If Market Street is where Philadelphians went to satisfy their mercantile needs, Broad Street is where they addressed their spiritual ones. This was especially true on the northern half, above City Hall. As Philadelphia's middle class migrated northward in the early decades of the 20th century, nearly every faith planted a stone-clad flagship on that famously long, straight street.

We know too well how the story has gone in recent years. Many of Broad Street's great religious citadels were torn down, and replaced by fast-food joints, fallow lots, and various other indignities. Others continue to hang on by a shoestring, thanks to the arrival of new denominations that adopted the old buildings and shaped them for their own faiths. But now, in a small, but enormously telling shift in the street's fortunes, one of the old-timers has begun to thrive again. So much so, it is planning a prominent new wing.

Rodeph Shalom, the crenellated limestone redoubt between Green and Mount Vernon Streets, has been one of the unexpected beneficiaries of Center City's recent surge in population. The stately synagogue has added so many new congregants in the last decade that its Sunday-school enrollment has doubled to 300 and its membership roster stands at 1,150 families. Despite having a location just outside the traditional boundaries of downtown, it now likes to boast that it is Center City's largest synagogue.

There is a certain bittersweet irony in this status: On a street that used to swell on the Sabbath with observant Jews, Rodeph Shalom has the sad distinction of being the last of the original synagogues. Its future once seemed shaky, too, and it opened an Elkins Park branch in the '50s to serve members who had decamped for the suburbs. But five years ago, Rodeph Shalom cast its lot fully with the city and sold the satellite. Now its Philadelphia building is bursting at the seams — further evidence that cities are always changing, and in ways you never expected.

To accommodate its continuing growth, Rodeph Shalom has hired one of the city's most respected architects, KieranTimberlake, to fashion a 28,000-square-foot addition to its historic building, designed in 1927 by Simon & Simon. The new wing, which will be built in the synagogue parking lot, will give the congregation a 250-seat chapel, a suite of classrooms and offices and a modern elevator-and-stair tower. The Historical Commission approved the $18 million expansion last week.

Although the architects' to-do list sounds straightforward enough, the assignment is an especially challenging one. Because of Rodeph Shalom's historic stature, architects are constrained in how they attach the addition to the building's south facade, which is graced by a series of slender, arched stained-glass windows. They must also balance a set of functional and urban demands that often seem at odds with one another. Despite the difficulties, which are common to many projects, it's not unreasonable to expect the architects to produce a design that stands up to the magnificence of the landmark shrine, an imposing block of a building with an interior modeled on the Great Synagogue of Florence.

The designers, unfortunately, have concentrated so much energy on solving the knotty functional and historic problems that they seem to have forgotten about making memorable architecture.

Much of the criticism so far has come from the Preservation Alliance's John Gallery, who argues that KieranTimberlake's modern style is so radically different from the vaguely Moorish, art-deco synagogue that it "visually competes" with the historic building.

For me the problem is somewhat different. The contrast of styles isn't a big issue; the proportions are. While the original synagogue is an assertive, vertical structure, rising up from Broad Street's surface lots like an acropolis, the addition is low and meek, burrowing into the ground. It seems to shrink away from the old synagogue as if the two had no familial relation. Placing the entrance in a low building on Green Street, with a driveway and drop-off, completes the impression that a suburban-style synagogue has invaded Broad Street.

The location of the one-story chapel on Broad Street is also troubling. The need to cocoon worshipers from the noise of the street means that the chapel will have a largely blank, windowless wall on Broad Street. Its low-slung proportions also recall the auto-supply stores and car dealerships that once lined the avenue.

In an interview, Richard L. Maimon, the KieranTimberlake principle in charge of the project, argued that there was little choice. The design was driven by two pressing needs: accessibility and safety. Because of the synagogue's multiple level changes, it is difficult for people with mobility issues to navigate between the street, the sanctuary, and the event space in the basement. Even the restrooms are inaccessible to people in wheelchairs. On top of that, the fire stairs fail to meet modern safety standards.

Improving the situation was complicated by the stained glass windows on the south facade, which bring light into the sanctuary and are an important historic feature. Since the windows come down quite far, the front part of the addition had to be kept to one story.

There is no such constraint in the back, however, because the synagogue wall is unadorned. Here, the proposed addition will rise to two stories, housing classrooms and offices. The stair tower, which will serve the main synagogue, will actually top out at 55 feet, about five stories.

Why not simply push the addition back, past the ornamental windows, and build one tall structure? It would eliminate the need for the awkward tiered design, and the empty space on Broad Street could be turned into a garden that would highlight views of the stained glass windows.

The short answer is that the architects wanted to take advantage of a door that had already been cut into the south facade, allowing handicapped people to enter the sanctuary without climbing the Broad Street steps. It's hard to believe there was no other way to maintain that connection other than designing an entire location around that one door. The swooping roof of the addition — reminiscent of the headhouses KieranTimberlake is designing at City Hall plaza — is meant to make the squat addition look taller, but it works about as well as elevator shoes.

Additions to great historic buildings are always fraught. The rules for historic preservation require new wings to be simultaneously deferential and different in style, yet also compatible. What these words mean has never been entirely clear. Maybe that's one reason we see so few extensions that are architectural equals of their parent building.

It's worth pointing out here that one of the best additions I've seen recently was KieranTimberlake's luminous party room at the Michener Art Museum in Doylestown. Although it is a second-generation addition, and therefore not a direct extension of the museum's original 19th-century building, the pristine glass cube works as a perfect counterpoint to that stone structure, designed by Philadelphia architect Addison Hutton. KieranTimberlake's modern cube accomplishes the astonishing feat of completing the original.

None of that was evident in the Michener renderings, by the way. Somehow in the alchemy of construction, the architecture came into its own. We can pray that the same happens at Rodeph Shalom.

Contact architecture critic Inga Saffron at 215-854-2213, isaffron@phillynews.com or on Twitter @ingasaffron.