Weeping Willow(20ft), 5 metres from property.Is this root risky?

Unfortunately, with willows, keeping the crown pruned does not restrict their root spread. The recommended planting distance for large willows (and the shrubby, coloured bark kind) is a minimum of 45 feet from any house or structure - my maths ain't great, but I think this one's too close to your house by about 30 feet. I'd be taking advice from a landscape architect or an arboreal expert as to whether it should be removed, or whether removing it will cause more problems than it prevents.

there even worse over clay soil.when theres a water shortage like summer they drink all the surrounding water shrinking the clay and making your house unstable.if you have a big garden and like the tree.weight till very early spring say febuary.cut of the size of new tree you want and push it into the ground a couple of feet.water it well and it will become a tree and just kill the parent tree before it kills your house.underpinning isnt cheap

See an Arboreal expert, as a good one WILL know about structural damage caused by roots of various trees in various soils.
Willows are very thirsty trees and will absorb lots of moisture from the surrounding soil.
If it's cut down what will be the consequence of NOT having that water drawn away?
I'm not trying to worry you, but these things need to be addressed when concerning felling close to properties.

I hope this helps. If it didn't, at least it gave you something to read ;-)

very true it does depend on the the type of soil and water content