Is this a bad idea for a green roof?
Is it a houseplant that got too big?
Is it a wheelchair accessible tree-house?
None of the above. It is just weird architecture, designed to preserve a rare Chilean wine palm. The tree was probably planted in the front garden of a Victorian home that was on this site before the site was redeveloped. Chilean wine palms were more popular back then; and this one seems to be about that age. Although it seems to be healthy now, the constriction in the trunk indicates that it had been stressed by the redevelopment, which undoubtedly covered much of the established root system. The time it took for the length of trunk above the constriction to grow coincides with the estimated age of the building below. The tree very likely had better access to rainwater before.
Because it is a palm, the trunk will not get any wider than it is. Because the trunk is so stout, it probably does not move much in the wind. However, the blue tarp around the trunk indicates that there is a problem with the roof leaking around the trunk. It is obviously difficult to get a good seal.
It is impossible to determine from this picture if the building is built on a slab or a simple foundation. A foundation would be a bit healthier for the tree. Perhaps it is more accurate to say that a foundation is less detrimental to the tree than a slab would be. It probably would be better for the building too, since a slab could have been displaced by new roots emerging from the base of the trunk.
Perhaps all this discussion is pointless. This weird but creative idea works. Although distressed and very likely embarrassed, the tree was preserved, and shades the building below.