The Buildings Department of Müller & Co, under Helene’s leadership, is on the top floor of Lange Voorhout no. 1. She has ambitious plans. Since her visit to Florence, she wants to show ‘how a merchant family from the turn of the century contributed to inner refinement’. She wants to build for the future. In June 1913, the architect H.P. Berlage joins the company. His first important commission is to design an office building for Müller & Co in London: Holland House.

For De Harscamp farm on the Veluwe, Berlage designs a new main building, chicken coops and a pigsty. He also works on the development of a model farm for another estate, De Schipborg near Anloo in Drenthe.

After the plans for Helene’s museum house on the 'Ellenwoude' estate come to a standstill, the Kröllers acquire Villa Groot Haesebroek in Wassenaar in 1915. Helene has had enough of the richly ornamented Huize ten Vijver and wants a more modern and austere home. Berlage produces a design for the renovation of the villa and also for the adjacent hunting house ‘Wildrust’, which is given a new function as living quarters for Sam. In late 1915, Berlage is commissioned to design a monumental country house where Anton can receive his guests: Saint Hubertus Hunting Lodge.

On the advice of Bremmer, Bart van der Leck also joins the Buildings Department in 1914. Helene and Bremmer see the artist as a great innovator and want to give him all the space he needs. Among other things, he designs the monumental stained-glass window ‘Mijnbouw’ (Mining Industry) for the entrance hall of the company’s headquarters, a poster for the Batavier Line, tile decorations and advertising material for De Schipborg farm. He also carries out private commissions for the Kröllers. He provides colour advice for their residences and designs the typography for Helene’s collection folders with reproductions of artworks. When his colour advice is rejected by Berlage in 1916, he terminates his contract. He wants more artistic freedom.

In 1915, Helene asks Joseph Mendes da Costa, whom she regards as the greatest sculptor of his time, to make a statue of the South-African general Christiaan de Wet. She has great respect for the men who fought to the bitter end for their country in the Second Boer War (1899-1902). Monument Christiaan de Wet is placed in ‘the Otterlo sand that most closely resembles some parts of South Africa’. She also asks Mendes da Costa to make a monument for Marthinus Steyn, another Boer leader and a close acquaintance of the Kröllers. With these commissions, Helene lays the foundations for her landscape park on the Veluwe as a ‘permanent monument where nature and art are singularly united’.

Helene has been making an inventory of her collection for some time. Four years earlier she writes to Sam of her hope ‘that it will be realized and I will just have to open my book to know every detail’. On 1 April 1917 the first official painting catalogue is complete. In it, Bremmer provides an alphabetical overview of the collection, which at that time consists of 408 works.