Around 1955, the number of assignments increases, resulting in the hiring of more employees. In 1961, a partnership was formed between Rietveld and Van Dillen, who had joined the firm in 1958, and Van Tricht, who joined in 1959. Rietveld functioned in this partnership primarily as the designer. Van Dillen and Van Tricht were the ones who took care of the organization of the growing practice and contributed their architectural knowledge when carrying out extensive assignments. They also produce their own designs, although to a lesser extent than Rietveld. The number of employees at the beginning of the 1960s is 22. In July 1964, Rietveld dies at the age of 76. Van Dillen and Van Tricht continue the work on the remaining commissions, some of which were already given before the association.