He was the brother of Mogens Lassen, also an architect. He trained as a mason before completing his education at the Technical School. 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.
Other striking features were windows that rolled down like car windows, a conveyor tube for the mail and a kitchen stocked with ready-made meals. Together with Jacobsen, he went on to design Søllerød Town Hall, completed in 1942 in a classical modernistic style inspired by Gunnar Asplund's extension of the city hall in Gothenburg. In the 1960s, Lassen completed a number of cultural centres and libraries including the Randers Cultural Centre, complete with a museum, a library and meeting rooms, in 1969.
The three-storey rectangular building in reinforced concrete is illuminated from a central courtyard while the walls along the streets are free of windows. All display strongly Cubist lines with rather raw exteriors but with well formed, welcoming rooms. In the 1930s and early 1940s, with his unconventional curved designs, Lassen contributed to the development of the Danish modern style.
Always intent on simple, clearly defined lines, he also designed lamps and silverware.