Lyons House, built 1967, is of state aesthetic significance as an excellent and intact example of Modern Movement architectural design by the eminent Australian architect, author and critic, Robin Boyd, who was principal of the Melbourne-based architectural firm Romberg & Boyd. The work included an innovative response to the site's constraints through the design of an airborne swimming pool as the structural centre of the house with cantilevered living areas arranged around it sufficiently elevated to access fine water views on three sides., The design included the use of traditional materials such as timber and clinker brick, internally and externally in combination with industrial materials such as post-tensioned concrete, and also the use of Japanese-inspired internal partitions. . . The visual expression of structural elements and the valorisation of the natural landscape setting are all important components of the design, which is a model of structural economy and site responsiveness. The significance of the place is further heightened by the design contributions of Marion Hall Best (interiors) and Bruce Mackenzie (landscaping). The only known intact example of Boyd's design work in NSW, the house has a distinctive architectural form and high quality detailing, both internally and externally, that results in a remarkable relationship between the house and its delightful landscape setting overlooking the Port Hacking River and Royal National Park. It has featured internationally in architectural journals and books about the life and work of Boyd. The place has a strong or special association with a particular community or cultural group in New South Wales for social, cultural or spiritual reasons. Lyons House is of state significance to architects and others who appreciate modern design (including architects). It has been listed on the community based heritage registers of DOCOMO Australia, the NSW Institute of Architects and the NSW National Trust. Lyons House was mentioned in every meeting in a series of consultations with NSW experts on Modern Movement design held by the Heritage Division in 2011. The place possesses uncommon, rare or endangered aspects of the cultural or natural history of New South Wales. Lyons House is of state significance as the only known intact example in NSW of the design work of renowned Melbourne architect Robin Boyd (since Boyd's Black Dolphin Hotel in Merimbula has been severely altered). Lyons House exhibits unusual construction and detailing. It is also the only Modern Movement place in NSW known to also incorporate the work of leading interior designer Marion Hall Best and leading landscape architect Bruce Mackenzie. The place is important in demonstrating the principal characteristics of a class of cultural or natural places/environments in New South Wales. Lyons House is of state significance as a highly regarded and intact, representative example of the mid twentieth century Modern Movement approach to architectural design in eastern Australia.