Ann Person, fashion sleuth and pattern mogul, was in a posh Texas store branch and spotted a chenille jacket. Price: $89. Her price, to sew yourself: $20.

Person is head of Stretch & Sew, Inc., a $90-million 200-store operation specializing in simplifying current styles and turning them into patterns to be made up in identical or similar knit fabrics. The do-it-yourself cost is at least half the cost of the ready-made, and as little as one fifth the cost.

When you do a cost analysis on any piece of clothing, the biggest chunk of cost goes for labor. So when you make your own clothes and save the labor costs - and who figures in their own labor? - the savings can be monumental.

The combination of increased clothing costs and the increased ease in making clothes with simplified patterns has no doubt caused the current upswing in home sewing. It declined when women increasingly took jobs outside the home, but when patterns and instructions became simpler (as did some of the styles) home sewing increased.

Some patterns are authorized designer copies, complete with the builtin bounty for the designer name. (Patterns are the biggest moneymaker for many designers...even more than the sale of their final products!)

Other patterns are straight "knock-offs" by Person and colleagues, always to be done with knitted fabrics and seldom, she claims, taking more than an hour to sew.

Fabrics Unlimited of Arlington has taken the simplified sewing technique in a different direction, interpreting popular styles from designers such as Calvin Klein and Julio. Their system requires no cutting and the sewing of only one seam and they claim a simple skirt, camisole or camisole dress can be made in 45 minutes.

An important rule of thumb, says Myriel Solomon of Fabrics Unlimited, is that fabric chosen must be the girth of the wearer at her widest point, plus four inches. Fabrics can be the identical silk crepe de chine that the designers use, "and you don't even have to be an accomplished sewer," says Solomon. "That's the best part." (Her sewing instructions are available free to customers.)

Dale Mayer, head of Charing Cross Kits, which have pattern lines screen-printed on the fabric, says her customers are mostly women who love to sew but are easily thrown by such things as buttonholes or zippers. The concept, called Clothkits, was developed by Londoner Anne Kennedy more than 10 years ago when she was a student at the Camberwell School of Art in London. The kits, first silkscreened and packaged at home, are now a worldwide business. Still made in England, they contain color-matched threads, lining, zippers, buttons or whatever else is needed to complete the garment.

Kennedy and staff start with the design, then create a fabric which is included, along with instructions screen-printed on the fabric.

"From our survey it was clear that many women had trouble selecting the right fabric for the design and only realized after they did all the work that the fabric they chose was inappropriate for the style," says Mayer.

Kit options range from a corduroy child's pinafore, plus matching doll dress, $12.50, to a winter-weight quilted jacket, $50, to a wool tweed coat, $85. Catalogue ( $1) from Charing Cross Kits, Inc., Meredith, N.H. 03253. CAPTION: Picture 1, Home-sewn ensemble, Fabrics Unlimited, about $43; by Larry Morris - The Washington Post; Picture 2, Designer jacket about $500; Picture 3, Stretch & Sew's pattern spinoff, about $20; photo by Vanessa R. Barnes