TWO KINDS of preservation are going on in Washington today.

In one kind, a historic structure is repaired and restored to its original condition, or at least as close to original as the restorers can figure out. This is proper when the structure is intended as a museum, a slice of history frozen to its time of glory.

But by far the most usual - and perhaps the most useful - kind of preservation is that called "extended use." There are hundreds of old houses in Washington that deserve to be saved not just because they were built in the 19th century but because they were built well and are beautiful. They live on in the 20th century because they make fine places to live.

Today, preservationists realize that most buildings will only survive if they work for a living. Few of the most devoted of old house fans would be willing to live with candlelight and outdoor plumbing.

The trick in remodeling historic structures is to keep the things about the house that make it great - the elaborate moldings, the curved wood details, the deep windows, the tall ceilings and the unified facades that give a neighborhood street its character. And to add to them, the 20th-century amenities such as good plumbing, air conditioning and lighting, convenient kitchens, an easy way to get to the garden, and above all, a sense of sun, space and air.

Two houses in Georgetown - where preservation began in Washington - will be honored tomorrow night for just those attributes. One is a 20-foot row house, the other a mansion. But remodelings are the work of architects George Hartman and Warren Cox. (One is Cox's own house.) The awards will be given by the Washington Metropolitan Chapter of the American Institute of Architects. (See related story on this page.)

Helen and David Kenney bought their house in Georgetown after coming back from a Foreign Service posting to Indonesia. They chose their house, they like to say, "because it fit our big Indonesian furniture, not because it fit our family."

The house is on historic Cooke's Row. The four double houses on Q Street were built in 1868 (allegedly for his 12 children) by Henry D. Cooke, first territorial governor of the District. Original architects were Starkweather and Plowman. The house has 13-foot ceilings, eight bedrooms and three or so floors, depending on how you count them.

The formal dining room is really grand - big enough to feed the multitudes - but the Kenneys wanted an informal dining room. "It just isn't our style to give big formal dinners," said Helen Kenney. The old kitchen wasn't adequate, the Kenneys thought, for either.

The living room, though immense, didn't have modern built-in lighting to show off the Kenneys' oriental collection, from their postings in Vietnam (where they met when she was with the British Foreign Service) and Indonesia.

So Hartman and Cox went to work.

They designed a 4-foot, 6-inch addition for the back of the house. They chose a vaulted shape, suggested by the house's double arch mullions on the windows. The arched addition has a curved glass roof and glass walls with glass doors onto the terrace. This area has become the heart of the house. "We all congregate here, even when we have company," Kenney said. A light above the glass roof contributes to the nighttime drama.

The whole kitchen was redone. Red quarry tile on the floor, and an orange ceiling with a built-in trough for lighting make the room seem cheerful and up to date. The cooking peninsula sticks out into the middle of the room so it can be used for buffet service. Shelves on the wall behind the cooking peninsula keep spices handy to the pot. All the cabinets are oak flooring.

A butler's pantry, complete with a bar sink, leads to the dining room. In the dining room, the under-the-windows cabinet acquired a glass top with lights underneath, a dramatic buffet or a place to show off objets d'art. The architects eliminated a hall at one end of the dining room to give it a new door to the outside terrace.

The living room had originally been front and back parlors. The dividing walls had been eliminated, but the twin doors and fireplaces remained. The architects moved one door to the center of the hall wall and eliminated the other. This gave room in the hall for the Kenney's proudest possession, a marvelous red and gold Thai altar carving, and a place in the living room for a curio cabinet made from an opium bed. A light valance around the room conceals plug-mold lighting.

The remodeling cost about $60,000, but that was about six years ago.

Warren and Claire Cox's house, though down the street in Georgetown, is not nearly so large. Though it has three stories, it's only 20 feet wide.

In his own house, Cox concentrated his remodeling on elegant cabinetwork and architectural detailing to display and store his large collection of architectural art and artifacts.

As you come in the front door, there's a circle cut through the hall wall into the living room. What looks like a square cube mirror is really a slide projector. It's aimed at the white wall over the fireplace. Cox likes to feed in architectural slides and let them run automatically during parties, giving a changing picture over the fireplace. Not a bad idea for a photography buff.

A shelf goes around three walls of the living room, holding a changing collection of modern art, often with architectural themes. Currently there's a Lowell Nesbit and a Robert Rauschenberg, among others. Scattered through the room are Cox's antique weathervanes.

One end of the living room is planned as a library. Cox rebuilt the bookcases with deep shelves for his ancient architectural books. Below are shadow boxes for objects.

The dining room was a rather boxy room when he bought the house, according to Cox, with a "slightly strange Palladian motif." He had built a dramatic vaulted ceiling suspended just below the original. The new vault conceals built-in lighting. Cox also added a narrow shelf along the wall to serve as a buffet. A heating unit is built in to keep the pasts warm for parties.

The most unusual of the room's ornaments are two terra-cotta and wood columns that Cox found at Artifacts in Alexandria. The columns aren't holding up anything. They're just standing there, being interesting. Two more free-standing columns, one of which has unfortunately lost its capital, stand on the canvas-covered terrace, just outside the door. As if that were not enough architectural jokes and comments, the pergola at the back of the garden has a trompe l'oeil door painted on it.

More architectural decorations are in the master bedroom. A ship's carving sits as decoration above the built-in bed and his side tables. Where the fireplace was boarded up by some unfeeling former owner are now two columns and an arch as impression of fireplace. A book box goes the length of one wall, becoming a flower box as it crosses the windows on the second wall.

E.A. Baker was the contractor for both Kenney and Cox houses with additional work on the Cox house by R.A. Grim Associates. Richard Hurst of Baker's did much of the elaborate carpentry in both houses.

Like most architect's houses, this one has been in progress for quite a while. And if Claire Cox can settle him down to the drawing board, there'll be a new kitchen, with a place for the sun to shine on Alexandra, the month-old addition.