Sewing at home is a $3.4-billion business have sewn up California design.

In just over one year, the nation's four largest pattern companies have signed up 11 Los Angeles designers and two from San Francisco to design for the 40 million Americans who sew.

Considering that it cost approximately $25,000 to produce and issue one pattern and that the California contingent will be designing not one but several patterns per designer, this investment in West Coast design talent is a financial commitment of the first order.

The man who masterminded McCall Patterns' signing of seven Los Angeles designers, Richard Segrin, explains his company's go-west attitude this way:

"Los Angeles represents a life-style that is admired and emulated throughout the world. It has a profound influence on middle America - much more so than Europe. And L.A. designers offer clothes that are distinct and original - much less european-influenced. Often, a trend that started in L.A. will be picked up faster than one from New York or Europe."

Los Angeles designers featured on the May catalog, on sale now, are Bob Mackie, Norma Fink for Theodore, Carole Little for St. Tropez west, Singer and Spicer and Bonnie Strauss for Strauss. The June catalog, to be issued Arpil 1, will introduce Dennis Goldsmith for La Chemise and Nancy Heller for Tea Shirts.

While all pattern company spokesmen mention California life-style in its designers, all are equally aware that Southern California is the country's biggest home-sewing area. Of the 125 million patterns sold annually by all four pattern companies, 25 million to 40 million are sold on the West Coast.

As Segrin puts it, "That's a lot of tissue paper."

Reaching this Southern California market with Southern California talent is only part of Simplicity's game plan, according to president Lilyan Affinito, who believes that California fashion is indeed transportable.

"You live it and we love it," she says. "The California designer expresses an individuality and straightforward kind of attractiveness that he believes has broad appeal across America. There is an energy, yet there is also a relaxed attitude that is very today."

As the anation's largest pattern company, Simplicity had no designer program until last year, when it introduced three west coast designers - holly Harp, Harriet Selwyn of Fragments and Jessica McClintock of Gunne Sax - along with two from New York - Charles Suppon of Intre-Sports and Cathy Hardwick. Board chairman Harold Cooper says his company's entry into the designer field reflects not only its belief in California design, but its recognition of a growing consumer acceptance of designer names.

"Until recently," he notes, " designer names were not widely recognized by most womena. We believed most women who sewed at home associated name designers with complicated patterns involving a lot of pattern pieces and a lot of work. Thanks to the profusion of designer names in non-apparel categories - textiles for bed and bath, eye wear, belts, scarves, cosmetics, fragrance, luggage, automobiles - the use of designer patterns now has more validity. The designer today is a celebrity status influences buying decision - at least for some."

Vogue-butterick, the jointly-owned company with the longest history in designer patterns, is also the one that started the current California wave when it introduced Jane Tzse patterns in January of 1975 and Edith Head in 1976 - Tzse for Butterick, Head for Vogue. CAPTION: Pictures 1 through 3, Los Angeles design sew-ups from left: Nancy Stolkin for Butterick, Edith Head for Vogue, and Bob Mackie for McCall; Los Angeles Times photos by Ken Chernus