I am buying a house that I will demolish & replace with a new one. In the garden, the existing lawn descends gently down to a nice stream/river. But, between the upper parts of the lawn and the stream is an area of 1,500 square meters that consists of soft wet ground. This entire area is covered with Giant Hogweed and some clumps of tall trees.

This area could look lovely if I could raise & clear the ground and extend the lawn from the house down to the stream.

This is what I am thinking about doing, but would be grateful for any comments or easier/cheaper suggestions...

1) Cut the stalks of the largest Hogweeds and lay flat. ( I am fully aware about how dangerous this stuff is).
2) Lay 1,500 sqm of weed fabric over the entire area to keep out the Hogweed and restrict the rubble fill from sinking into the mud..
3) Demolish the house and use the brick rubble to raise the lawn by about 40cm and incorporate a French Drain system to the stream.
4) Lay another layer of weed fabric over the brick rubble.
5) Lay down 30cm of top soil
6) Seed for the lawn.
Am I going over the top here? I need to create a drain'able surface but do I need so much fabric? I don't want to give the Hogweed a chance to come up. Will the top soil remain in place over the weed fabric or could it wash away?

Thanks, Guy.

New Forest, Hampshire, UK