Question
How do you kill a Willow Tree that has got too big for the garden, and is twice the size of the house

Answer
They are fairly easy to kill. Buy a herbicide called Roundup. It will not take much so get the smallest container you can find. Cut the tree at the stump and immediately paint the freshly cut stump with Roundup. The herbicide will be taken up by the tree stump and transported to the root system and kill the tree and its roots. Roundup is a herbicide that will kill almost any green plant so when you paint the stump do not get it on any other plant not wanted to be killed. The active ingredient in Roundup is Glyphosate. You can find and use other name herbicide products with Glyphosate and these maybe less expensive than Roundup. The herbicide will not go from the tree's roots to other plants there so in that way it is a safe herbicide to use around other plants. The tree can be cut down in sections if it is too big for one cut at the stump. When you make the final stump cut at near ground level then apply the herbicide.