Washington architects cut their teeth on them. Real estate agents attribute great "potential" to them. The screened porch, a commonplace Washington architectural feature, is one of those wonderful spaces that can be enclosed to accommodate one's expanding living needs. Unlike most additions, many porches boast a ready-made roof and foundation, making the process of converting them to year-around usable space less expensive than new additions.

With the advent of air conditioning, those flimsy screened-in hot boxes have lost their appeal to many householders. They seem to just cry out to be filled in -- precisely what thousands of Washington homeowners have done. A front veranda may still hold some charm, but those funny little spaces off the living room, at the entrance or upstairs off the bedroom simply no longer serve the same function that once made such porches de rigueur for Washington builders.

The design challenge posed by enclosing a porch is to integrate it with the rest of the house. The common strategy is to fill in between the columns with the same siding, finish and windows as the original house, or to create open, glass walls between the vertical supports on what usually is an otherwise dark, traditional house.

Probably the least expensive solution is to install standard windows and insulate walls and ceilings. Those proud owners of a jalousied porch -- known in the 1950s as a "Florida room" -- soom learn that narrow glass slats which open and close en masse do not a truly enclosed porch make. To convert a jalousied porch into a real room, one needs to spend a little more money. The average cost of enclosing a porch, without too many frills, in the Washington area runs about $8,500 for a room about 10 feet by 12 feet. Building a room the same size as an addition can cost as much as $20,000, according to Ernie McCoy of McIntire Construction Co.

Heating an enclosed porch can be a problem. Generally, the porch rests on pilings a few feet off the ground; usually, the floor has a not-so-subtle slope so rainwater will run off. The floor has to be leveled and frequently reinforced as well.Once the new crawl space has been established, wires for electricity, additional plumbing or duct work can be run underneath, but the crawl space then should be closed in and insulated so that you don't suddenly find yourself walking on a freezing cold floor in winter. If the porch rests on a cement slab, a new raised floor usually will have to be built to bring the enclosure's floor up to the level of the floors in the rest of the house. In that case the air pocket between can be stuffed with insulation.

Lighting a side porch can be a problem; often that porch is as close to the property line as is allowed and is perpetually shaded by the neighbor's house. Not infrequently an addition the size of the porch would not be permitted if one were to build anew, owing to the property line or to set-back rules. Be warned, though, that in some area jurisdictions you may need a special zoning variance as well as the usual building permits in order to enclose a porch.

When architect Jim Greenwell bought his two-bedroom bungalow in Brookland, in Northeast Washington, he acquired the predictable jumble of tiny spaces. In addition to blowing out the inside of his home, Greenwell took a useless back porch and turned it into a delightful dining room and sitting area by sanding down the floors and adding heating and insulation. The big alteration in the porch came with the addition of glass french doors and a wide set of steps leading to the outside. Now when you walk into the end room of the house, you are immediately drawn visually into the back yard. The new leaded glass transom provides a transition from the kitchen area to the porch and brings light into the kitchen. The total price of the porch enclosure was about $2,300.

Two more ambitious enclosures involved adding second floors to what began as single-story porches.Bill and Eve Lilley's house in Wesley Heights in the District came complete with two screened porches. To reach the front door, one had to enter a small side porch and work around to the front door, which opened directly into the living room. The side porch, a modest 8 feet by 12 feet, was handsomely and effectively enclosed by simply placing a new front door in the center with fixed glass windows on either side of the door. One opening was left on the side. The other door, at the back, was closed up to make way for a much-needed downstairs closet. One now enters a well-lighted vestibule. At the back is a set of sliding glass doors leading to a deck. The old door to the house is gone and instead, a doorway leads the visitor into the living room. What had been an awkward entrance is now sun-filled and gracious.

At the back of the house, just off the kitchen, was yet another porch, this one resting on pilings a full story above ground level. After closing in, the back porch became an informal television room and glassed-in garden overlook. Sliding glass doors lead to the same deck visible as one enters the house.

A few years after the TV room was created, the Lilleys called in architect Susan Woodward Notkins to design a second-story addition to top the old back porch. Result: a memorable new master bedroom surrounded with closets, a dressing room and a new second-floor bath. The house still has only three bedrooms, but the old bedroom at the top of the stairs is now a transition space leading into the new area.

The new bedroom, complete with cathedral ceilings, rests on top of the old enclosed porch. The marriage of old and new spaces is so successful that it is impossible to pick the addition off as a mere proch enclosure. In part, the two-story enclosed porch successrully presents itself as an original part of the house because of its neight. Enclosed porches often miss the mark from a design point of view because they are one-story additions to two or three-story homes. As such they give the impression that they are afterthoughts.

To solve the "after-thought" problem, Philadelphia architect Michael Kihn designed a three-story porch enclosure plus an addition for Tim and Andrea Corcoran. The spacious stucco home in upper Northwest had a small porch off the living room. But the couple really wanted more first-floor space, a new kitchen, a better way to relate the back of the house to the yard and a new master bedroom suite, complete with bath. That's asking a lot of a mere porch enclosure. But in fact the old porch served merely as a departure point for an ambitious remodeling project. Even the foundations had to be rebuilt in order to support the new addition. Kihn developed a three-story wing that added three rooms and a bath in the area that had been taken up by the old porch.

"One of the difficult problems in using a porch enclosure, or even adding on a second story on a three-story house," says Kihn, an architect with the prestigious Philadelphia firm of Geddes, Brecher, Qualls & Cunningham, "is that they never look a part of the original house -- they always have a kind of stuck-on look."

To soften the stuck-on look, Kihn decided to continue the roof line on the third floor -- a sort of outline version of the roof that might have been over the second story master bedroom addition. Rather than enclose the entire roof, he created a third-floor patio, protected by lattice-work walls, with a dramatic beam which extends from the main peak of the roof of the house, carrying the eye through to give the impression of a three-story addition. Instead of filling in between the columns of the original porch Kihn repeated the multi-paned french doors that had characterized the entrance from the main house to the porch and used the columns as a decorative element. This recalls the original porch and separates the enclosed porch area from a lower-level addition that was once a patio off the back of the house.

On the lower level, Kihn installed a fireplace with a double flue (so that a fireplace could be placed in the master-bedroom as well) and a wall of tall, narrow windows and doors opening onto the back yard and a central patio. The addition that resulted is light and airy in a contemporary way, but it does not deny the Old World charm of the original house. CAPTION: Cover photo, no caption; Pictures 1 through 3, The Wesley Heights home of Bill and Eve Lilley has been dramatically expanded by enclosing a front porch to create a gracious entry and much-needed closet and then enclosing a back porch for a sitting room or family room off the kitchen. This enclosed porch the couple added a second-story bedroom and converted the original room to a dressing room; Pictures 4 and 5, An enclosed porch can cost as little as half the price of putting on a new addition of comparable size. The three-story addition designed by architect Michael Kihn for Tim and Andrea Corcoran began as a simple porch enclosure and expanded to create an entire new wing for the house. The columns were part of the original porch and now demarcate the old porch area from a former patio that, enclosed, is the Corcorans' dining room. On the second floor a new master bedroom suite emerged. Above that the architect continued the roof line of the main part of the house out to the end of the new roof deck on top of the addition, thus marrying the new wing to the old house.; Picture 6, For $2,300 the back porch of the Brookland home of architect Jim Greenwell acquired walls and french doors and became a combination dining room and sitting area. A leaded glass transom over the archway brings additional light into the kitchen, from which this photograph was taken. The french doors and adjacent windows flood the former porch with sunshine and draw the eye into the Greenwells's back garden on which the doors open.; Picture 7, French doors taken from the original porch highlight the first floor of this enclosure. Sun floods in through the skylights to illuminate a new dining area.; Photos by Breton Littlehales