Q.Are The Roots Of A 20ft Monkey Puzzle Tree Likely To Affect House Foundations?
The tree stands on the border between the neighbour’s garden and the garden of a property we want to buy and is approx 8 feet from the house. It has been there for years.

Certified GKH Gardening Expert
The article below says to give them "plenty of room." Trees ideally should not be closer than 15 feet from a structure. However, this is such as unusual tree (and endangered) that has merit. I did find an article that said surface roots would make it hard to mow and could upend sidewalks.
https://www.gardeningknowhow.com/houseplants/monkey-puzzle/growing-monkey-puzzle-outdoors.htm
https://landscapeplants.oregonstate.edu/plants/araucaria-araucana