It may have been converted from an existing structure, but it belongs to a class of structures known as hearth-passage homes, which in design have a common characteristic of the main hall fireplace and chimney backing onto a central corridor, which serves to partition the house into two pieces. Alongside some minor modifications, extensions were added to the front and rear of the building in the later part of the eighteenth and in the nineteenth centuries. All bar one of the extensions has been removed.

The central corridor is entered by a storeyed porch that was enlarged from a smaller entrance and provides access to the hall on the north (upslope) side and two service rooms on the south (the eastern of which is apsidal to the east). A single chamber opens to the roof above the hall on the second level, while a Great Chamber above the service end is hidden behind a loft.

The stairwells to the top floors are contained in walls and illuminated by little arched lamps carved out of a single construction block. The original porch and corners of the main building are adorned with quoins made of Quarella stone, while most of the rooms’ door frames are made of Sutton stone. Other notable architectural features are the squared lights in the porch and the herring-bone shaping in the main hearth wall.

The ground floor is stone slabs, while the top story is timbered. Many of the building’s original timbers have been preserved, although the top floors, as well as some purlins and beams, were replaced in the late twentieth century during a prior attempt to repurpose the structure.

The Old Buildings Preservation & Restoration Trust, St Albans, now owns St John’s House. Attempts have been made in the past to purchase the building and prevent it from being converted to residential usage, which would mean it would be lost to public view.