Thorney Mill Sidings, Thorney Mill Road, Iver, Buckinghamshire, UB7 7EZ Data via plota.co.uk

Green belt policy allows a short list of exceptions, such as limited extensions and replacement buildings. Anything else needs very special circumstances that clearly outweigh the harm. Some green belt land can count as grey belt, where it does not strongly serve the green belt's purposes. The council decides this in its green belt assessment or on each application, so we do not mark it here.

Where development of farmland is needed, policy prefers poorer land over the best and most versatile (grades 1, 2 and 3a). Three sources: Natural England's post-1988 field surveys, which are definitive where they exist; the Defra predictive map for England (2025) and the Welsh Government predictive map, modelled from the soil map with 3a and 3b split; and the 1970s provisional map, which planners and other services still show, coarser and with grade 3 undivided. Where the provisional and predicted grades disagree, treat the predicted grade as the current estimate and expect an application to carry its own survey.

Schemes that add traffic, parking or combustion plant are assessed against the area's action plan and may need an air quality assessment.

Nationally significant infrastructure projects (power stations, electricity lines, pipelines, major roads and railways, harbours, airports and reservoirs) are consented by a Secretary of State under the Planning Act 2008, after an examination by the Planning Inspectorate, rather than by the council. The boundary is the one the Inspectorate consults on and can change as the scheme does. Projects are shown within 2 km of the site.