As one of his most controversial projects as Stadtpräsident (mayor) of the city of Zürich, in 1971 Widmer initiated the plans for the satellite town Waldstatt (literally "city in the woods", also called Sigi-Ville) on the Adlisberg hill plateau for about 100,000 inhabitants, comprising an area of about 4.5 square kilometres (2 sq mi).

In addition to 30,000 relatively affordable housing, 230 classrooms, 10 double gyms, swimming pools, shops, cinemas, churches, a hospital, hotels, a convention centre and a theatre were planned, as well as a subway and underground motorway connections, thus a completely car-free settlement on the surface would have occurred.

[1] Widmer was honoured to hold the Federal Assembly speech on occasion of the 700th anniversary celebration of Switzerland on 3 May 1991.

He published numerous newspaper articles, gave lectures and wrote books on contemporary and historical issues.

[1] Sigmund Widmer died on 11 August 2003 after a short disease at the cantonal hospital of Valais,[5] and was buried at the Fluntern Cemetery.