Even before President Carter gave his energy message on television, Seventh Avenue designers were stiching up clothes suitable for the coldest winter, inside the house and out.

Some designers see a way to beat the cold in shearling, storm coats, quilted jackets and quilted pants, and fur-lined or fur overcoats. Others have taken to layers of shirts, sweaters, jackets, shawls and coats, leg warmers and boots.

Only Halston starts from the skin with one-piece dance leotards in wool knit, cashmere knit, and even stretchy satin versions for evening, worn under dresses, with skirts, or just alone under a big fur coat. If you are afraid of being cold with the indoor thermostats turned down, wiggle into a woolknit leotard, put an ultrasuede dress over it, and watch the sweat start to run.

While keeping warm may be a phony reason for buying new clothes next fall, the designers have injected enough fresh ideas into their collections to lure credit cards and probably cash of great quantities from customers' wallets.

Clothes do best when they are sensuous-feeling, pampering wearers with velvet coats, satin blouses, cashmere dresses, and anything else that feels good to the touch.It has worked well in underwear, it's bound to succeed in clothes generally, and designers are using it for both daytime and evening.

And while the clothes for next fall feel good, they are often loose, almost to the point of being sloppy. Big blousons, full dresses, unlined full coats, jackets and sweaters that look like they came out of your boyfriend's closet make the point that comfort and ease are big this season.

Clare Pritchard, who buys dresses for Lord & Taylor, expects customers to like the freedom of full styles as long as the dresses, as at Gil Aimbez for Genre, fit closely across the shoulders and chest. "The fullness has to be controlled to be flattering," says Pritchard. "Then it can be as big and full as you want."

Pants are still very much around, and the assortment is endless: pegged, pleated, narrow, wrapped at the ankle, bloused to the knee. Noticeably missing, of course, is the kind most of us have in the closet, the fuller-cut variety, but that's to be expected. (Wear them with flat shoes and a soft blouse and hardly anyone will notice that you are a season behind.)

What's newer are softer skirts, often in a dirndl shape, in supple light fabrics like wool challis or silk crepe de chine. Calvin Klein refuses to abandon his elasticized waistline style, which is not only soft in effect but also very comfortable.

The softer skirt is one way designers take the hard tailoring edge off haberdashery clothes. Soft blouses are another. Ralph Lauren has put ruffles, tucks, and even an edge of lace on his shirts. (If a shirt becomes a blouse when it has such feminine touches, then these surely are blouses.)

Real blouses abound, too, and thanks to Yves Saint Laurent many of them have Pierrot ruffled necklines. That collar treatment is everywhere, with Bill Blass' version in black broadtail perhaps the ultimate.

As essentially casual as most of the clothes are today, some designers have struck out for total opulence for evening. Blass is big on luxury fabrics dipped in gold and sable-trimmed velvet, while others like Kasper and Scott Barrie like the look of lace.

Few things are so sure as the fact of next year's cold winter (says the Farmer's Almanac,) and with it the increasing cost of clothes, if for no other reason than the fullness that demands more fabric and luxury fabrics that cost more money.

"We haven't started looking at prices yet, but things are guaranteed to be more expensive this coming fall," says Koko Hashim, fashion director of ready-to-wear at Blooming dale's. "Prices are always the hardest part."