if the trees are so close to addition that you would be digging into the roots, you are building too close to the trees. the trees will continue to grow and overshadow the house. you should not have any tree that has limbs close enough to rub the siding or roof at any time. in high winds and storms, trees bend more that most people think, and can cause damage to the house. IMO build on another side of the house, or remove the trees.