In 1896 the eldest son, Maria Adrianus van Nieukerken (1879-1963) joined his father at the firm; from 1903 he worked with his father as an independent architect. A younger son, Johan van Nieukerken (1885-1962) joined the partnership some years later. The sons were also self-taught, although M.A. van Nieukerken also attended lectures at the Technical High School in Delft for several years. A number of major commissions followed. -1898; the restoration of the vestibule of the Paleis het Loo in Apeldoorn. -1898; the first phase of the construction of the Academic Hospital in Groningen. -1899; the construction of De Wittenburg castle in Wassenaar. -1906; the construction of the house de Duin en Kruidberg on the estate of the same name near Santpoort. Mr. Cremer, former Minister of Colonies and director of the Nederlandse Handelmaatschappij was the client. Father and sons Van Nieukerken worked together on this project, designing both the exterior and the entire interior (down to furniture designs). -From 1910 construction of the first pavilions on the grounds of the Martha Foundation in Alphen aan de Rijn began. In 1911, the architects were invited to participate in a competition for a new building for the Colonial Institute (now the Royal Tropical Institute); J.Th.J. Cuypers and B.J. Ouëndag were the other candidates. The board of directors chose the Van Nieukerkens' design because it most closely met the program of requirements and the requirement of beauty. However, the design had to be revised on a number of points so that the construction could be realized within a set budget of a maximum of Fl. 1,200,000. The board of directors finally agreed to the revised plan and the final contract was awarded to the Van Nieukerkens. For J.J. van Nieukerken, this was the last project he worked on; he died Oct. 5, 1913. The Colonial Institute would become the largest project the van Nieukerkens worked on, but it also became a controversial project. The building plans had been submitted to the Municipal Executive, which submitted them to the municipal beauty commission for its opinion. The beauty commission, headed by architect A. Salm, rejected the plans outright, stating that the building complex lacked unity and did not fit in with its surroundings. A conversation followed between the beauty committee and M.A. van Nieukerken, which did not lead to a solution. The board of directors asked for an opinion from a 'recognized authority on architectural matters', Jhr. V. de Stuers, chief of the department of Arts and Sciences of the Department of the Interior and founder of the care of monuments in the Netherlands. Jhr. de Stuers evaluated the design of the Van Nieukerkens positively and he found the report of the beauty committee substandard. The college of B and W apparently valued the opinion of Mr. de Stuers more than that of the beauty committee because the building plan was approved by the college on February 27, 1914. The entire beauty commission resigned. In 1915 the actual construction could begin, which was planned to take two years and another year for the furnishing. However, due to unforeseen circumstances, construction would be continually delayed. The outbreak of World War I soon resulted in scarcity of materials and consequently price increases. The budget set soon proved too small. Much time was needed to raise new funds to continue construction. By 1918, only the substructure had been completed. Preparations for the construction of the superstructure had been completed in 1920; it was now hoped to complete the work in a few years. A storm in 1921, which caused much damage, including to the scaffolding, again caused a considerable delay and an increase in costs. All in all, it took until 1926 before the building could be officially opened by Queen Wilhelmina. The construction and furnishing costs eventually amounted to Fl. 6,000,000. The architects Van Nieukerken encountered resistance more often when presenting their designs. In 1914, the Nederlandse Handelmaatschappij commissioned the construction of a new office on the corner of Lange Vijverberg and Kneuterdijk in The Hague. However, the Van Nieukerkens' design disappeared into a drawer for several years because of the outbreak of World War I and because the head management in Amsterdam eventually had other priorities. In 1920, the plans for construction became topical again and the Van Nieukerken brothers were commissioned to develop their designs further. Now it was the beauty committee of the municipality of The Hague (headed by Prof. K. Sluyterman and Ir. Suyver) that raised objections. The committee disapproved of the design and gave a negative advice to the Municipal Executive, which subsequently refused the building permit. The beauty committee formulated its opinion as follows (according to M.A. van Nieukerken): 'the chosen style of the design does not suit such a large building; []the design lacks the character of a distinguished office building; []the beauty committee did not wish this character, renovation of the 17th century Renaissance, for this building; [] the architects did wrong to continue in the character of their work; []the currents are now entirely different, and one must go with the flow and not against it'.(29) The client, the Nederlandse Handelmaatschappij, supported the design and they rejected the suggestion to choose another architect for the facades and leave only the interior to the Van Nieukerkens. 'For the Hague office they had deliberately turned to the architects Van Nieukerken as they had deliberately turned to the architect De Bazel for the Amsterdam office; both had their own distinct art.' Either Van Nieukerken's design was built or there was no building at all, and this while the existing buildings had already been demolished. In short, quite a conflict. The matter was submitted to the City Council, which finally voted in favor of the Van Nieukerkens' design in a second session by a few votes, 21 to 19. The Municipal College acquiesced to the council's decision. The NHM office could be built. The Hague beauty commission reacted in the same way as the Amsterdam commission had reacted to the Colonial Institute affair; it resigned en bloc. Between the years 1918-1930 the architects Van Nieukerken were awarded several more major commissions. -1915-1930; the construction of the office of the Bataafsche Petroleum Maatschappij (now Shell) on the Carel van Bylandtlaan in The Hague. Construction of the office complex began in 1915; two expansions followed in the 1920s. -1928-1931; the construction of the Diaconessenhuis Bronovo also in The Hague. -1925; the construction of the country house Op Hoevelaken in Hoevelaken for the Van Aalst family. Yet the firm was getting past its prime. The historical direction in Architecture to which the architects Van Nieukerken belonged was losing more and more ground. In 1930, the municipality of The Hague held a competition for a design for a new city hall on Alexanderveld. A number of architects representing different movements were invited to participate; not the Van Nieukerken brothers, as representatives of the "historical direction. They wrote a letter of protest to the Council House Committee, the Municipal Executive and the City Council; however, without result. The architects remained excluded from the competition. But private clients also turned their backs on the Van Nieukerkens. For example, when the Bataafsche Petroleum Maatschappij (Bataafsche Petroleum Company) was building a new office for the BIM (Bataafsche Import Maatschappij) on the Wassenaarseweg in The Hague, the Van Nieukerkens were bypassed and the commission went to the architect J.J.P. Oud. After World War II, commissions declined further. The last major project the architects worked on was the construction of a school building on Sweelinkstraat as an annex to the Household School on Laan van Meerdervoort, 1950-1955. Mainly restoration work remained. In Leiden the restoration of the Pieter- and Marekerk, 1940-1950; in Gorssel the restoration of the Nederlandse-Hervormde Kerk, 1946-1947: in Breda the restoration of the OLV-Kerk which had already been started in 1911; in The Hague the restoration of the Catshuis in Park Zorgvliet, 1949 The health of the two brothers deteriorated and, as there was no successor, the Van Nieukerken architectural firm was closed in 1960.