In 1899, Simonsen started his own company which traded in building materials. The company grew rapidly. It purchased
Niverød Brickworks in 1911, Snesere Brickworks in 1916 and later also Kollerup Brickworks. He was also a major importer of timber from Sweden and Finland. In 1918, when
World War I had resulted in a shortage of cargo ships, he attracted media attention for converting a delivery of timber from Finland into a gigantic raft, naming it Refanut and towing it to Denmark. In 1926, he acquired a large piece of land at Svanemøllen in Copenhagen from the estate of S. N. Meyer. He constructed Svanemøllegården and Ulfsgård and sold off the rest of the land in lots. He was later also involved in a number of other development projects, both alone and in collaboration with others. These included Lindevangsgården,
Hostrups Have, Ringkøbinggården, Hollændervænget and Thorsgård. In 1936, Simonsen converted his company to a limited company under the name Harald Simonsen A/S with an initial capital of
DKK 1 million. The property management division was placed in a separate company. ==Personal life==