HOMES BY MAIL. You've seen the magazines, brimming with floor plans, at supermarkets and home improvement centers. For some people, sending away for a set of detailed blueprints has proven to be a cost-saving alternative.

Ordering house plans by mail may sound a little risky. It's not like ordering a sweater in an unbecoming shade of chartruese that can easily be returned. Sending away for a house design seems too weighty a matter to leave to the sometimes chancy world of mail order. Yet thousands of people apparently are willing to take the risk. One of the largest home design companies, Home Planners Inc. in Farmingham Hills, Mich., sells ``in excess of $4 million'' in plans each year through plan books and magazine advertisements, according to its president, Charles W. Talcott.

Why such demand? The biggest reason is money. Having an architect design a residence can cost anywhere from a few hundred to a few thousand dollars, depending on where you live. By comparison, most ``pre-designed'' or stock plans are priced between $95 and $130 a set. For some people, this can make the difference between being able to build a home this year, or even building at all.

Dennis and Linda Bunt built their home in upstate New York from a stock plan designed by Sun-Tel Designs of Portland, Ore. They were impressed with the detail of their blueprints, particularly the electrical layout.

``We saved about $2,000 on the design costs,'' says Mrs. Bunt. ``What we liked most was the variety there was to choose from. In fact, that's what helped us decide on what type of house to build. It would have been much more difficult to go to an architect and say, `Here's what we want.' By looking through plan books we formulated what we wanted.''

Despite the tract-house implications of ordering a design from a book, it's not likely that any two plans will yield the same finished house. Dozens of variations are available. Many plans offer a basic floor layout with assorted exterior ``overcoats''; others use one style with interchangeable interiors and various square footages.

For example, you could choose a ranch house with a Spanish-style exterior or a Victorian fa,cade equipped with an open, modern floor plan. Basically, whatever you can imagine you can probably order through one of the more than 20 plans books on the market, which are published quarterly and are often available at larger newsstands and home-improvement centers.

Sally and Ray Gilbert of Aledo, Texas, sent away for and built a plan designed by Larry Garnett of Pasadena, Texas. Mr. Garnett has been designing and selling stock plans for many years through the mail. ``It not only was exactly what we had in mind, but we saved a lot of money too - I'd say between $5,000 and $10,000,'' says Mrs. Gilbert. She adds that ``the framers were so impressed with the detailed blueprints, they said the plans were one of the most complete they had ever seen.''

That kind of customer satisfaction has a long history. Pattern books for homes abounded in the 19th century, and by 1908 Sears, Roebuck & Co. was selling actual construction kits for 22 different house models by mail.

Virtually all the plans sold today require some modifications after purchase. Some are formalities, like adapting certain elements to meet local codes; others are simply enlarging windows or relocating doors.

Real success in buying and building from a stock plan comes from doing your homework. Careful scrutiny of these pre-designed layouts is important - there is a lot of bad design clothed in an attractive exterior. Be sure you know exactly what you want: how many rooms, how many bathrooms, what amenities you can't live without, and how well your plan integrates with your lot, if you have one. Many of the plans companies suggest that an architect or builder be consulted for major modifications. Most also suggest you at least check with a contractor before buying a plan. Since most plans are not returnable, you are pretty much stuck with what you order.

There may also be a few unforeseeable problems when building from a stock plan. The Bunt's house, for example, has multiple rooflines, which present no problem in milder climates, but are prone to ice buildup along the seams during the harsh Northeastern winters, a situation that might have been alleviated by using a local architect.

If money is a factor - and it usually is with buyers of pre-designed plans - check to see that overall dimensions of the rooms coincide with standard lumber and building material sizes. Many plans offer separate materials lists to give you a rough estimate of supply costs.

Also, having a good idea of your family's needs and habits is very helpful in recognizing what floor plans will work for your daily living situation. People who entertain frequently may want a very open plan, where the guests can visit in the kitchen while they cook. Families with children may want to have plenty of separation of play and work spaces. The more you research on your own, the less you will have to pay someone else to do it.

The pre-designed route may not be for everybody, however. ``Although some recent designs have been targeted to an upscale crowd, the reality is that these are stock plans,'' says Ann Hope, publishing director of Best-Selling Home Plans, an offshoot of Home Magazine. ``People who have very specific ideas are better off with architects,'' she adds. Tips from the designers

``Circulation should be the focal point of any good working plan,'' says architect Charles Koty of National Home Plans in Long Island, N.Y., a longtime designer of stock plans. ``Watch out for long hallways - it means the designer has wasted space,'' he adds.

``Secondly, the zoning or placement of rooms is very important,'' he says. ``Activity rooms like kitchen, dining, and living rooms should be separated from quiet areas like bedrooms and studies.''

Ken Gephart, of Piercy and Barclay Designs in Tigard, Ore., agrees that balance and traffic flow are of primary importance. ``Rooms should be in proportion to the overall plan. The bedrooms shouldn't seem tiny compared to the activity areas,'' he says. Exterior integration is also a factor. ``The exterior should have `curb appeal' and fit into the community, as well as be properly oriented to the topography of your lot.''

``The energy efficiency of the house needs to be addressed,'' Mr. Gephart continues.``The heating unit should be central in the plan, and the windows needn't be oversized, since this is a major area of unwanted heat gain for the Sun Belt and heat loss for the rest of the country.''