BACK WHEN most rowhouses in Washington were being built, sitting outside was for the birds - or the horse, the laundry, the chickens and the garbage cans. Outside, to put it as gently as possible, smelled.

Unless you had a porch, rocking chairs and a view of the Potomac, you stayed inside. In this kind of weather, you closed the shutters during the heat of the day, brought out your funeral parlor fan and the lemonade (cooled with ice from the ice wagon) and read "Ivanhoe."

SO it is that in most of the old sections of Washington, many houses have only rudimentary backyards, just big enough for the laundry. After all, city folk had real honest-to-goodness up-to-date trolleys, so they didn't have to bother with keeping a team of horses or a big carriage. And they didn't really even need a cow, what with the milk wagon coming by so convenient-like.

Today, many people, from the Henry Kissingers to the John Murphys, prize such handkerchief or postage stamp-sized plots to park their car. But they also have a feeling the space should do more. As Luncinda Murphy, a landscape architect, puts it, "Everybody wants a place for flowers and vegetables and place to sit, with no maintenance."

Murphy and her law-professor husband, John, started out with a car, a determination to live right downtown, four children and a strong desire to sit outside now and then.

When they found the old house in the Dupont Circle area, "It was just exactly what we wanted - six bedrooms, enough so everybody could have one plus a studio for me and a study for Jack," Lucinda Murphy explained on a hot morning last week. "The only thing it lacked was a patio."

The house took up the whole lot, with just a bit of space in the front for a petunia, and in the back, just a driveway with room for the garbage cans.

The Murphys, undaunted, bought the house and installed their collections from tours abroad. And she took to her drawing board. What she came up with was a simple, workable plan that didn't cost an amazing amount of money but did have (if in miniature) all the necessities to handle a hot Washington summer.

First off, she designed and good-looking wooden fence between the house and the neighbor's backyard. For the street side, she had made two large gates so, if need be, the car or a delivery van could pull all the way in. The rest of the time, the gates pretend to be fence. They swing on heavy rollers, which makes them easy to move and helps keep them from sagging. The fencing was made by Mill-To-You of Forestville, Md. It cost $750 and took about two days to install.

Murphy designed the patio to be covered with a 6-inch slab of concrete, into which handsome rectangular stones of varying shades were set and grouted. To her plan, masons also built a small planter with a stone top for seating, a low brick wall (also with a stone top) and stone steps down into the lower terrace, and a tiny pool with a fountain. The fountain and pool, hardly bigger than a basin, is in the funny sort of triangle section just below the kitchen steps and two steps down from the rest of the patio.

In one spot, the stone was left out in favor of a sandpile just big enough for the 2-year-old. A wood cover goes over it to keep cats, dogs and such out. The masonry work cost $3,100 and took about a week.

The electrical work for the fountain and lighting cost $200.

There's even enough room leftover between fence and sidewalk to park a European-size car.

Murphy manages to stick solidly to her drawing board, even with the 2-year-old playing hide and seek underneath. She apprenticed with landscape architect Eric Paepcke after majoring in art and botany at American University. Now she takes on small residential jobs such as the patio she designed in a small space for Richard and Katherine Bull in Georgetown. The Bulls currently rent the house to the Kissingers, who from all reports like it fine.

The Kissingers' patio is a bit fancier. There is a small terrace, reached through glass doors, big enough for two or three chairs and drinks. Again there are raised planters - people in the city think gardeners shouldn't have to get on their knees to push in petunia - with the Murphy signature: the sittable top edge. A recirculating fountain makes a pleasant noise (incidently helping to block out traffic noises). This fountain is what Murphy describes as an "African bowl shape," with water coming out two sides into the small pool.

Matthew and Ellie Fink, who live in the Kalorama Triangle area, also have a Murphy marvel, made from a former driveway. For the Finks, it makes city living possible with two children. They did much of their own work - taking up old rough brick, regrading the area, laying cinders, sand and finishing off with brick. They got the full treatment: sandbox, fountain, sittable walls and even room enough, outside the gate, to park the car. Here the gates are actually a sliding section of fence.

In another such project, Murphy removed the garden side wall from an existing garge to make a summer house, a dramatic sort of stage for the end of the garden.

Murphy suggest that people who want want to do such projects on their own (she charges from $400 to $1,000 depending on the complexity of the design) should start with a measuring tape and graph paper.

"The important thing is to plan the whole project from the beginning, including everything you are going to do. Otherwise, you may find you have a 15-foot-high tree where you want the fountain. You don't have to execute the plan all at once - you can do it in stages as you get the money. But do figure out what you need to do.

"Try for a unity of materials. Use only one or two paving materials, for instance. Brick, for instance, can make both walls and paving. Too many textures, especially in small gardens, are too busy.

"If you plan to drive your car onto the patio, you'd better lay a substantial (6-inch) concrete slab below your paving materials. Brick on sand is all right if you're just walking on it.

"Access to the kitchen is very important; make it easy, because you'll be carrying food and drink often.

"Steps, gates, entryways can be handsome elements, so plan them well.

"Tops of planter walls as well as steps work well as garden seats. Such unobtrusive seating can double the number of people who can be comfortable in a tiny garden."