IANAL but I think you will find that does not obviate the need to seek planning permission, which can still be refused. The condition as I understand it is how and where the water drains, not whether you have a right to let it do so. To put it another way, waters need to drain somewhere (unless it is contained until it evaporates, bottled and drunk, electrolysed into hydrogen and oxygen which is release, sent to the moon, ...........)