I am planning an addition to my house and will need to cut some roots of a grand Maple in my back yard. Whilst I am reasonably confident that the tree will not suffer catastrophically, I am wondering if the root will continue to grow and should I therefore cut it further back from the planned foundation. I believe the tree to be about 70 years old, it is about 20" in diameter and the foundation will be about 10' from the trunk.

Thanks, bobekernahan.

Hi! bobekernahan, and Welcome to AMHD :)

In all likelihood yes.
I have seen roots grow and push up through the ground after cutting, and I have seen them die.
Because of your particular situation (building an addition) I would opt for complete eradication.

To further help you in your decision see:

Ask Me Help Desk - Search Results
And
Cutting maple roots - Google Search