With more and more people choosing to live in cities, there is less and less affordable housing available, meaning that some municipalities are trying out things like micro-housing or relaxed zoning laws to meet up with the demand.
Of course, not all of these micro-developments have to sit on vacant land; Danish designers Mateusz Mastalski and Ole Robin Storjohann have created a series of clever urban infill concepts that could occupy the residual spaces between buildings, yet remain lit with natural daylighting and looking surprisingly spacious.
Titled "Live Between Buildings," the project proposes several designs sited in various cities like New York, Tokyo, Amsterdam, Helsinki and London. Coming in various configurations, the surfaces are all covered with transparent roof windows to let the maximum amount of light in. 
The designs are definitely not for the disabled, as access to the various levels requires some nimble navigation up ladders and stairs. But the spatial overlapping allows for a lot of different functions to be potentially packed in, while leaving some room for fun things (climbing wall, swing and hammock? Why not).
The project recently won an award in New Vision of the Loft 2 competition, which was organized by roof window maker Fakro and A10 Magazine. According to Dezeen, winning entries had to include Fakro products and would feature urban loft spaces that are functional, energy-efficient, space-efficient and filled with natural light. Say the designers:
The LIVE BETWEEN BUILDINGS project is a new way of living in the city. Infills between existing buildings that consist almost entirely of Fakro window technology enable a life hyper-close to nature and city life, while on the same time exploiting the qualities of the already existing blind walls of the city.
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