Hi Julia,
you can have you Natural Swimming Pool anywhere. Some are even built right up to the back door.
Have a look at Bill's pool. He came on one of my courses and went off to build this in his garden.
http://www.organicpools.co.uk/bill.htm
You could build it next to your existing rain garden but in order to minimise the possibility of nutrients being introduced into the pool water (nutrients=algae) it would best best not to have them hydraulically connected. However this can easily be achieved without seeing the pool edge by covering the join with gravel.
Hi I’m David. I am probably the only person to have two Organic Pools (aka Natural Swimming Pools). The reason for this over indulgence was to make a DIY film of the process. Loads of these pools have been made all over the place, helping make the world slightly wetter, pond by pond. I have also made a short film for the BBC about Natural Swimming Pools introducing the concept to a wider audience here in the UK. As a consequence of all of this, I have started giving courses to fellow self-builders who have traveled from far and wide to come and sit in my garden and talk about pools and ponds for two days. What could be nicer?
David Pagan Butler organicpools.co.uk