The oldest part of the existing Dargavel House is a Z plan tower house built by the Maxwells of Dargavel who acquired the property by 1514. MacGibbon and Ross illustrate a 1584 panel with Maxwell coat of arms and initials, but Nigel Tranter describes it as renewed although it is not extant in c.1890 photographs held at RCAHMS. Its whereabouts are unknown. There is a 1670 dated sundial on the southwest tower. MacGibbon and Ross illustrate a plan by David Bryce. The plan consisted of two cellars and a kitchen on the lowest floor with the main stair in the southwest tower leading to the hall and chamber on the floor above. A secondary staircase accessed the chambers on the floor above.
In 1849 John Hall Maxwell commissioned a new wing to the northwest of the original building from David Bryce the completion of which was commemorated in a dormer head dated 1851 with the initials of Mr and Mrs Maxwell in the adjacent dormer heads. The new block contained a new entrance, staircase and service accommodation on the ground floor. The two principal reception rooms were designed on the first floor with the drawing room opening into a corner bay window. The original great hall was converted into the principal bedroom suite and the original chamber became a boudoir. The latter retains an elaborated mid 19th-century plaster ceiling. The floor above contained suites of bedrooms with servants’ accommodation in the attic. Service accommodation had already been added to the south of the original tower as shown in the MacGibbon and Ross illustrations.
In 1910 Peter MacGregor Chalmers was employed to alter and extend the house. He added a new stair tower in the re-entrant angle between the original house and the Bryce staircase, added a new entrance porch, extended the south service range and added a large two storey bay window. Chalmers created a dining room opening into the new bay, a library and remodelled the entrance hall on the ground floor of the Bryce block with new fireplaces and panelling. The original dining room on the floor above was refitted and also opens into the new bay and the adjacent drawing room was remodelled with a new fireplace and cornice. The first floor of the original house was opened up into a single space with a large new fireplace. A beamed ceiling was inserted but the Bryce plaster ceiling over the eastern portion of the new room was retained. The bedrooms on the floors above remained little altered but new bathrooms with elaborate tiling were installed. Peter MacGregor Chalmers exhibited his designs for Dargavel in 1912.
The castle was used for meetings and training when it became part of the defence industry site during the 20th century. In January 2012 its future is uncertain.
Information from RCAHMS (STG)