Even my best friends shudder when I say built-in. They should know better. Built-ins don't have to be the most expensive way to store possessions or to make work or play surfaces. Indeed, if you use a bit of common sense and imagination, built-ins can be a real bargain.

Built-ins are particularly economic in a child's room where spaces are so tiny that conventional furniture can't fit. Even a custom carpentry job can be an economy.

But if the built-in is simplified -- and all this takes is common sense -- you can do most of it yourself and really save a bundle.

In a tiny room for a teen-age girl, I tried in vain to scale the usual bureau, desk and dressing table into my plan. So, I decided to take the built-in route instead.

After measuring the space, I decided it was best to use a continuous surface on two adjoining walls. The shelf is supported by a continuous wood strip along the wall and by right-angle iron brackets at frequent intervals. The shelf under the single small window became an elegant dressing table with an illuminated mirror. A double unpainted chest under it, on the right, makes ideal storage space for jewelry, as well as underwear, sweaters and the miscellaneous T-shirts a girl can acquire.

The longer shelf became a generous work surface, long enough for a typewriter and reference books. Under this I used two mobile file cabinets, available at most office suppliers and even some home furnishing shops. Since the shelf is self-supported, one cabinet can even become a handy night table, rolled alongside the bed at night.

Over the long work shelf, I used continuous, stock-lumber shelving, supported underneath by the same 1-by-4 strip I used for the work surface, and by the right angle wall at either end of the shelves.

These long shelves hold books, tapes, tapedeck, speakers, toys, dolls and storage boxes for all those accumulated priceless treasures.

A third strip, 1-by-6 this time, is notched to receive three rods, merely the unfinished, sturdy kind used to hang clothing in closets. This was the only piece made to order, except for the laminated surfaces; it is mounted on the two long, facing walls. From the rod over the dressing table, I hung two glass shades to light the end of the room. The other rods can easily be relocated as needed. One made a splendid spot, for example, for a bunch of party balloons.

The same 1-by-6 strip molding is used visually. A white work counter, white painted shelving and a white ceiling stretch the space. The bureau and file cabinets are gloss white, too. The rods and the pulls on the drawers are brilliant green, for a pleasant contrast to the pale robin's egg-blue of the wall and wall-to-wall carpeting.