An international tech entrepreneur with a fondness for architecture asks Rem Koolhaas to build a house on an impossibly small piece of mountainside in Zell am See in Austria. The architect of the celebrated book S,M,L,XL seizes the challenge: how to draw light into a house less than four metres wide that is mostly underground? Photographer and filmmaker Frans Parthesius followed the building process and offers insight into Koolhaas’s way of working and the special relationship with his client.