SO&CO Threads a Concrete Headquarters onto a Two-Meter Flagpole Lot in Tokyo's Koenji

Tokyo has long been good at small buildings on awkward land. Even so, the Koenji Office Building by SO&CO., led by So Teruuchi, starts from an unusually hard brief. The site is a flagpole-shaped lot near Koenji Station, and only about two meters of it touches the street. On it stands a headquarters for a company that has kept its offices and shops in the neighborhood for years. It sells action figures, balance bikes, apparel and accessories. The building rises three stories, adds one basement level, and lists a total area of just 50 m².

The narrow lot is not the most interesting part. The real story is how the architects use reinforced concrete. Japanese RC buildings usually rely on a frame stiffened by shear walls. SO&CO. chose not to work that way. Every column and wall is 300 mm thick or less. Beams change in width and depth from place to place, and the design fixes only the minimum headroom beneath them. The result is a structure that follows a set of broken-up, interlocking rooms instead of forcing them into a regular grid. We think that reversal is worth studying closely.

Two Meters of Street, One Long Climb

From the street, the building barely registers. It shows a tiled sliver between its neighbors, with a van, bicycles and overhead wires in front. This is how a flagpole lot works: the narrow handle carries you in, and the wider body of the plot sits behind the street wall. SO&CO. turns that hidden depth into a sequence of outdoor stair flights. They step past a glazed meeting room and climb toward the upper terraces.

The exterior stair is more than a fire route. Its thin steel railings cast hard shadows. Its landings look out onto the back of the block: apartment walls, a neighbor's fire stair, laundry and air-conditioning units. The building does not hide this view. It treats the messy interior of a Tokyo block as its real context, and we find that more honest than putting on a polished street facade.

The Stair Void as Structural Spine

Inside, the stair void organizes everything. Cantilevered concrete beams support H-steel members running along its edge. Those members in turn carry steel treads that seem to hang next to slender columns. Look down from a landing and the dark treads spiral below you, framed by exposed beams and grey-framed glass doors.

The mix of materials is deliberate. Concrete does the heavy work and sets the proportions. Steel takes over where thinness and reach matter more than mass. Neither material pretends to be the other, and the joints between them stay legible. On a site this tight, that clarity is useful: you can understand how the building is carried while you climb it.

A Concrete Grid That Refuses to Be Regular

With the furniture taken out, the structure shows plainly. A double-height room reveals a lattice of columns and beams of different depths, lit from high windows. The thin columns would normally raise slenderness concerns. Here, intermediate beams act as buckling restraints, cutting each column's effective length so it can stay within the 300 mm limit.

That engineering choice, developed with LOW FAT structure, is what makes the space possible. Because the beams and columns brace each other, there is no need for continuous shear walls. The grid can therefore open, close and shift to suit the rooms. A walkway can slip past a built-in bookshelf. A wall can become a field of square openings. The building reads less like a frame filled with partitions and more like a three-dimensional trellis.

Meeting Rooms Seen Through the Wall

The gridded concrete walls work as structure and as views between rooms. A timber conference table faces one of these walls, and through its square openings you see lounge chairs and sunlight in the next space. In another room, a meeting table sits under a concrete stair that seems to float, with a timber stair and a glazed lounge visible through gaps in the grey wall.

This is what the architects mean by a three-dimensionally fragmented space. No room is fully sealed, and no room is simply open-plan. Each one borrows views, light and sound from its neighbors through openings that are part of the structure. In a company of modest size, that partial visibility probably does more for daily collaboration than any open floor could.

Work Floors Stacked Along the Stair

The work areas are more conventional, and they need to be. Rows of mesh chairs sit at timber desks, and an open stair is visible behind glass. A timber stair rises beside a glass wall. A narrow corridor squeezes between that glazing and oak shelving with a frosted door.

What stands out is how much circulation is on view. On many small office floors, stairs are walled off into a core. Here, people moving between levels pass through the visual field of those at their desks. That could be distracting. It also makes the building's vertical layout easy to grasp, which matters when the plan is split across five levels.

Double-Height Rooms with Borrowed Views

The double-height rooms are where the project feels most generous. A white table with folding chairs, a potted ficus and woven pendant lamps sits under a tall ceiling, facing straight at the apartment block next door. Seen from the top of a stair, the same kind of space opens out ahead of you, softened by plants and pale daylight.

These rooms show how the structural strategy pays off. Because no shear walls take up the perimeter, the section can open vertically where the program allows. A 50 m² footprint might suggest cramped rooms. Instead, the project trades floor area for volume in key places, and that trade works.

Kitchen, Lounge and the Basement Below

The lower levels hold the building's social spaces. In the basement, a stainless steel kitchen counter sits in a concrete room with two hammock chairs and a single pendant lamp. A steel-railed stair leads up and out. On the ground floor, another steel kitchen unit sits against concrete walls, with a grey stair rising toward a glazed door. Elsewhere, a black leather sofa and oak shelving furnish a double-height lounge in front of the gridded wall.

The basement plan does not match the floors above, so a transfer beam at the lower level reconciles the two. This lets the event and work space below run more freely than the stacked rooms overhead. The client's products are playful, and these relaxed rooms suit a company that probably uses its headquarters for gatherings as much as for desk work.

Plans and Drawings

The drawings make the building's logic clear. The site plan shows how small the footprint is within a dense grid of city blocks. The basement holds a large event and work space beside a mechanical room. The ground floor sets an entrance terrace and a compact work space next to a paved grid. The second floor links a meeting space and a work area with internal stairs, and an exterior stair terrace extends to the west. The third floor puts the president's room next to a shared work space. The roof levels shrink to a small terrace with diagonal bracing lines marked across the structural bays.

The section is the key drawing. It stacks event, work, meeting and president rooms, ties them together with internal stairs, and runs an exterior stair up the sloping edge of the site. Read with the plans, it shows a building organized around movement as much as around rooms.

Why This Project Matters

Most small urban buildings in Japan treat structure as a fixed starting point: choose the frame, add shear walls, then fit the program inside. SO&CO. reverses that order. Columns stay thin, beams vary, and only headroom is fixed, so the concrete can respond to the rooms instead of defining them. The building's title on Divisare, "Emergent Connections and Expanded Roles of Concrete," sums up the ambition well. Here concrete encloses and carries loads, but it also frames views, braces neighboring members and shapes how people move.

This matters well beyond Koenji. Cities are full of leftover lots: flagpole plots, narrow infill sites, irregular parcels that standard approaches handle badly. This project shows that an ordinary material, used with careful engineering, can turn those constraints into spatial richness rather than compromise. For anyone designing on tight urban land, it makes a clear case. Rethink the structure first, and the rest of the building has room to follow.

Koenji Office Building by SO&CO. (led by So Teruuchi), Koenji, Suginami Ward, Tokyo, Japan. Area: 50 m². Completed: 2025. Structural engineering: LOW FAT structure. MEP: Y.M.O and Sakuragumi. General contractor: Maekawa Construction. Photography by Hayato Wakabayashi.

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