by ALOC needed a showcase.

The quality of Burn Brae's productions soon brought offers to produce musicals in Virginia, Texas, Florida and New Jersey.

Now, without having to run the catering, which Levin oversees in Burtonsville, the firm produces for three Philadelphia-area dinner theaters, the 900-seat City Line, the in-town Riverfront and Mickey Rooney's Downingtown Inn. Each of the four theaters now mounts two productions a year for six-month runs. At Burn Brae "1776" put them off schedule. It ran 18 months during the Bicentennial, an East Coast record. All of which means that Kinnamon and Levin produce eight musicals annually, a high average.

From that initial $750 invested in Burn Brae, the partners have poured back almost every box office cent into their plant, productions and food. From 150 initial seats, structural alterations have expanded the place to seat 350 and more expansion is on the calendar.

The opening bill on May 24, 1968, was "Brigadoon" and, as I recall, ham on beaten biscuits. Currently it's "Show Boat," but the 10th anniversary audience, which for this night will include scores of former performers, will find a spread that progresses from clams and oysters to mountainous, gooey desserts.

For that, the Burn Brae audience has the rival Hayloft's Frank Matthews to thank.

Matthews, whose nontheatrical activities have included inventing a germ-free - and evidently profitable - hospital bed, had been president of the amateur Reston Players when he decided to look into dinner theaters. A Naval Academy graduate, he explored the Washington area systematically, finally settling on an abandoned carpet factory building in Manassas for a dead-of-winter opening six years ago.

Matthews had studied the area's demography, learning - and using - how long it would take patrons to rim the Beltway and head out to his location off Route 66. He saw a potentially vast crowd for 300 seats. And he instituted a subscription plan with savings for the faithful.

"They call them dinner theaters, but where's the dinner?" Matthews asked. He set out to find a first-class chef, and from the Belgian Embassy lured young Georges Richa.

Matthews encouraged the Lebanese-born Richa to make his buffet a show in itself - curtains, spotlights, trumpets and all - and gave him equal billing with the stage attraction. Instantly the other area theaters had to improve their menus.

Hayloft and Burn Brae differ in their approach to Equity, the stage performers' union. Hayloft works under union regulations; Burn Brae does not. Across the country about 70 dinner theaters use one of four Equity contracts, depending on their seating capacity.But there are far, far more non-Equity dinner theaters. The best of them and the Equity theaters are linked into ADTI, the American Dinner Theater Institute, of which Matthews is a former president.

In the Equity vs. non-Equity national conflict, salaries are not the major dispute. Equity doesn't like stage personnel waiting on table, though exceptions are made for interns-in-training - tips then can be absolutely vital when working for peanuts.

Working conditions are a major reason dinner theaters prefer to stay out of the union. "When we have a show to get on in two nights, we can't stop for a break every three hours. Our plants, almost always adapted from previous uses, don't allow for the specific square foot of dressing room space and shower facilities demanded by Equity," observes Kinnamon.

"As it is, our salary scales are often higher, far higher, for principals, than Equity's. It's very complex but even the four Equity categories aren't elastic enough for what we've been developing. Most of our people have daytime jobs or are on leave from them.Intended to protect, Equity regulations sometimes end jobs."

High Art is the dinner theater dream but not its immediate aim. There have been efforts at O'Neill, Albee and Williams. In some areas, especially here, popular musicals are the top draw. These also have their drawbacks, a half-dozen musicians never capturing the orchestral detail of the originals.

Kinnamon has been experimenting, effectively I think, with taped orchestras for his live singers, a trick introduced locally by that sou-saving Frenchman, Maurice Chevalier.Since most of the dinner theater bands are amplified anyway, the sound comes out electrified. With quality taping, Kinnamon's conductors work with orchestras as large as 40 pieces, at a cost of up to $16,000 for a full recording session.That's a big dent in the budget, but the orchestrations can be rich. And tapes are lasting.

Sex farces or even light comedies may flop on Broadway but seem to be manna to a dominant dinner theater sector.

"Norman, Is That You?," about a suburban father who visits his son in the city to find the towels labelled His & His, had made a fortune on the circuit for author Ron Clark, who endured Broadway disaster. Into its seventh London year but a Broadway loser after 16 performances, "No Sex, Please, We're British" has made dollars for its authors in the dinner theaters, as have Paul Zindel's New York failure, "The Secret Affairs of Mildred Wild" and John Patrick's several "Opal" plays.

Composer, Jerry Herman says that his "Mack and Mabel," costarring Lobster Newburg, has been better staged in a dinner theater than on Broadway.

Another facet is the touring star, often with spouse and children in the supporting cast. Such famous names as Van Johnson, Myrna Loy, Pat O'Brien, Dana Andrews, Joan Fontaine, Eddie Bracken, Tab Hunter and Virginia Graham earn thousands of dollars a week while meeting admirers face to face. The question here is that once a dinner theater has adopted the star policy, it usually has to be followed up.

Local favorites can be nurtured. Matthews has been developing the impressively gifted Pat Karpen and Mickey Hartnett, now in "Same Time, Next Year," although his brave sortie into the new play field with a Diana Churchill script was a harrowing disaster.

Radio's Johnny Holliday has transferred his WJMD-FM audience to Harlequin where, for a time, a marvellously gifted set of young leads, the Shues, Michele Mundell and Jack Kyrieleison held sway. The value of performing regularly has made an accomplished comic out of Harlequin's Buddy Piccolino.

Kinnamon and Levin literally light up in searching out undiscovered talents. In Philadelphia they tapped a 10-year-old Andrea McArdle, later Broadway's first "Annie" and now capturing London in the same role, and Lisa Peluso, who plays John Travolta's sister in the hit flick, "Saturday Night Fever."

"And don't forget Richard Stillwell," puts in Levin. "He was out here the other night after his season with the Metropolitan Opera on his way to sing Pelleas at the Paris Opera. Next fall the Met will revive 'Billy Budd' for Richard. We had him first."

Which says something about the value of dinner theater in developing talents as well as audiences and appetites.