Having done some of each, dropping a large quantity of water in an urban situation isn't the same as doing so on open ground. There are extra hazards to both the aircraft and occupants and to those in the vicinity on the ground.
Besides, unless the roof of the building has already gone, the water won't get to the seat of the fire.
If you even mention it to the CAA, they will need up to 28 days to consider each written application....