THE SMALLER the garden the harder it is for a gardener to grant a square foot for anything besides his roses or whatnot.

But I noticed one Saturday on Capitol Hill, where the gardens are often no larger than a modest kitchen, that men were surprisingly busy in the alley poking garbage cans.

My friend, who had already finished kicking his garbage can, said it was a territorial sort of thing. They like to bask in every last inch, he said, and valued the little strip in back of their fence.

It can be surprisingly pleasant to have a second "house" even in a small garden.

If there is a gate into the alley for the trash cans, and the garden is only, say, 15 or 20 feet wide, it is a shame to waste that space, though it is commonly wasted, by leaving it with a low wire fence and a little gate beyond which sit the cans.

A solid fence or, if the gardener is rich, a wall can easily be built, with a solid wood gate set in it. The entire width can be turned into an arbor of 4 by 4 posts with 2 by 6 cross members, and a grape or rose or wisteria or clematis, etc., grown over the top for shade.

Even the simplest arbor of posts, set against a solid fence, serves the purpose of an enclosue, and the gardener can sit there contentedly surveying his lands.

Of course if the garden is only 15 feet deep, it would be foolish to try to build a garden house in it, but even there it can often be effective to glorify the gate, by building a trellised hood over it, with a vine on it.

The sentry box can be put to loftier used than holding a sentry. In a very small garden, in the angle of two walls, an area as small as 2 by 4 feet can contain a bench for the gardener, and a drop-down table can be fixed to the wall and this will do for a plate and a glass. Merely getting out of the house and sitting outside is a joy that should never be overlooked even in a tiny garden.

Once I recommended to a friend, whose garden was only 12 feet wide, that he convert a concrete-block compost bin at the end of the garden into a place to sit.

In 12-foot gardens it is dumb to fiddle with compost.

In vast gardens (35 to 50 feet wide) there is no end to the things that can be built for sitting outside. Four posts and a roof for vines, made of an occasional cross piece or some 5/4-inch slats, or 1 by 2-inch slats, just enouth to hold the vine, is the basic thing.

Beyond that, there can be fancier treatments.

At Bodnant, the finest garden of Wales, the rich gardener the rich gardner moved in an old pin-mill. In the 18th century they made pins in the building. He moved the thing into his garden, at the end of a plain and rather magnificent pool of water. The mill was two stories high, with pediment and various Palladian touches.

That is the way to do it.

But often an extension of the garage into an arbor, or a gazebo, maybe with three steps up to it, or an arrangement of trellises, or 2 by 4 frames with bamboo screening or canvas or panels of exterior wallboard painted, can serve the dual function of the major garden ornament and a doll house.

We do not like to say we admire doll houses, so it is important for the garden house to be sober enough and generous enough in scal -- even if it is no more than a sentry box -- to avoid the appearance of a toy.

A big niche to hold once chair looks better than a miniature palace.

It works best if the garden structure is connected to an exixting building or is set at the boundary against a wall or fence. It should have a built-in look, rather than the appearance of being stuck in for the sake of ornament.

This may be a good place to mention that all timber set in the ground should be pressure-treated with chemicals against decay. Sometimes at lumber yards they say cedar and redwood do not need this treatment. They lie.

Life is too short and dollars are too scrace to put up framing that lasts six years.

Few things make me angrier -- because I know the disappointment the gardener will reap-than flimsy trellises.

They should be made of 1-by 2-inch redwood of high quality, without knots, and should be mortised together and rust-proof nails used. They should have one coat of primer paint and two finish coats. They often look better black than white, too.

Rather than use the flimsy ready-made trellises commonly sold at stores, it is better to fix screw eyes into the wall and run wires between them.

It is hard to persuade a gardener that in a small garden the more barriers there are (within reason) the larger it will look. If the gardener sees through an arbor, then a group of posts supporting vines (spaced at least four to six feet apart, naturally, othewise you won't see through them) and then go up maybe a step or two to the back gate which is treated like an arbor, the garden will look twice as roomy as the same garden left completely soild with flowers.

It is important, finally, not to undertake even the simplest arbor without plenty of thought. It is not a bad idea to stick in poles or 2-by-2's to get the idea of how the thing will look.

I find it better always to err on the size of over-generous scale, rather than any hint of miniaturization.

Graph paper, which has never been properly recognized as the greatest invention of civilization thus far, is perfect for getting proportions rsight. But always stick up some poles on the proposed site itself, since an arbor that seems right on paper may look ridiculous when seen in the open air.

Even ther most modest arbor -- 5 by 12 feet, say, and 8 feet high, against a fence or garage -- must be seriously constructed, its posts set 30 inches or so deep, in concrete, and firm sturdy timber used. It is always -- always -- always an error to use a 2 by 4 when a 2 by 6 is required. It is always an error to space the supporting posts more than 6 feet apart if 4 by 4's are used as beams.

A grape vine or a wisteria in full leaf, in a 60-mile-an hour wind, will take down any flimsy structure. The place to play with little slats is the kindling room, not the garden.

Finally, the temptation to erect a great assortment of arbors, trellises, gazebos, roofed gates, pin mills and statues of Venus or George Washington, must of course be resisted.

Thomas Jefferson, if life and fortune had gone father, would certainly have filled up his entire mountain top with additional pavilions, grottoes, loops of chain on posts, and so on. Some of us know all too well the itch to set up posts and beams ad infinitum. But too much is too much.

If there is an arbor with chairs, we should not build another arbor with chairs in sight of it. But I do not mind saying that for most town gardeners, their garden will come into focus for them at precisely the moment they build it, so to speak, rather than plant it.

For a gardener like me, who goes quite berserk with determination to work in, somehow, one more peony, that is a pretty tribute to the power of architecture.