Fourteen different people owned Koopmans-De Wet House and the land it was built on until the de Wet family acquired the property in 1806. This family would own and occupy the house for just over a century and would be the last family to own the building privately before it became a museum.

The Dutch East India Company made grants of building plots in Cape Town in the early 18th century. Streets were laid out in a gridiron plan and were divided up in blocks. Block J was bordered by Strand Street, Long Street, Castle Street and Burg Street and was divided into 10 erven. Erven 7 and 8, on the Strand Street side were granted in full freehold by Governor Willem Adriaan van der Stel to Reijnier Smedinga in 1699 and 1701 respectively. Reijnier Smedinga was originally from Friesland, Holland and was appointed as an official silver assayer at the Cape.

He probably built a single-storeyed rectangular building with a thatched roof as was common at that time. There was probably a wing at the back for a kitchen area resulting in a L-shaped house plan.

In 1722 the house and erf 8 were transferred to Anthonij Hoesemans, lessee of the Company's wine licence. A German, Johan FW Bottiger, acquired the property in 1748 as well as erf 10 in 1760 behind the house. Presumably he intended to build on it - possibly quarters for his household slaves. Bottiger, originally a carpenter, probably enlarged the southwest wing. He was prosperous and owned several other erven and properties in Cape Town.

Pieter Malet from Amsterdam, became the next owner in 1771. He acquired the two remaining portions of erf 10 bordering on Long Street. This provided him with a wide entry from Long Street into the yard in order for a carriage to pass. It is therefore possible that he built a coach house and stable which he later transformed into a warehouse. The frame of the blocked-up doorway is still visible in the wall on the western side of the yard.

Malet enlarged the house considerably in order to house his family of 16 children. This probably comprised the lengthening of the east wing, the addition of the west wing, the heightening of the ceiling of the main part of the house, and a second storey with a flat roof.

Margaretha Jacoba Smuts was the widow of Hendrik J de Wet, President of the Burgher Council during the first British Occupation. She bought the house in 1806 and brought up her five children there. Shortly before her death in 1840 she transferred the property jointly to three of her sons, the fourth one having left the Cape. The eldest son, Johannes (1794-1875), read law at the University of Leyden in Holland. He would later practise as an advocate in Cape Town and involve himself in aspects of cultural, political and educational life of the colony. He was a founder member of the South African College and was a member of the Legislative Council for 15 years. Johannes married Adriana D Horak whose maternal grandfather was Martin Melck, responsible for building the Lutheran Church in Strand Street. They had two daughters Marie (1834-1906) and Margaretha (1836-1911). Johannes bought his brothers' share of the property so that the property became entirely his own. He bequeathed everything jointly to his two daughters.

Marie and her sister, Margaretha were given the best education available at that time: they were taught several languages, music and painting and travelled widely. It serves as no surprise that Marie followed in her father's footsteps to take her own place in the social and cultural life of Cape Town. She married Johan Koopmans in 1864, an officer in the German legion. He worked as a foreign correspondence clerk in the General Post Office until the post was abolished in 1867. Marie and her husband, who were living in Wale Street at that time, subsequently moved to Marie's parental home in Strand Street. In 1879 Johan Koopmans died and Marie wore black for the rest of her life. In memory of him, she referred to herself as Marie Koopmans-De Wet.

Events in her youth such as the anti-convict agitation in 1849, in which her father played a prominent part, had developed within her a strong sense of patriotism. She offered extensive service to the Republics during the South African War (1899-1902). She organised petitions, convened women's meetings and received 2 000 boxes of goods from the Netherlands which she personally sorted, packed and sent to women in the concentration camps. The house in Strand Street served as a depot for all this material. At one stage she was placed under house arrest.

Marie continued to add to the fine collection of antiques, objets d'art and books which her father had collected. She furthered the advancement of the Dutch and Afrikaans languages and worked towards the establishment of a woman's movement. She played a valuable role in the preservation of elements of South Africa's heritage long before any conservation body was established. Thanks to her personal influence she saved the Castle from partial demolition to make way for the railway from Cape Town and she prevented unsympathetic alterations to the Groot Constantia Homestead. Marie helped prevent the demolition of old trees in the Company's Garden as well as the closure of a Malay cemetery at the foot of Signal Hill.

During the 17th century slaves were imported into the Cape. They were either captured in their homeland or bartered before being auctioned on the slave market. The monetary value of a slave depended on age, gender and health. Most slaves originated from the east coast of Africa, Ceylon, India, Indonesia and Madagascar.

In Cape Town, many white adults owned slaves and wealth and status were often determined according to the number of slaves an individual owned. The town slaves were allocated to do domestic work such as cleaning, fetching firewood and water, nursing, escorting the family to church, carrying ladies in sedan chairs to make social calls, etc. In the country the farm labour was performed by the slaves. Some excellent craftsmen were included amongst them such as masons, carpenters, smiths, tailors, furniture makers, and musicians. The quality of living quarters occupied by slaves varied considerably from separate sleeping quarters to odd corners in the farmstead. Proper housing of slaves in Cape Town posed a problem from the very beginning. The Slave Lodge museum was originally built as sleeping quarters for those slaves who worked in the Gardens or were hired out for services.

Domestic slaves of Cape Town were generally well treated. Slave owners were responsible for their conditions and had to provide food, drink and medicine. However, stringent laws existed regulating slaves' behaviour. The law allowed the owner to punish his slave for so-called domestic offences. Desertion and theft were the most frequently committed crimes by slaves who were punished severely, even with death.

Marie Koopmans-De Wet's grandfather owned about 20 slaves. In the inventory of Hendrik de Wet's estate dated 1802 the slaves were carefully described as their value as asset or investment had risen.

Slavery at the Cape was abolished in 1834.