The ground
West Melbury Marly Chalk
Crowmarsh Gifford is mapped on West Melbury Marly Chalk, which the British Geological Survey describes as soft, marly chalk. River gravel and alluvium are mapped within half a kilometre, towards the Thames.
The RHS notes that chalky soils are alkaline and may be light or heavy, and that they hold more water where clay is present. Soil can change markedly over a short distance, so a garden nearer the river may behave differently from one further from it.
Planning
Chilterns boundary · no conservation area
The village centre sits just outside the Chilterns National Landscape — about 140 metres from the boundary — but about 89% of Crowmarsh parish is inside it. There, an outbuilding beside the house falls outside permitted development and needs planning permission, and outbuildings more than 20 metres from the house are limited to 10 square metres in total under permitted development, so check your address before planning one.
Crowmarsh Gifford has no conservation area of its own. The usual limit without planning permission is two metres for a fence, wall or gate — one metre next to a road, and new front-garden paving over five square metres must let water through, or run off onto a lawn or border, or it needs permission.