A collection of 90 original architectural drawings of the interiors and exteriors of British country houses goes on view tomorrow at the American Institute of Architects' Octagon Museum, 1799 New York Ave. NW. Complementing the National Gallery of Art's "Treasure Houses of Britain" exhibition, which features paintings, objets d'art and other furnishings, "The Architect and the British Country House, 1620-1920" focuses on the contribution of the people who created the houses themselves.

"After all, if it wasn't for the architect and designer, the country house wouldn't exist," said John Harris, curator of drawings for the Royal Institute of British Architects and organizer of the exhibit. "The exhibit tells the story of the container that would ultimately hold all of these great treasures," he said.

Spanning 300 years of British architectural history, these drawings trace the style, character and development of the British country house, and reflect a large diversity of architectural styles. While some of the drawings come from the houses themselves and other private collections, most belong to the Royal Institute of British Architects' drawings collection. Harris will lecture on the evolution of country house architecture at 8 p.m. tomorrow at the American Red Cross, 2025 E St. NW; the Octagon Museum will be open from 6:30 to 7:30 p.m. so the exhibit can be viewed prior to the lecture. For information, call the AIA at 627-7300.

There's also the "Treasure House"-related exhibit at the Folger Shakespeare Library: "The Compleat Gentleman: Books From English Country Houses." More than 100 books, manuscripts and engravings from the Folger collection are on display, providing viewers with a sense of how country house owners lived and what concerned them. "The Compleat Gentleman," the book from which the exhibition takes its title, outlines the qualities and knowledge essential to the 16th- and 17th-century country gentleman. In addition, there are account ledgers showing food purchases for household residents and staff, a book that depicts what would be served for a two-course meal, and books on genealogy and heraldry, important subjects for country house dwellers.

Both exhibits run until April. Painting the Corcoran

The Corcoran School of Art had its own version of Labor Day on Saturday -- "Labor Day II: Corcoran Construction Core." Ursula Meese; National Endowment for the Arts Chairman Frank Hodsoll and his wife Mimi, U.S. Ambassador at Large Daniel Terra and other Corcoran supporters showed up to help paint the school's exhibition hall and make other necessary repairs. Besides donating their time, Corcoran Construction Core members paid at least $25 each to participate.

"It's a little reverse psychology. People pay to work," said Elaine Silverstein, a Corcoran board member and organizer of the fundraiser. Local suppliers and contractors provided the paint and rollers and also promised their services to make other much needed renovations over the next few months. Arts Notes

Beginning Wednesday, the Smithsonian's National Museum of American History presents "America on Film," a series of feature films reflecting the American experience. Each month the series will focus on a selected theme or topic related to the museum's collections and exhibitions. This month's theme is India as seen through the eyes of Hollywood. Call 357-2000 . . .

Maryland photographers chosen from the Slide Registry of the Maryland State Arts Council are featured at the Montpelier Invitational Photography Exhibition opening today at the Montpelier Cultural Arts Center in Laurel. The exhibit runs through Nov. 30. Call 953-1993 for information . . .

Painter May Stevens, whose works are in the collections of the Whitney Museum and the San Francisco Museum of Modern Art, will lecture at 8 p.m. Friday at the Washington Women's Arts Center, 420 Seventh St. NW. She will discuss her "Ordinary/Extraordinary" exhibition, which appeared earlier this year at the Art Gallery of the University of Maryland . . .

Washington Area Lawyers for the Arts (WALA) will present an arts law conference, "Art Law in Theory and Practice," Nov. 22 in the Smithsonian Castle and the Hirshhorn Museum. Topics for discussion will be copyrights, artist contract negotiations and meeting the legal needs of artists. WALA also provides arts organizations and artists with limited incomes with free in-house counseling or referrals to volunteer attorneys on legal problems. For further information, contact WALA at 861-0055 . . .

First Class Inc. will sponsor two seminars on art collecting. The Nov. 14 seminars will be on how to make your first art purchase and on oriental rugs. For details call 797-5102. Seventh St. NW. She will discuss her "Ordinary/Extraordinary" exhibition, which appeared earlier this year at the Art Gallery of the University of Maryland . . .

Washington Area Lawyers for the Arts (WALA) will present an arts law conference, "Art Law in Theory and Practice," Nov. 22 in the Smithsonian Castle and the Hirshhorn Museum. Topics for discussion will be copyrights, artist contract negotiations and meeting the legal needs of artists. WALA also provides arts organizations and artists with limited incomes with free in-house counseling or referrals to volunteer attorneys on legal problems. For further information, contact WALA at 861-0055 . . .

First Class Inc. will sponsor two seminars on art collecting. The Nov. 14 seminars will be on how to make your first art purchase and on oriental rugs. For details call 797-5102.