First, it is very unlikely that the building is finished. When there is combat in a city, the buildings in it take damage, even the "partial" one that is under construction. Even if this damage is just a few percent, it is usually enough to make sure that the building is taking THAT much longer to construct until half an hour later the next combat round, when it will take damage again. In other words, the destruction by the battle is going faster than the build process.
Second, this usually isn't so very important. The partial building (say, 95% of a fort or a recruitment center) is ALREADY functioning; the building doesn't have to be finished to suddenly get the full boost from 0 to 100%.
The only thing that springs to mind where this is really important is an airfield: when it gets to level one, any planes in the city are suddenly in the air again, and they can either flee or use their air stats in the battle, for example by patrolling right overhead. Given the first point, this is probably academic: the airfield needed to get under 100% before the battle started, but not damaged enough below it to take construction off the table completely.
