Are you allowed to modify the space? In that case it's probably better to post a question in the studio construction area of this forum. There are many very knowledgeable persons there who can help you with ideas how to address this problem.
It sounds like you would need to build another insulated wall (plasterboards + rockwool) to block the noice. Exactly how thick it should be and how many layers is hard to say without seeing some pictures of the place and measurements.
Gear: FocusritePro40+Octopre2, MackieOnyx32, Korg TritonExtr+MOSS+EX800+TRRack, Roland XV5080+D550+MKS7+MKS50+MKS70+MKS80+S550+JP8080+Juno60+JD990+DR660, NordRack2, ATC1, ESQM, Yamaha A5k+An1x+TX802+TG77+TX7+MotifRackES+RY30, Akai S5k+MPC1k, Blofeld, Pulse, Mopho, Indigo2, Emax, Rogue, MachineDrum, KawaiR100, ProOne, Drumulator, M1AM1, Linndrum, CZ101, CR68
Effects: API 512C+525, Boss SE50+SE70+CE300, UAD2Duo+Solo, Powercore, SRV330, MPX550, Rev2496, 1204, DP2, Filterbank, 9010