Doubling up on space is a must these days. If you still cling to the idea of a guest room, study, sewing room or library - or even a separate dinning room - it helps to combine two or more of these functions in the same space.

Unless you own an old house that has each of its many rooms geared to an individual use (and few of us have that luxury today), you probably live in a typical two-or three-bedroom home as a study or workroom or music room.

In a home I designed, for example, two little girls shared one large bedroom, their parents had another and the third room - a smaller room - became a guest room for a sleepover child or adult friend.

Though the room was small, it had ample wall space for a gest bed that, with many thick pillows, also could be used as a sofa.

The real need, however, was for a work and play space for mother, father and daughters. To provide this, I had used a long shelf, two feet deep, under the picture window - large enough to accomodate a typewrtier for my mother, who is a novelist, and still leave plenty of surface for father to use for handling finances, and for the daughters to use for occsaional homework and letter-writing or hobbies.

I used plywood for the workspace countertop, choosing the three-quarter-inch thickness for stability, and I attached this to the wall, at proper work height (29inches), with brackets. I edged the plywood on the front with a stock one-by-two-inch piece of lumber to stablize the edge, give the top a thickness tht makes it seem stronger and finished the raw edge of the plywood at the same time.

I made the guest bed from a 36-by-72-inch piece of plywood that rests on a box made of one-by-12 sections forming the rectangular shape. Corner braces at the bottom make a place for the concealed casters to be attached so that the whole thing can be rolled out easier bedmaking and less confined sleeping quarters.

he mattress is foam in a standard twin size (39 by 75 inches), and I allowed a bit of it to hang over all the sides of the platform box, softening its edges. Two mobile file cabinets hold his and her work materials. These can be rolles out for easier access, and a clamp lamp swings readily from typewriter to bed.

I used terrycloth, bought by the yard, for a contour spread; if you prefer, you can switch two large towels together for the same effect. White mini-ve netian blinds in the large window control the light for work, play or sleep.

A white plastic-laminate-topped round table, matching the white painted work counter and bed platform, is a convenient spot for games or art, and the desk chair can join the others to make this dual-purpose room a convivial center for the whole family.