The house as it stands today presents Neoclassicism at its best.

There is no documentary evidence as to who designed the facade but some historians have attributed the work to the French architect Louis Thibault who worked at the Cape during the late 1780's and to the German sculptor Anton Anreith.

Features such as large sash windows, and large entrance doors are reminiscent of Dutch architecture in the 18th century. Owing to the climatic conditions and the outdoor life style of the Cape, certain features developed which are characteristic of the architecture of the region, amongst others large rooms with high ceilings, shuttered windows and a stoep.

The facade of the house possibly dates from 1790 and is characterised by its four fluted pilasters, some of which are made of wood, others of plaster. The pediment spans three windows instead of the usual one. An architrave crowns the entrance and a triglyph and metope frieze lies directly underneath. There is a lantern in the fanlight of the entrance door. A candle in the lantern was lit every evening as soon as it grew dark. Rectangular panels with plaster garlands fill the spaces between the windows of the ground floor and those of the first floor.

In front of the house, taking the full width of the building, is a raised platform or stoep made out of klinkers which are hard burnt bricks imported from Holland. The stoep ends on each side with a brick plastered seat. A stoep sometimes contained basement rooms or cellars but there is no evidence of this at the Koopmans-De Wet House.

The courtyard is paved with shale from Table Mountain and square slate tiles from Robben Island. Part of the courtyard was only paved in 1918!

The outer buildings at the back were probably used as a coach house and stables, while the first floor is said to have accommodated the slaves. There is, however, no definite evidence of this. The rooms are today used partly as sheds and partly as rest rooms for staff members.

It is interesting to note that the iron railings of the steep stairs leading up to the first floor of the outer buildings originally belonged to the so-called White House across the road from Koopmans-De Wet House. The White House was once the residence of Prof. Changuion, one of the pioneers of Afrikaans. The railings were installed in 1919.The ornate lantern seen in the courtyard once belonged in the Castle. The vine is reputedly one of the oldest in South Africa.
 
The roof of the house was originally constructed with teak beams carrying yellowwood boarding, supporting a layer of lime concrete on which bricks or tiles were laid. These heavy roofs kept rooms cool. This roof had to be replaced due to its condition and is now made of corrugated iron. Some of the old teak ceiling boards are part of the original construction. Occasionally there was a dakkamer or room on top of the roof from which ships entering Table Bay could be seen. There is no loft or attic, but a pencil drawing of the house dated 1889 shows a chimney which no longer exists.

A narrow passage separates the house from its neighbour serving as a drain for rainwater coming down from the roof and as an additional precaution in case of fire.

Finally, some interesting remarks on the colour of the facade can be made. There is pictorial evidence that 18th century townhouses were often painted in different shades whilst the exterior woodwork was green. There are no early contemporary depictions of the Koopmans-De Wet House in existence except for black and white photographs dating from the beginning of this century and one watercolour painting, recently acquired by the SACHM. It is presently in the temporary exhibition room in the museum and shows the front of the house as it was in 1907.

The artist, Leonard A Brimble, depicts the front with a reddish-brown shade and a yellow and orange shade for its neighbours respectively. Dr Purcell who conducted the first large-scale restoration of the house in 1913 noted then that the house was painted from the start. The original colour appears to have been a dark brown.