After working in a number of different architecture studios, in the 1930s Lassen set up office with Arne Jacobsen with whom in 1929 he had won a Danish Architects Association competition for designing the "House of the Future".
Built full scale at the subsequent exhibition in Copenhagen's Forum, it was a spiral-shaped, flat-roofed house in glass and concrete, incorporating a private garage, a boathouse and a helicopter pad.
In partnership with Erik Møller, he designed the Nyborg Library (1940), for which he was awarded the Eckersberg Medal.
The three-storey rectangular building in reinforced concrete is illuminated from a central courtyard while the walls along the streets are free of windows.
[3] In the 1930s and early 1940s, with his unconventional curved designs, Lassen contributed to the development of the Danish modern style.