In the late 1960s, Lagaay went to work for Simca, then from 1971 for Porsche. He worked in the team designing the Porsche 911 and designed the Porsche 924. From 1977 Lagaay worked as Chief of Design for Ford in Cologne, and moved to BMW in 1985.
Lagaay was behind the design of the BMW Z1 (designed 1985-1986), an experimental small roadster where Z was taken from Zukunft (German: Future), the body presented for the first time in 1987 and produced between 1989-1991; Z-1 had an aerodynamic design, removable body panels with an idea of simple color change of the whole car, side panels made from plastic, electric doors which open by sliding down into the car's body; Z1 development led BMW to several patents; however despite an initial enthusiasm and speculative buying, the car felt underpowered and did not sell - only 8000 cars were made
Lagaay - an avid sports car driver himself - returned to Porsche in 1989 as head of the "Style Porsche" department in Weissach after Urlich Bez, director of BMW M-Technik Gmbh, who led the Z-1 project, also moved there.
As well as the Porsches of the period — the Porsche 968, Porsche 993 (the 1993-1997 generation of the 911 for which Englishman Tony Hatter is also credited), Boxster, Cayenne, 996 (the all-new 911 of 1997), Carrera GT — he also oversaw the company's work for external clients.