Multi-generational living is already popular in Japan, the country with the world’s oldest population (over one-quarter of its inhabitants are aged 65 or above). And with home ownership becoming increasingly unattainable in many parts of the world and an aging global population at risk of social isolation, the trend is likely to take off in other areas.

Having had a head start, Japan can offer inspiration for those in locations less familiar with the concept. Nendo’s Stairway House, for instance, is a two-family home in one of Tokyo’s quieter residential zones. The project highlights how just one interior element – a staircase – can both segregate and connect a shared living arrangement while making the most of a built-up situation. By positioning the architectural volume to the north of the site, the designers could capitalize on the available daylight and ventilation. The resulting glass-fronted south-facing façade allows greenery to flourish in parts of the interior.

Since stairs aren’t the easiest interior elements to scale for aging residents, the older couple’s rooms are located on the ground floor while the younger couple and their child live on the two levels above. The central staircase – which begins in the garden and pierces the interior membrane before continuing through the interior – unites the spaces and their inhabitants and incorporates various functional elements, bathrooms among them. A spokesperson for the studio says that the connection goes beyond the interior to include the environs and the city: the line of the staircase links to the road that extends southwards on the ground level, and extends through to the building’s skylight to serve as a portal to the world beyond.