There is another thread which touches on this but suffice to say that, as far as I am aware, permission is not needed and the safety rules regarding swimming pools do not apply.
Based on that assumption I dug 3 ponds in my garden, originally all connected, some 18 odd years ago, the highest, and largest one, is in the shade of the surrounding forest. This is the one I use for swimming throughout the year and is 7 metres long and about 3 metres wide but, as it was dug out of solid rock, the shape of the width and depth (about 2 metres) is variable.