I would suggest in such a case that you try installing a very narrow spot beam X-pole antenna on a near-by base station illuminating the building from the outside. As a bonus, dropped calls due to “hard handoffs” in the upper floors can be reduced drastically by the Dominant signal provided this way.
Such a very narrow beam (in both H&V) antenna can do the job without adding interferences on either side of the target building.
I have seen such a solution in many varied situations worldwide and the actual test results were beyond expectations.
What do you think will be the Pros and Cons in using this solution for providing a signal to such buildings where service is problematic or where QOS is poor?