The Koopmans-De Wet House is, as far as is known, the first private townhouse in South Africa to be opened to the public. The house was opened as a museum on 10 March 1914.

Although Mrs Marie Koopmans-De Wet had expressed her wish in a letter to Sir Gordon Sprigg, then Prime Minister of the Cape, to have some of the antiquities in the house preserved for future generations, she and her sister in their joint will (1902) did not make provision for this, nor for the preservation of the house itself.

Margaretha de Wet, Marie's sister, did, shortly before her death in 1911, state in a codicil to their joint will that certain items of the collection should go to the Theological Seminar of the Dutch Reformed Church in Stellenbosch and that other items be given to the Old Town House in Greenmarket Square, Cape Town. These antiquities would have to be put on display, in an area appropriately called "Het De Wet's Museum". However, there was never any mention of the house to be used for that purpose. The codicil was rejected by the Supreme Court and the house and its contents were put on a public auction. Shortly after Margaretha's death, the South African News reported "…the National Society of South Africa is strongly of the opinion that the residence ought to be secured by the State…".

A committee was set up to help save and secure the house and its contents for the nation. The committee was established by a.o. Lady Lionel Phillips, Mr JR Finch, then town-clerk, and Dr WF Purcell, a scientist connected to the SA Museum and a personal friend of Mrs Koopmans-De Wet.

In April 1913 the house and some objects from the collection were acquired on a public auction and subsequently handed over to the Trustees of the South African Museum for preservation and maintenance. The house was bought for the sum of 2 800 with financial grants from the Cape Town City Council, the Union Government and from public subscriptions across South Africa.

The opening of the house as a national museum on 10 March 1914 was attended by numerous political and cultural personalities and received extensive attention in the local press. The amount of visitors received during its first year as a museum was nearly 17 000.

Rumours in the press that the City Council intended to alter the stoep of the house in order to facilitate traffic in the vicinity precipitated the proclamation of the house as a protected national monument in 1940. At the same time the issue of the correctness of the name of the museum was raised: should the house be proclaimed in the Government Gazette as the "De Wet's house" or retain the name, Koopmans-De Wet House, which was used spontaneously from the beginning? The debate raged on until it was decided to retain the old name. The house has been under the auspices of the SA Cultural History Museum, Cape Town since 1964.