The latest additions are an East Kilbride house, featuring unusual five-sided wedge-shaped rooms, and a single-storey dwelling with an L-plan layout in East Lothian.
Morris and Steedman was prolific in the 1950s, '60s and '70s. Its progressive style was characterised by imaginative use of space and bold Modernist features.
Although responsible for a number of public and commercial buildings, it was the practice's hallmark houses that attracted most attention.
Described as 'beautiful' by Pevsner, Morris and Steedman's designs typically included open-plan living spaces, flat roofs, floor-to-ceiling windows and dramatic interiors with exposed stone and timber panelling.
Deborah Mays, head of listing at Historic Scotland, said:
'Morris and Steedman's radical approach was fuelled by a shared idealism and a desire to rethink the way we live, maximise the practicality and functionality of the home, and emphasise the relationship houses have with the landscape in which they're set.'
She added: 'Morris and Steedman made a very significant contribution to the development of architecture in Scotland and its cutting-edge designs have inspired the work of many contemporary architects.'