You may want the tenants to vacate the houses so you can renovate and afterwards hike up the rent. Do all leases become invalid when someone buys tenanted property or will the owner have to terminate the leases himself?
It is that way here, when a new owner takes over the property, the present tenant has to enter into a new contract. There was a case when a landlord sold his apartment to his relative just to get rid of a nasty tenant. Legally, the tenant was ejected by the new owner and there was no recourse but to obey the law. When the tenant was out, the administration was given back to the landlord (although the title was not changed anymore).
I have been in two buildings in the past that have been sold. In one case I had to resign the lease under the new owner. The rent did go up when the new owner took over. In the second case I was given notice to move because the person who bought the building wanted to live in my apartment and rent out the rest. I was given the option to move to another apartment in the same building but I chose just to move instead.
Each country has their own laws to deal with this. However in most cases there is some sort of a timeframe... you can't simply kick the tenant out instantly. You will have to notify him that he has to move out by a certain date, once again that date depends on the local laws.
Depending on what country your in, depends on the laws you have to abide by. Here in the UK, asking as we as the landlord give the appropriate notice to the tenant then we don't have to keep them in the property for any longer than we have to.
An appropriate notice is 30 days according to the law. However, it seems that tenants have so many rights here that the landlord sometimes loses control of his property. Suppose the landlord had sold the apartment but the tenant would fight the eviction, the tenant can secure a motion for reconsideration in the local court. There, the tenant stays and what happens to the buyer of the property?