Suppose our party has happened upon a damaged tower. It's got a chunk missing out of an upper wall, and the floors are in awful shape. But it's well placed, and we could really use a defensible location.
We have a Wizard on hand who is able to cast Fabricate. If we can supply sufficient 'raw materials' for the Wizard, can they repair the tower over a couple days?
Ideally, the path to do this here would be that the party manually busts out the rotted floors, then the Wizard Fabricates new ones. Then they clean up the hole in the wall, and the Wizard Fabricates new chunks of stone wall until they have plugged the hole.
My concern with this is that the Fabricate spell says
You can fabricate a Large or smaller object (contained within a 10-foot cube, or eight connected 5-foot cubes), given a sufficient quantity of raw material.
A part of a wall wouldn't necessarily be an object...but if I Fabricated a 'small piece of stone wall, shaped like this' and just happened to Fabricate it in a location where it meshes in with the rest of the wall...I could see that working. Same idea with the floor...I position it so that it just happens to line up with where it would be installed.
The only thing I have on-hand that would really support this idea is that way back in AD&D 2E, the Spelljammer campaign setting specified that you could use Fabricate to repair damage to a ship. (Concordance of Arcane Space p82) Saying...
Fabricate: This spell can be used to repair lost hull points or maneuverability class for a ship. For every 10 cubic yards of material repaired 1 hull point, 1 AR, or 1 Manueverability Class is regained, up to their original ratings
By that logic...then yeah, you could fix buildings with this as well. But I don't know if that holds through to 5E
So, does this work?