Question
I am planning an addition to my house. My architect wants to build within 3-4 ft. of a 30 ft. pine tree that is irrigated with flood irrigation in AZ. If I do the pie calculation that you have mentioned in other answers, it looks like my foundation will be within 3-4 ft. and the addition will cover/take out approx. 25% of the roots. My architect says never alter a great house plan for a tree. I love the shade the tree provides and hate to lose it. Although I am considering planting a fast-growing Chinese Elm or even some Giant Bamboo (with a root barrier) a bit further away, for shade. Do you think this addition is likely to damage the tree? Will it die? What if I built a basement under the new addition, effectively taking out all of the roots under the addition? Thank you.

Answer
The roots of a tree spread out about twice the width of the foliage area. The roots are within the first 24 inches of the top of the soil so once that area is cut a basement would have no further effect on the roots. Pines do have a tap root as well as the side roots so this type cut will not be as bad on pine as a hardwood tree. The cutting of the roots will damage the tree how much will depend on not only the root cutting but also the construction. If the architect does not care for the trees what do you think the contractor will do. Most contractors first want to cut the trees so they will not be bothered with the tree dying later or be bothered with protection methods for the tree. Since it is a pine and only 30 feet tall you may see some doe back of some of the limbs but it will more than likely survive IF you protect the remaining root zone of the tree and let NO machinery or digging cross the rest of the roots. Put up barriers so the crew will not "forget". You need all the root system you can save for the tree to continue being healthy.
Here is a web link to more infromation on protecting the trees during construction. http://www.ext.colostate.edu/PUBS/GARDEN/07420.html