The fundamental issue in the short-term is what your lease says -- whether you in fact are renting the entire premises including the second building, or whether the landlord either limited the lease to certain areas (e.g., the building you rent with the grounds as a common area) or otherwise provided for the possible lease of the second building. The secondary issue is whether you have a lease term (and when it ends) or if you're a month-to-month tenant.
For a month-to-month tenancy, your landlord can modify your lease on relatively short notice (generally one full rental period); and when your lease expires, your landlord may opt not to renew or otherwise seek to renegotiate your lease. Even if you can prevent the second building from being rented to somebody else in the short-term, assuming it is a legal and habitable building that can be approved as a rental under the ordinances applicable to the property -- in the longer term the landlord will be able to rent it to other tenants.

