MODEL ROOMS, where the windows can be faked at the best spot to show off the furniture, are one thing. But interior designers really have a chance to show how they can cope when they tackle the real problems we all have: What to do when there are too many small rooms and no closets, long, dull stairways, unfinished basement ceilings.

Every year, the National Symphony Orchestra's annual Decorators Showhouse gives us the chance to see how 23 or so interior designers cope with the vagaries of a real house. Everything is for sale except the professional ideas, which are free for the stealing. The catalog itself is a good compendium of house pictures and decorating sources.

This year the Decorators Showhouse is a house designed by Rocco V. Tricarico after the Carter's Grove Plantation on the James River near Williamsburg.

Shown here are three of the rooms. The top-floor ballroom, currently serving as the lecture hall, wad decorated by design students of the Northern Virginia Community College-Loudoun (directed by Edith Murray and Fran Wallingford of The Design Factory). The exercise room, designed by Keith Babcock with multi-levels for exercise and massage, has walls of bronze acousticord to silence the bumps. The kitchen by Milo Hoots and Bob Waldron has handmate Mexican tile from the Tile Gallery, metal pots and pan racks also handmade by Nol Putnam and ceramics by Ann Friend Clark.

Showhouse, a benefit by the Women's Committee for the National Symphony Orchestra, is at 650 Chain Bridge Rd., McLean. Open 10 a.m. to 3:30 p.m. Monday-Saturday, 12:30 to 6:30 p.m. today, and next Sunday, when it closes. Tickets, $5.

Among the other notable rooms; the gray, sisal-covered platform bed and study area for a young gentleman by Magenta Yglesias of Designare Ltd.; the black and silver high modern-style family room, designed by Tom Backner and George Barber of the Hecht Co.