In the UK, when is planning permission required for a pool in the garden?

Only when the garden is in a Heritage area or National Park or when the building is listed, otherwise they come within the Permitted Development Regulations that limit the eaves height to 2.5m and the ridge height to 4m.

The pool hall also has to be behind the line of the front wall of the house.

Please click here to see the Government Guidance on Permitted Development Rights.