Elsie

[Elsie]

[maybelateron]

If the crown does not actually extend as far as the boundary line there are unlikely to be any major roots at the boundary. There will be lots of small/very small roots around the boundary, but I would be surprised if an arboricultural report expressed any major concerns. You say the tree is about 85ft tall, and (presumably the trunk) is within 20 ft of the boundary. If the crown does not reach the boundary that would suggest a tree that is twice as tall as it is wide. Are there other trees close to and around it? Do you think perhaps it is rather less than 85ft tall? 

Photos may be useful.

[Spideylj]

The tree report should be done to British Standard (BS 5837:2012).  As a general rule of thumb the root protection area of the tree will be 12 times the diameter of the tree.  The exception of this is in the case of a veteran where it should be 15 times the diameter or 5m past the drip line of the tree, whichever is greatest.

 

As for your extension, it will all depend on how close your extension is to the tree, what foundation design is being used, drainage and construction method.  I would also consider carefully what the extension is being used as the tree may affect light into it (again depending on how close it is).

 

Your Arboricultural Consultant should be able to advise you on these matters.