[dragonwriter]

> Take a mature tree, turn it in to a building. Grow another mature tree in its spot and you've now sequestered effectively twice as much as having never turned the tree into a building.

It takes several (in many cases 5+) decades for a tree to mature (in the sense of growth levelling off.) The average proportion of original wood building remaining after 50 years is, well, less than 100%.

Probably significantly, more than if the old tree died and fell, so you could have some net gain, but it's not going to be 100%. (And its going to be even less if you are clearing land for the building.)