SO&CO has slotted a skinny office building into an alley between two buildings in Tokyo, Japan, which contains five tiny concrete workspaces.

The office block, named the Multi-Tenant Building in Ginza, is located on a narrow L-shaped plot on a densely-packed backstreet in one of the city's leading shopping districts.

It has a stark concrete facade designed by SO&CO to stand out against the backdrop of the "solid and unattractive" buildings that typically occupy these side streets to grab the attention of passersby.

"From the main streets in Ginza, there are uncountable alleys that radically differ from the idea people usually have of this area – transparent facades and fashionable – and are mainly composed by solid and unattractive buildings," explained the studio's founder, So Teruuchi.

"Being the site in one of those, we felt that, in the middle of this packed area it would necessary to create an iconic building that make people look up, like a bell tower."

To maximise space within the tight constraints of the site, SO&CO divided the building into two adjoining volumes that slot into the site's L-shaped form. The smallest volume, which faces the street, is just 2.7 metres wide.

Here, the entrance is marked by a narrow walkway that punctures the facade, designed to evoke the feeling of entering the alley that has been replaced.

This sensation is enhanced by the giant windows placed throughout the building, which frame the walls of the buildings either side.

The office's narrow entrance leads into a bright circulation space at the heart of the office block, which connects the two volumes via a four-storey open-tread staircase.

Lined with glass walls and topped by a skylight, this stairwell also doubles as a lightwell, feeding light into the five offices spaces that are split over the four storeys.

Like the facade, the interior finishes of each office is characterised by the exposed concrete structure.

This is designed by SO&CO to form a minimal backdrop to the belongings of each tenant, while the shared stairwell is hoped to be used as a shared showroom or exhibition space.

As cities around the world become denser, architects are increasingly having to design buildings that squeeze into the narrowest of gaps.

Other skinny buildings in Tokyo on Dezeen include Width House by YUUA Architects, and Love2 House by Takeshi Hosaka, which featured in our round up of 12 skinny houses from around the world.

Photography is by Takumi Ota.