There is an old saying in the fashion business that comes up at the start of each season: "Every coat must have two sleeves."

Yet hard as it is for designers to come up with something fresh, the very enterprising still do. Midway into this week's collections for next fall, the Fendis sent down the runway a coat (and jackets and dresses) with only one real sleeve. In place of the other sleeve was an extension of fabric cut like a shawl.

There is hardly any danger that we will start wearing one-sleeve clothes, except for those who can afford the Fendis' lavish designs. But there are lots of other changes in fashion that will touch those who care about such things. Among them:

* Gray will start to take the place of the black that has been around for so long. It will start with a gray almost as dark as black and get lighter. And gray and black together in tweeds and bold patterns, sometimes with a dose of white, show up as the newest color range for fall.

* Big coats that swamp the figure--occasionally enlarged with huge sleeves and large collars--offer a way to hide all of your old clothes if you just want to set out with one thing new.

* The magnified plaid or check will replace all the dots and stripes. Sometimes they are as simple as blocks of color but other times are enormous houndstooth checks.

* The neckline is covered again, sometimes with huge cowl collars, funnel necks or mufflers tied around the shoulders to look like a cowl neckline. Shawls are back to doing the same thing.

* Nothing is simple. Leather jackets have inlays of wool. Silk blouses may be backed in knit. Dresses combine satin and suede.

* Lame' and sequins add shine to every collection for evening.

It doesn't seem to matter what length skirt is worn, or if one wears a skirt or pants. The bottom of every outfit seems unimportant and changeable. But certainly the narrow leather skirt, short or long, is a favorite choice of many of the designers and the fashion crowd attending the shows. (A favorite detail on skirts long or short, as well as on coats, is snaps down the side, which leave the possibility of making the skirt as revealing as you want.)

Belts, when they are used, are back at the waist for the most part. High-heel pumps, gloves and a hat are required. The coatdress, a pet of Seventh Avenue and Paris designers, is on the runway here and so is the chemise, particularly in knit.

"These clothes are money in the bank," said a confident Ellin Saltzman, fashion director of Saks Fifth Avenue, as the shows were winding down today. "They are new enough to be enticing but not funny-looking," said Saltzman, who recalled that a year ago she sat through the collections shaking her head and saying, "Where would you wear these clothes?"

For Saltzman, the hands-down winner of the season is Gianfranco Ferre. Fendi she admires as "an incredible work of art." To Gianni Versace she tosses "the most improved" award. And to the Missonis she gives an affectionate nod "for making the superb knitwear which a special kind of man and woman adores."

"I came to Italy planning to spend a little money and liked the clothes so much I'll probably spend a lot," said Thelma Weiser, owner of Charivari, a group of avant-garde boutiques in New York. "I bought so much I may have to open one more store."