The city of Derry, or Londonderry as it was known at the time of the plan, is situated on the River Foyle at the neck of the Inishowen Peninsula, five miles south of the river's confluence with the large sea inlet of Lough Foyle. While no precise records exist, it is known that people have settled in the area at least since the Bronze Age. In 1965 it had a population of around 56,000. By contrast the larger rural district had a population of about 25,000 and extended south-eastwards from the city in an isosceles triangular-like form for a distance of some twenty miles towards the Sperrin Mountains. Its northern boundary ran along the border with County Donegal and the Irish Republic while its western boundary followed that of County Tyrone. The eastern boundary aligned with that of Limavady Rural District and the remainder of County Londonderry. The total area of the county borough and rural district combined is about 134 square miles, and the rural district is bisected by the smaller River Faughan, which terminates in the Foyle Estuary. The Foyle itself flows majestically in a north-easterly direction, then embraced by its lough, reaches out to meet the broad expanse of the Atlantic Ocean.

Dominating the valley of the Foyle lie the foothills of the Sperrin Mountains to the east and the Donegal Hills to the west. From these mountains and hills flow the River Faughan and the smaller tributaries of the Foyle, first racing through the narrow upland glens and then slowing as they wend their way towards the rich, flat floodplain near the Foyle Estuary. About one half of the area lies at heights of 400 feet or more to as high as 1,500 feet in the mountains. The valley of the Foyle is overlooked by high, rounded hills in the vicinity of the city itself and especially to the south where the valley slopes rise steeply from the tidal river.The topography around Derry and the River Foyle contrasts sharply with the flat reclaimed mudflats (‘slobland’) on the east shore of Lough Foyle protected by sea dykes.