Many Washingtonians who need more space in their homes for growing families have been hit by a double whammy: they are unable to trade up to bigger houses because of high mortgage interest rates, yet find the cost of building a sizable addition prohibitive.

Redecorating may help, but tends to be largely therapeutic -- taking the same spaces and making them look better. So a new boom in innovation is underway -- the pop-out. Architects refer to such jobs as blowing out spaces, bumping them out or popping them up. Laymen talk of dormers, greenhouse windows, bay windows or just plain raising the roof.

A thoughtfully executed pop-up or blow-outs can expand spaces impressively at costs per square foot far below that of adding an addition. A major design advantage of the approach is that the character of the house usually is retained. One of the more modest examples of a blown-out wall approach is the breakfast nook in the home of John and Suki Sargeant. The couple had a small kitchen which had neither enough nor eat-in capacity. Architect Calvert Bowie eyed the low brick wall outside the kitchen door. The wall was used to hide garbage cans. Bowie decided to use that wall and bring the house to it. Masons built up the low wall about 18 more inches and then a glass and a doublehung traditional window that echoed the windows in the rest of the house.

The old kitchen wall and door were removed and glass double doors leading to the backyard were added. The space is just big enough for a table and chairs and a small storage area for children's coats and art work. The cost was $5,000.

Architect Jim Greenwell wanted to retain the exterior appearance of his turn-of-the century bungalow in the Brookland section of Northeast. But he needed more space in the two-bedroom home. So he had two rooms in the front demolished -- what had been the dining room and living room, both small spaces, one behind the other. The joists in the ceiling (all of which were marked Sears, Roebuck because the house had been a Sears kit house) were cut out, opening up the attic in the front of the house. The total room count of the house may have diminished with the vanished partitions, but the sense of spaciousness is way up.

A second story remains over a bedroom located to the right to the living room and a loft space has been carved out of the rear of the house in what had been unusable attic space.

The back stairs curve around to the low-ceiling space which overlooks the living room. Unfortunately, the ceiling is so low that the room might have been made for Munchkins if it weren't for Greenwell's clever placement of a skylight right at the top of the stairs, giving just enough head room to enter the bedroom.

At the back of the house, the kitchen space was expanded by a bump-out -- a cantilevered greenhouse window which not only provides more space in the kitchen, but allows more light to pour in. Two tiny rooms across the back of the house were made into a single space that provides a table area and storage space and access to the enclosed porch at the back of the house (see the article on page 44) where the dining room now rests.

Greenwell managed to retain the exterior appearance of the house without giving any indication of the explosion of light and space achieved on the interior.

Sometimes an attic can be made useable by simply popping up a dormer space. Architects Kent and Robbie Cooper of Cooper Lecky Partnership, devised a clever scheme for a Northwest Washington couple who added 20 percent space to their home. A multilevel brick Colonial, with the living room, dining room and kitchen on a second floor and two bedrooms on the third, the house had a stairway leading to a narrow, low-ceiling attic.

The architects lifted up a piece of the roof and planted a dormer with a difference -- the corners increase one's sense of space by the use of glass-on-glass seams. The 12-foot-wide space is the focal point for the new room, bringing light into what had been a narrow, unworkable space. It is now transformed into a guest room with bath (stacked over a second floor bath to conserve money and take advantage of existing plumbing), and a sitting room. Two built-in desks, one that can be rolled away into the wall and one free-standing, make it possible for two people to work in this sun-filled space. A small window, traditionally placed to bring a little ventilation into the peak of the roof, has been extended into a floor-to-ceiling window to bring in light at the end of the tunnel-like guest room. The project cost about $15,000.

An ambitious bedroom addition was designed for a two-bedroom house in Spring Valley by architect Don Hawkins. The house has a view of a park and was designed by an artist to have a huge second floor deck. One downstairs room, now a den, had been a bedroom, and upstairs there were two additional rooms. By extending the roof line of the house outward, Hawkins made a big bath out of a tiny office, added a bedroom and a master bedroom complete with a loft office.

Rather than completely close in the old deck, the architect retained a small, comfortable patio just outside the master bedroom suite. The overall impression is that the rooms always had their current configuration. The master bedroom takes up about half of the second floor space, making it a marvelous retreat for the parents of three active children.