We are looking to purchase an apartment and spoke to the occupier of the ground floor apt who is a Rabbi. He advised that the Aron Kodesh in the Beis Knesset (in the basement) is underneath his salon and therefore underneath our’s too.
I’m assuming that this is permissible as it is two floors above the Beis Knesset and none of the bedrooms or toilets are above the Aron Kodesh.
Is there anything else that we need to consider if the salon is above the Aron Kodesh?
Any living quarters above a Beit Knesset is questionable. Shulchan Aruch [151:12] forbids sleeping quarters above a Beit Knesset and remains doubtful with regards to other activities. However various leniencies have been applied to this halacha by the poskim. The Rama quotes Mahari Vyel that any place that was originally a home, and later converted to a Beit Kneset would not be included in this prohibition. The Mishna Brura [sk 39] quotes form the Rambam in his Tshuvos Peer Hador that the main concern is above the Aron Kodesh and not the rest of the shul. Some are lenient with regards to the second floor above the Beit Kneset, see Shevet Halevi [1:27], however this is a matter of dispute [see Shu”t Avnei Nezer 32]. In any event the MIshan Brura brings from the Taz that any usage such as a bathroom should be avoided. So while there is certainly room leniency in your case, living above a Beit Kneset should be a consideration when purchasing an apartment.