NOW THAT we're all grown up and not too young to admit that grandmother was often right, we can not only appreciate but use good ideas from other times. The bay window may well be one of the most important inheritances from the past. It is no accident that you will find bays full of green, prosperous plants. Every bay worth the name gets sun from three directions, an efficient passive solar collector, as our predecessors knew. And if the bay windows are tall and deep, they capture every sun ray from sunrise to sunset.

Bay windows also have the charm of what is sometimes called lagniappe - something extra thrown in for free. The bay steals just that bit more from the outside. In some jurisdictions, bays are even allowed to cantilever out beyound the building line, a big help in cities where every inch of land costs too many dollars.

In Chevy Chase, Maryland, Victorian is quite a respectable age for a house to be. So it seemed a shame that the pleasant house on the corner had splendid gingerbread gables but faked federal windows and a silly porch with "wrought iron" supports where its bays should have been.

But there are people who can look at a house that's had many masters and instantly see how it could be made to work better for them.

When Robert and Judy Elliott with their two young daughters came home from three years in London, they brought with them a magnificient John Broadwood & Sons burled walnut grand piano, circa 1870 - and tastes to match.

When they saw the funny old Victorian house, about the same age as the piano, and noted its ten-foot high ceilings, they knew they'd found a home. They didn't dare tell Cathy, 11, and Meg, 10, about the other icing on the cake - the "Wendy" house (as playhouses are called in England), until they were sure their bid was accepted.

There was, of course, a small drawback. The house was about half the size the Elliotts needed. "We knew from the beginning," said Judy Elliott, that the house was too small for us." But they also knew from the beginning whom to call to make it grow: Hugh Newell Jacobsen, the architect who had remodeled an earlier house for them on Upton Street, and a master at making modern additions mind their manners when meeting their elders. "Hugh had also given us a great many ideas - or we stole them from him, I can't remember which - for the remodeling of our London house."

Jacobsen, according to Mrs. Elliott's story, didn't hesitate a second. He proposed a design which has much in common with the paper-doll chain people cut out when they'r old enough to be trusted with scissors. The two additions and the old house are the twin paper dolls. A glass entry links the two together like a paperdoll chain holding hands. Jacobsen and the Elliotts painted the house three exterior colors, a great habit of the period. And they embelished the nineteenth-century charms, including romantic woodwork and rather magic forest-type roof.

But the new entry hall is very much of today. You come up three steps to double glass doors, set between panes of glass. You may note that the handles on the doors are set rather lower than usual to make the tall doors seem even taller.

Once you're inside and look up, it's rather surprising to find what you thought was a two-story high entry way isn't. The upper section is the bridge for the top floor, set neatly between one old gable and one new.

Beyond the opposing glass wall at the entry level is a plant perch, paved with the same stone as the glass entryway. Standing there, you note the vistas, so important to the house: left/west to the living room in the new wing or righ/east to the old house. Either way the doors again are super high."They make me feel like Alice in Wonderland," said Mrs. Elliott.

The living room has two bays, plus extra windows, brilliantly lighting the white-walled room and delighting the plants. The side bay makes a dramatic setting for a clear plexiglass sculpture by Rockne Krebs. In the front bay, a fine antique desk makes it possible for tax lawyer Elliott to bask in the sun while working. Ah, but there's more here than meets the eye. On either side of the bay are closets, one for the master's files, the other for the family's record player and records.

On the opposing wall is the fireplace, with what seem to be closets in formal balance. Open up one and there's the bar, decorated with the Elliott's extensive collection of photographs. On the other side is actually a secret room leading to the new basement steps and a secret door (cut into the clapboard of the house so as not to be obvious) leading to the plant perch.

(When everyone noticed that there was a crack line on one door and not the other, Lou Stovall, the artist, was commissioned to come out and paint a line to pretend to be a crack. He did and, by request, signed it.)

To the right, in the old house, is a music room for the Broadwood piano, and to keep it company two great Art Nouveau objets d'art from Mrs. Elliott's family set in a typical Jacobsen library wall with its trademark shadowline molding.

The dining room has a magnificient Victorian waterfall chandelier above the classic Eero Saarinen marble-topped round dining table. When there's a big crowd, Mrs. Elliott puts leaves in the game table in the big adjacent family room and puts more guests in there. "But the ones at the round table always have the most fun. I suppose because it's cosier." In both rooms are permanent flowers - the latest art medium of Mrs. Elliott's mother, an early collector of photography who helped give her daughter a background for her present collection. In all the rooms are fine examples of contemporary art by Washington artists mixed with such grace notes as the wonderful portrait of Mrs. Elliott's grandmother.

There's also a guest bedroom with twin shining brass four-poster beds, bought in London, and a bath. Upstairs, the girls' rooms are in the old house, with amazing coat closets whose doors are cut in strange shapes to fit the steeply sloped ceilings. The master bedroom, in blissful privacy across the glass bridge in the new house, has what might be considered the ultimate in architectural detail: a one-of-a-kind fabric sculpture for a bedspread, commissioned for the Elliotts by Jacobsen. The bath has not only two I-spy porthole windows but a skylight.

Elliott acted as his own general contractor on the remodeling. He is experienced in these matters since he helped his father build a house as a youngster. How much did it all cost? "He told me I would be happier if I didn't know," said Mrs. Elliott.