Major garden overhaul - drainage etc

DrPhilG

Posted this in the gardening section, but due to the probable need for significant ground works the Mods there suggested adding it here too.

My house is on a 0.84 acre site but about a third of an acre of it is currently a wilderness of grass, soil, rubbish etc which is behind what is currently the back fence.

Panoramic shot of the area


Various pictures from around the sides





In part of this area, surrounded by a man made banking, is the septic tank.




And this is all currently hidden behind my back fence





And now the point of all these pics...

In the not too distant future, probably at the end of the year (or early next year more likely due to the weather), we want to turn that mangled heap of wasteland into a bigger back garden, with decking, BBQ, seating, decorative stone area and maybe even a pond.

Anyone have any experience of this kind of mass clearance? Drainage will be a major factor. During the winter that raised area around the septic tank tends to be waterlogged and I assume that the banking is there to stop the tank from flooding. The banking rises up on the sides and back, you can walk to the tank from the front via a gate in the fence. To clarify, the area outside/behind the banking gets waterlogged. Not inside the banking where the septic tank is.

It's a terrible waste of nearly half of the site currently and has huge potential so I'm determined to make it work. I do realise it will cost a lot of money but I'm trying to get an idea of roughly how much, and who should I be contacting?

BryanF

if it were me id be contacting a civil engineer to detail/inspect/certify the waste treatment tank and separately I'd get that Engineer to engage/liaise-with one of those football pitch* (*type) specialists who would typically understand and detail drainage underneath the intended leveled off garden