We are building a granny flat and have had to lose half of a two forked gum tree. The edge of the flat will sit over the stump with the rest of the tree behind it. Is there anything I can paint on, or do to the stump which will prevent regrowth without harming the root system, as we don't want to lose the rest of the tree.
If you are erecting a building that close, the tree should be removed completely, or killed off. There is no way of treating the stump you've cut with a stumpkiller and have that not affect the other side. Further, eucalypts are perfectly capable of regrowing at a rate of 8 feet in a season if cut to the ground, so given that's the case, you should either cut the whole thing down and apply stumpkiller, or have it cut down and the roots bored out prior to construction.