yawningdog writes:

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.

In the US you'd be messing with wetlands and need some serious permits.

I've never seen hogweed here in the US. I just read about it though.
Doesn't sound like something I'd want to get up close to.
Spray it from a distance, let it die, work your way down the hill.