Even a garden no bigger than an old-fashioned bathroom is a priceless addition to any dwelling, but it cannot be treated as if it were a suburban lot.

Sometimes owners of row houses are so discouraged by the tiny size of their back yards that they think it's not worth fooling with, and thus they needlessly deny themselves one of the major pleasures of life.

Sometimes the plot has a broken-down fence, patched over the years with odd lengths of hog wire; sometimes half or all the yard is paved with concrete that over the years has cracked and crumbled in places to afford a snug home for thriving weeds, which are the only ornament for the trash cans that have the place of honor.

Suppose the occupant lacks the ready cash to have the concrete removed. It may still be possible to resurface it with brick or stone or planks or gravel or mortar. The fence may be repaired; big branches of overhanging mulberries or other weedy trees may be sawed out, and some vigorous creature like a lace vine (Polygonum aubertii) may be planted to cover the fence all the way round the garden, and the trash cans may be set outside the fence in the alley.

Such a small patch will be great for giving the kid, the cat and the pup some fresh air. More than that, it can become a delightful garden. I have seen it done admirably in a space no larger than 12 by 14 feet.

The general trick is to confine plants to the boundary walls or fences, leaving the entire center paved in some neutral color. Although brick and stone are costly, the space is so slight that they may well be used even by the most budget-conscious.

If the paving is tan or gray or rose and the boundary fences or walls are carried up to six feet and covered with vines, a small outdoor room will result, and a rich effect will come from plain terra cotta pots grouped here and there, filled with plants interesting for foliage or flower. No matter how small the garden may be, I would always have a table and at least two chairs in it, or an arbor built against the house wall with a comfortable wooden bench beneath a grapevine, say. A place to sit is more important than a blaze of roses.

Also I would have a fish pool in even the smallest garden, at least 2 by 4 feet of water surface, but as large as possible. A pool this size might have six goldfish, some underwater seaweed, a submerged tub holding pickerel weed or some water rushes or an acorus or water iris or small waterlily (such as the little yellow 'Helvola' or pink 'Johanna Pring,' among hardy winter-enduring types, or a small tropical waterlily such as the blue 'Dauben' or violet Nymphaea colorata.

You would have to make up your mind, and would do well to settle for one of these and not try to have the pickerel weed and the iris and the waterlily and everything else. There is no point having a pool if you can't see the water surface, and there is no point having goldfish if they're so crowded they struggle to stay alive. No more than six in a 2-by-4-foot pool.

In a garden so small I would let the pool and the sitting place be the main ornaments, and would confine plants to pots or tubs or boxes against the walls or fences.

"Container Gardening" is a soft-cover 112-page book lavishly illustrated in color (Sunset Books and Magazine editors have put it together, published at $5.95 by Lane Publishing, Menlo Park, Calif. 94025). There are endless ideas for things to be grown in pots, and although the pictures show a lot of flowers in various containers, an important point of the book is the variety of paving shown, in every sort of material. The range of containers is great, including some you can make yourself.

If you worry about the cost, remember things like nasturtiums, petunias, lantanas, verbenas and other bright annuals grow easily from a packet of seed.

It is not nearly as much trouble as you might think to keep things growing in big pots (or smallish ones) even in the Washington sum- mer. One year I grew bush cucumbers in six- inch pots and hardly ever watered them, but got plenty of cucumbers. Despite what books may say, a lot of things will flourish in containers with watering once a week or even less.

A half-barrel will accommodate a young fig tree or a grapevine -- I often wonder why people with no place for beds or borders do not grow the grape in tubs or large pots.

The critical thing is unity of effect. A fairly high wall of green leaves, a neutral pavement (stoutly resisting the temptation to mix different materials or to inset colored tiles or to include urns or sculpture) with a pool and a bench. Even if you move away after a couple of years, the investment will have been minimal. Dogs, by the way, who love to investigate anything you do in a flower bed, are strangely unmoved by your projects in tubs and pots.

An important book, though not as immediately practical for gardeners with tiny yards behind row houses, is the ambitious "Herbaceous Plants of Maryland," a fine reference work that is the result of decades of work by Melvin L. Brown and Russell G. Brown of the University of Maryland's botany department (The Book Certer, University of Maryland, College Park 20742, and the price is a hefty but justifiable $56.50). There are 2,255 drawings, 272 small color illustrations and an interesting text on the climate, soil and ecology of Maryland. A great many introduced (not originally indigenous to the state) plants are dealt with. This is a grand 1,127-page addition to any reference library.

A plain-dirt book, so to speak, is George Boggs Roscoe's "Here's the Dirt" (Sense Publications, Mount Vernon, Va. 22121, $13.95). Its 130 pages deal with vegetables, with plenty of drawings and photographs, with a folksy text. As in all books written by a gardener who has actually gardened, there are occasional unexpected bonuses here, such as two recipes for soil mixes you can whip up in the garage and store in bags for eventual improvement of your soil. You take 12 cubic feet of sphagnum peat moss, 6 cubic feet of vermiculite, 6 pounds of 5-10-5 fertilizer, 6 cubic feet of perlite, 2 pounds superphosphate and 5 pounds of ground limestone. You mix it thoroughly four times and wind up with some very handy potting soil. There is also a brief account of his tool shed modestly presented as "the best little hoe house in Virginia" that I found irresistible.