The Community Assembly of the Chilterns is located in the Chiltern Hills, a chalk escarpment to the northwest of London. It covers 660 square miles across Oxfordshire, Buckinghamshire, Hertfordshire, and Bedfordshire, stretching 45 miles from Goring-on-Thames in the southwest to Hitchin in the northeast. The hills are 12 miles at their widest. The built-up area is around Sheffield and Rotherham, with over half the county's population living within it.

The chalk escarpment of the Chiltern Hills overlooks the Vale of Aylesbury and roughly coincides with the southern most extent of the ice sheet during the Anglian glacial maximum. The Chiltern Hills are part of a system of chalk down lands throughout eastern and southern England, formed between 65 and 95 million years ago, comprising rocks of the Chalk Group which also  includes Salisbury Plain, Cranborne Chase, the Isle of Wight and the South Downs in the south. In the north. The chalk formations continue north-eastwards across Hertfordshire, Norfolk and the Lincolnshire Wolds.