I am often asked to design gardens on sloping sites, and many of these are also on clay. The designs can be either traditional (rectilinear), which are simpler and cheaper for the landscaper to construct, or curvilinear to lead to a more flowing garden with sloping paths and fewer retaining walls.  The planting can then determine the flavour of the garden such as tropical, romantic....
Ann Balmforth
ann balmforth GARDEN DESIGNS - Your local Dorset Garden Designer/Landscape Architect
If you have any questions regarding the designs please don’t hesitate to contact me.