FIRST Daughter Amy Carter may not be planning to move in January -- she just recently received an innerspring mattress to go with her antique canopy poster bed in the family quarters of the White House. The feather mattresses that once were standard on the antique wood beds in the Lincoln Room at the White House, Blair House and Mount Vernon are long gone. Instead, these valuable old beds sport firm innerspring mattresses.

Robert Shone, president of Truman Boyle Bedding, who made the mattresses for these historic beds, claims that they are one of the last manufacturers of custom bedding in the area. "There used to be about 12 companies that specialized in bedding. But one by one they stopped. The larger mattress companies, like Serta and Sealy, can't afford to upset their production to make a one-of-a-kind mattress."

Boyle charges anywhere between $200-$400. Shone says they handle an average of seven mattresses per week. They make both innerspring and foam mattresses in whatever firmness the customer requests. Shone adds that they also manufacture feather mattresses, "but generally the trend is towards the tighter, posturepedic style."

The American Foam Center in Arlington also makes custom mattresses, but only of foam.

Sometimes it takes more than the proper mattress to make your antique bed usable.

Imagine: Your long-lost great-aunt has passed away, leaving you with an antique brass bed to remember her and her $5 million estate by. It's probably be too short for your 6-foot-4-inch husband and not wide enough for the two of you unless you enjoy an elbow in your right cheek.

Antique brass and wood beds can both be enlarged, but warns Bella Ross, president of the J.B. Ross Co. in New Jersey, the customer should realize that the bed's value as an antique diminishes considerably once new parts are added -- except for lengthening the bed rails. If you cut into the head and foot, you're destroying the antique value. The question is whether the piece is more valuable as an antique or a bed.

If you are looking for an investment with high returns for the future, don't alter the bed, says Ross. But if you are more interested in its attractiveness and comfort than its monetary value, by all means, enlarge it.

Ross's father founded the J.B. Ross Co. in the 1930s and the company went on to become one of the largest antique brass bed importers on the East Coast. Since 1973, they have been manufacturing reproduction brass beds, often using old parts from beds they have in stock.

When beds were first made in Europe, no one bed was the same size. Full beds ranged in width from 4 to 4 1/2 feet, while twin beds ranged from 32 to 39 inches. "In those days," says Ross, "the blacksmith made your bed according to the size of the room it was to be placed in and according to you and your wife's size. There was no uniformity."

People have gotten larger since the days of the first beds. "Today's adult American is," according to the pamphlet "Facts About Bedding" by the Spring Air Mattress Co. in Chicago, "on the average, 2 inches taller and 20 pounds heavier than his 1900 counterpart. . . . In 1900, only one out of every 25 men topped the 6-foot mark. Now, one in five is over 6 feet." To avoid cramped quarters and bad backs, adjustment of your antique bed makes sense.

In adapting brass beds, Ross says, you don't want to use the usual box-spring mattress since the beds are high enough to begin with. "Instead," she says "we suggest one of two things: using custom made rabbit-edge bedding or a bunky board."

Rabbit-edge bedding is the more expensive of the two. With rabbit-edge bedding the bulk of the custom-made box spring is sunk about 4 inches below the frame. The mattress surface has a so-called rabbit edge that grips the frame on all four sides. Your mattress fits on top and is no higher off the floor than it would have been before the adjustment.

The cheaper procedure (and "one that most mattress companies don't like since it eliminates half their profits," says Ross) is to use a plywood base also known as a "bunky board," instead of a box spring. A piece of 3/4-inch plywood is bolted onto the frame and a mattress is placed on top.

For people who prefer less firmness, Ross suggests an extra-soft mattress, since the plywood will provide much of the support. For a better looking bunky board bed, the customer may want to cover the plywood in the same fabric as the mattress. Bunky boards are often used in children's bedding.

To widen a bed to queen-size, Ross suggests taking off the top and bottom rails that go horizontally across the foot and head boards. They're usually about 4-foot-6-inches. Buy the additional length of brass you need for a queen bed -- about six more inches. Take the parts to a brass craftperson, ("not a welder!" stresses Ross -- if welded, brass will melt), who will attach the two pieces by drilling holes into the brass tubing at the same intervals that the original had. This adds about three inches to either side of the bed and does not disturb its balance, says Ross.

To widen a bed to king-size is a more difficult undertaking. Ross suggests you divide the footboard in half and attach it to either side of the headboard to make the king-size width. A professional welder (if the bed's made of brass and iron), or a brass craftsperson (if the bed is all brass), will attach the parts for you. Besides widening the horizontal bars as you would for a queen, you might want to add a few vertical bars for balance.

Metal sculptor Todd Pendleton supplements his income by modifying brass beds for Rice's Antiques, 4208 Howard St. in Kensington, Md. When Pendleton lengthens a bed he first takes the bed apart to get to the side rails. Once he knows the new dimensions he attaches the additional amount of brass (or a combination of brass and steel) to the original bed rail. aHe then attaches the entire extended rail to the bed by brazing (similar to welding but at a lower temperature) it to the frame, being careful not to damage the bed, itself.

Pendleton feels that customers should be aware of what they's buying when they buy brass beds. "The old brass beds were actually made of steel pipes with a brass sleeve wrapped around them. It wasn't until the Industrial Revolution that people developed a more sophisticated metallurgy, which enabled them to make brass in a tube shape -- strong enough to support a brass bed. This is why, Pendleton explains, "the older beds are much heavier than their newer counterparts." (The new brass beds are all brass.)

Ted Rice, owner of Rice Antiques, claims that 99 out of 100 antique brass beds that he sees are just steel pipe with brass wraped around it. Since the invention of electroplating in mid-19th century, small parts of brass beds, such as the phenaels or cannonballs (the knobs that decorate the head and footboards of brass beds), were sometimes brass-plated, meaning that the brass was bonded to the steel. However, says Pendleton, the major parts of the bed were always steel or brass.

Bella Ross adds that American brass beds cannot be considered antiques since they really weren't manufactured here until 1895. "European beds were popular in the 1860s and 70s and these are the ones that can truly be labeled 'antique.' The first time a brass bed was shown in England was only 1851 -- not that long ago."

A piece of advice when working with a brass piece: Never tap antique brass directly with a hammer, otherwise it might shatter, says Ross. Instead, suggests Ross, place a piece of wood on top of the brass and then hammer it.

Antique wooden beds may also require enlarging. Dale Gillilland of American Foam Centers describes the sort of bed you might find: "Instead of a coiled boxspring, all there is to support your mattress and your back is rope -- pulled taut from width to width on the bed's underside. A sort of hammock-effect. The bed is probably suited at most for someone about 5-feet-2-inches" (and not very heavy).

Gillilland suggests two ways to make yesteryear's rope bed suitable for sleeping today. The first and easiest solution is to make the bed into a platform bed. This is done, he says, by mounting a piece of plywood inside the bed, so that the top is flush with the rail. This avoids cutting the frame of the bed. "Use a foam mattress -- whatever size you need -- on top of the plywood. You will be more comfortable than if you had used an innerspring mattress," says Gillilland (who has a slight bias since his business is foam mattresses).

Why? "Because," he says, "A foam mattress catches you when you sit down on a bed -- before you hit the plywood. Foam mattresses are what make platform beds possible." And by setting the mattress atop the plywood you are making use of the true width of the bed.

If you're determined to cut the wooden frame, extend the side rails, as with a brass bed. Measure the length of wood needed for the extension. Then splice the two pieces -- the original and the addition -- together. Re-attach the rails to the bed frame. Most antique dealers advise you to go to a professional cabinet maker or furniture maker if you intend to do this.

Claude Huret, owner of a cabinet shop bearing his name, does a good deal of work with antique beds.

"Lengthening the bed is no problem," he says. "We try to match the new rails to the bed. If you do the job right you should have maple rails with a maple frame and cherry wood rails with a cherry wood frame." After matching the wood and cutting it to the proper length, he then attaches the new rails to the bed frame with wood dowels, screwing the rails into place. "I advise customers to have new rails put in rather than splice in an additional piece. The old rails are usually straight -- not very decorative -- and if the bed is high and given to swaying, the old rails may not be too sturdy. New rails are more attractive and make for a stronger foundation, especially if your bed is high or you plan to use a box spring."

Because some woods are scarcer than others, the job of altering an antique bed can get expensive.

Widening the bed is more difficult and Huret prefers not to meddle with it unless the bed is a simple four-poster, with no fancy head or footboards. "It's almost cheaper to have a new mattress made and use the bed in its present width, than to have it widened," he says.

Widening involves splicing new wood pieces into the original width, as well as into the foot and headboards. By splicing wood, you're cutting another piece of wood and overlapping it with the old part. Elizabeth Webb, owner of the Early American Shop, warns that the seams will probably show no matter how well they're blended. Stan Krupsaw, owner of Krupsaw Antiques, agrees. "The parts that were cut will be pronounced. But," he adds, " a little carving of the foot and headboards may help."

Advice for the rope/wooden bed owner: Weave in additional rope on the bottom for increased back support.

Prices for the above alterations vary, since most antique stores commission all the work out to a number of craftspeople. For instance, adjusting the size of a wooden bed may require the expertise of a cabinetmaker, a refinisher, and a bedding manufacturer. Antique dealer Krupsaw says "I couldn't touch a bed alteration for less than $150."

The price differs depending on whether the bed you're adjusting is brass or wood. Some sample estimates: Wayne Cooperman, owner of Cooperman Antiques, where four or five brass beds are converted monthly, charges about $300-$350. Claude Huret converts wood beds for about $200-$300. And for custom bedding, prices range from $135 to $185 at American Foam Center, depending on the size of the bed. And at Truman Boyle's -- anywhere between $200-$400.