They are called "in-fill" building sites, and we are all familiar with them: places in built-up areas throughout the metropolitan area that for one reason or another are attracting new construction.

In the large scheme of things, this process is good. For sound economic, environmental and social reasons, it is usually best to locate new buildings close to existing services such as roads, trains, electrical lines and so on.

But every in-fill site presents its own special set of challenges. In matters both large and small the existing architectural context has to be taken into consideration. Questions of fittingness always come up. Are these the right land uses for a specific area? Are the new buildings the right size and shape? Are bricks the proper material, and are these bricks the right color?

Such questions, of course, can have more than one good answer--and more than one bad answer, as well. A number of new buildings and building proposals along a two-mile stretch of Connecticut Avenue NW concisely illustrate a broad range of responses to demanding in-fill conditions.

To start with a high point, both literally and figuratively, take a look at a new condominium apartment building at 4025 Connecticut, close to the crest of the Van Ness hill. As designed by Cunningham + Quill Architects of Washington, this 90-foot-tall structure is a refreshing if flawed addition to our area's abundant inventory of skillful contextual architecture.

What is refreshing about the design is the generally successful aim of partners Ralph Cunningham and Lee Quill to expand the local contextual lexicon a bit. Also, Maryland developer P.N. Hoffman spotted a market niche for duplex apartment units saturated with natural light.

The building is in the middle of a block. Abutting it to the south is a five-story red-brick apartment house with art deco stonework; to the north are conventional two-story row houses, also brick. Across the street is a park with high, old trees and Australian architect John Andrews's 15-year-old Intelsat headquarters with its distinctive octagonal pods, cylindrical stairwells and facades of silvery glass.

Cunningham and Quill responded to this mixed context with a couple of sure-fire moves and a nice surprise in the middle. Using the art deco building to the south as a starting point, they designed two red-brick, "stone"-trimmed (actually, precast concrete) five-story wings. Then in the middle they set a metal-and-glass tower that rises from the base of the building all the way to the top.

It is a sort of toothpaste tube effect--the wings appear to squeeze out that light-colored, narrow top. This concentration on the middle and the top is all to the good, for it is the top--carefully shaped and detailed with a certain precision--that gives the building its charm. As passersby we share just a bit in the romance of penthouse living behind all that glass.

Fundamentally, then, this is a modern building that adapts politely to its circumstances. Rather too politely, perhaps--the stone cornices atop the two brick wings, looking as if they had been lifted out of a handbook of traditional moldings, are formulaic and jarring. Compare them, as you pass by, to the sleek steel fins of the building's upper section, and you will see that they do not belong. They detract emphatically from the design's integrity.

One other fault bears mentioning. The metal-sheathed elevator out front providing handicapped access from ground level to the entrance porch looks like a garbage bin, demeaning to the people who use it and harmful to the design as a whole.

"On the Avenue, Over the Park" is the marketing refrain for another new apartment building that is nearing completion several blocks away, at 4411 Connecticut. The refrain is literally true--the building faces the avenue on one side and on the other, it towers over Soapstone Valley Park, one of those fragmentary extensions of Rock Creek Park. That the apartment house is being built at all testifies to strong demand for luxury housing in Northwest Washington: Given the plunging escarpment at the rear, the site had long been considered unbuildable at a reasonable price.

Normally, this building, developed by Charles E. Smith Residential Reality Inc., wouldn't merit mention in an architecture column. It is chock-full of high-end treats (thick carpets, 24-hour concierge), but the architecture customarily would not be considered good enough--or awful enough--to single out. In a discussion of Connecticut Avenue's context, however, it is a pertinent example--and quite a bad one. The building is a passionless, thoroughly undistinguished rehash of the tried and true.

Old formulas can be given vigorous new life--witness the Saratoga at 4601 Connecticut, architect David Schwarz's decisive 1989 recasting of a gabled, brick-and-stone Washington standard. By comparison, the new Park Connecticut shows nothing better than halfhearted cleverness in its arrangement of familiar motifs--base-middle-top divisions, incised ornament, projecting bays, niched balconies and so on.

In-fill construction, by its very nature, is controversial. Even positive change can be upsetting, and sometimes proposed changes are just downright bad. Almost always, there is a variety of strong opinions. Not infrequently, in Washington's lawyer-rich neighborhoods, disagreement leads straight to the courts.

One such case in the Connecticut Avenue corridor involves the Kennedy-Warren, that extraordinary art deco apartment building designed by Joseph Younger in the early '30s but never completed. A proposal to finish the complex (at 3133 Connecticut) with a wing of Younger's design, including appropriate changes for today's needs, ran afoul of some in the neighborhood and is being contested with a lawsuit.

A flier distributed some time ago by opponents of the expansion showed images of the site with and without the added wing. It was, in a way, an extraordinary document--although the images were not labeled "bad" or "good," the assumption clearly was that any fool could tell the unbuilt site was preferable to any building that might go up.

Yet I am sure I was not alone in thinking the picture looked a lot better with the proposed building in it. If ever there was an urban site that seemed destined to receive a large-scale residential building, this is it--a highly desirable location to lure tax-paying middle-class citizens back from the suburbs, close connections to mass transit, plenty of businesses and services within walking distance, fine architecture that already has passed the test of time, and a distinguished firm--Washington's Hartman-Cox--to oversee the improvements. So, we'll see. Perhaps someday the Kennedy-Warren will get out of court and be completed as beneficently planned.

Not all in-fill sites are equal, however, and as a reminder we should conclude with the strange, cautionary case of Cathedral Mansions South.

This is a lovely apartment house at 2900 Connecticut designed by Harry Wardman in the '20s, a building that sprawls across a lawn in an inviting, relaxed manner. Despite the obvious evidence that the lawn and the building clearly go together, a decade or so ago the owner decided that the best thing to do was to build eight town houses on that stretch of grass--a preposterous notion that has been consistently rejected, most recently by the D.C. Circuit Court of Appeals.

The case is a useful reminder that, although building wisely at in-fill sites is important to the future of metropolitan regions such as Washington, it does not follow automatically that all of them should be filled in. The adverb is the key: Build wisely.

CAPTION: The tower at 4025 Connecticut Ave., above and in a rear view below, took some cues from a nearby art deco building.

CAPTION: Fitting right in: The 4025 Connecticut Ave. NW apartments.