building regulations state that a new soak a way has to be at least 5.5m away from the house, and at 1 square cubic meter per 15 square meters of roof area the water falls from. you are now not allowed to run storm water in to fowel drains.
You can get advice on the design for the soak a way from the water authority. For heavy clay its best to have a deep rectangular soak a way filled with clean crushed hard core.( broken clean bricks only). Its not how deep or how big it is. Its how its designed that matters.
Spending a LOT of my time doing land drainage system and soil improvement work for people that are stuck on clay, can you tell me exactly how that will work then?
as for planting conifers, well they will struggle on a heavy clay soil anyway