If your roof is connected to the combined sewerage network, your downpipe will drain into the ground through a drain grate.
Disconnecting this pipe can be quite a simple process and if you are competent at DIY, it is something you could consider doing for yourself. Alternatively it would be quite a straightforward job for a builder or landscape gardener.
There are different options to suit your garden. The diagrams below show the different ways you can do it, while the factsheets on the right give more detail about some of the considerations.
Rain garden planter with geocellular soakaway