Zonnestraal, on the wooded edge of Hilversum, is a 1928 tuberculosis sanatorium designed by Jan Duiker and Bernard Bijvoet. It was always meant to be temporary — built to last just 30 years, the lifespan of the cure they expected to find — and the lightness of its concrete frame, glass walls, and pavilion plan still feels radical a century later.
The site was nearly demolished but was painstakingly restored from the 1990s onwards and is now on UNESCO's tentative list. Guided tours run on Sundays and take you through the main building and the surrounding park; the medical pavilions, set in the trees, have an almost monastic calm. It is one of the founding monuments of modern architecture and one of the most under-visited.