Subsidiarity is the principle that a higher tier authority should only perform those tasks which cannot be performed by a lower tier authority. He has used a history of planning in the City of Adelaide in South Australia as a case study of subsidiarity. He observes that for a long period from 1840 the Adelaide City Council was able to maintain its own control over planning the city, primarily because it was able to exert influence in the Upper House of the State Parliament. Then from 1972 there was a successful model of a joint approach by the State and the Council to city development. But from 1993 there has been a gradual erosion of the Council’s power and an increase in control exercised by the State. Dr. Llewellyn-Smith has then compared this case with planning in Britain. Visits were made to a range of cities – Birmingham, Bristol, Cambridge, London, Manchester Newcastle, Oxford and Westminster – where he held discussions with city planning officials. He examined the historic arrangements between central and local government tiers, and the current relationship between the National Planning Policy Framework and the Localism Act, 2011 in England, in terms of subsidiarity. The national government’s decision to devolve power to Councils and neighbourhoods was reviewed in terms of the implications for strategic planning, local plans and development assessment.