(Adapted by Rabbi Meir Orlian from the writings of Harav Chaim Kohn, shlita)
Q: I installed an air conditioner and constructed a built-in, custom bookcase in my rented apartment. Does the owner have to reimburse me when I leave?
A: The renter can make property improvements that are beneficial to the landlord as well. If the improvement was important, the renter can demand reimbursement of expenses, up to the amount it benefited the owner, even if the owner initially refused (C.M. 178:3; Nesivos 341:15, 264:6).
If the landlord would not have done the improvement because he lacks funds, he does not have to reimburse the renter, but the renter can deduct the relevant amount from the remaining rent.
However, if the improvement does not add to the rental value, so that the landlord would not have done the improvement, he is exempt from reimbursing. He can tell the renter: “Take what you installed” (C.M. 375:6-7; Imrei Yosher 2:200; Minchas Pittim and Shiurei Minchah 375:6-7).
Thus, the owner would have to reimburse for the air conditioner, but not for the bookcase, unless it increases the rent.