Manhattan online.

In New York, new offices for a giant electronics communication company built on top of a famous old department store reflect the nature of modern business.

Sidnam Petrone Architects is a youthful New York practice responsible for a series of graceful Modernist inspired schemes. They include private and spec-built houses, offices and shops, and their clients include corporate industries as well as the single family. But in spite of their diversity, the schemes express a consistent architectural intelligence informed by regard for site, purpose and clear form, and an evident interest in materials.

Their design of offices in Manhattan for America Online - a company supplying Internet services to roughly eight million people - achieves the difficult feat of creating a model of functional clarity and at the same time a lyrical place to work. The nature of the business means that the company operates 24 hours a day: and there is always a proportion of visiting staff from America Online's headquarters in Virginia. The brief asked for flexible offices and a similarly adaptable central space that could serve as the office living room and be used for presentations and parties.

The site was lent romance by history and situation, if not by architecture; for it was a decrepit brick box, formerly a skylit restaurant, on the roof of the old Siegel-Cooper Department Store at the corner of 6th Avenue and 18th Street in Chelsea. This enormous palace with its wonderfully exuberant exterior is one of five such stores built at the turn of the century that together made up Ladies' Mile, before the Great War the smartest place to shop in all Manhattan. The War signalled the demise of the store (and the area) and the building was converted into a military hospital.

In 1989 Ladies' Mile was designated a historic conservation area and the various buildings, gradually rehabilitated by private developers, were let out floor by floor. The lower levels of Siegel-Cooper are occupied by various well-known retailers with the upper ones reserved as offices.

On the roof, Sidnam Petrone were confronted by what looked like an old air-raid shelter requiring extensive structural repair and stripping out, as well as new subfloors and services. But there were fabulous views to north and south, an existing skylight that the architects could extend the building's length, and a picturesque greenhouse structure extending about 30m from the shell to the elevator on the east side of the building. This structure was found to be illicit, put up at some point in the remote past and could not be absorbed into the main part of the scheme or enlarged - though it is possible that the structure might form part of a second phase which would encompass the remaining part of the roof. For the moment it has been kept as a corridor, reinforced and lined down its length with translucent panels which can be used for display and serve to screen the newly installed service ducts. Out of ramshackle ingredients the architects have created a luminous arcade that retains the picturesque quality of the original and makes an event out of entrance.

Inside the building they have used an equally light hand allowing its intrinsic qualities to predominate - partly necessarily for the budget was limited. On plan and in section, arrangements are simple, designed to exploit the building's linearity and great light-filled volume. Two levels of offices, connected at the upper level by a bridge, line the perimeter of the building and give onto inner corridors. The kernel of the scheme is a double-height atrium, enclosed overhead by a high glazed roof and, on either side, by gently curving wood framed walls faced with perforated metal panels.

Different intensities of light alter the transparency of these walls. Under daylight they are opaque imparting a silvery quality to the atmosphere, but at night when the place is artificially illuminated, they become delicately transparent revealing the structural grid. An existing spiral staircase which once led to an old water tower has been clad in similarly silvery sheet metal and now leads to an informal meeting room with extraordinary views of the surrounding city.

Apart from its lyrical properties, the curving wall structure is designed to inhibit visitors' views into offices; while the coffering and metal perforations dampen sound, making a large open space quiet and free of echoes. The few pieces of furniture, set on warm wood floors, add the only flashes of brilliant colour. Elsewhere in the offices finishes are simple: pine floors and white painted walls, with proprietary systems used to furnish and divide the workspaces on both levels.

It is hard to imagine, confronted by the wonderfully overbearing presence of the old store, that this essentially modest and delightful scheme is part of it, and it was perhaps the architects' good fortune to have been freed of the baroque constraints that would have inevitably been imposed by the main building envelope. But then the rooftop restaurant is a department store tradition all over the world. It is the hidden surprise, the means by which physically and metaphorically you can break free from the city into light and air, and with some adjustments to circumstance Sidnam Petrone have simply appropriated the tradition.

Architect

Sidnam Petrone Architects, New York

Photographs

Michael Moran