Buying an apartment with an existing rental contract

Correct: A change of ownership of an apartment does not change a rental contract for that apartment. You as buyer just become the new landlord, with the exact same rights and responsibilities as the old one.
You should get a copy of the rental contract and the deposit from the seller. Then you just need to inform the tenant about this change and the new payment destination (i.e. your bank account).

Hallo beppi,

Thank you for the reply.  I have just searched on the internet about giving notice to the tenant.  It seems that the previous owner needs to write a letter to the tenant to inform him/her :
- about the change of property ownership;
- that the new owner will have all rights and claims in relation to the existing rental contract;
- about the effective date and new payment detail (e.g. name and bank account of the new owner).

It suggests that the tenant may deny such kind of letter coming from the new owner.

Is this the usual practice for the seller to do so?   
Shall such a letter be included as a formal document to be transferred to the new owner? 
Shall the Purchase and Sales agreement between the old and new owners mention such a letter as well as the existing rental contract?

Yes, having the old owner inform the tenant is certainly a good idea - and I believe this can also be included in the purchase agreement (Ask the notary public who writes the contract for you!).

Please note that this thread was dormant for 7 months - I am not sure the OP still reads it or needs the advice.
However, for completeness I add the following:

davidpmcguinness2 wrote:

3)  get the deposit ("Kaution") from the existing tenant?
You should request it from the tenant.

You cannot ask the tenant to pay the deposit again - the previous owner (who collected it at the start of the tenancy and, by law, must have kept it separate from his own finances) should hand it over to you. If he doesn't, that has nothing to do with the tenant but is only your problem (and potentially your loss, because you as new landlord will have to reimburse the deposit to the tenant at the end of tenancy - even if you didn't received it from the old owner!).

I am now the new owner of the existing apartment and I received only partial deposit from the previous owner. the rest half is still left to be paid by him. What can be done about it? Even after repeated messages he has still not paid to me the rest half deposit

@paritaorama I understand from your message (which is very brief), that you bought an apartment that is rented out to a tenant with existing and valid rental contract. In that case, the rental contract remains valid and must be honoured by you as new landlord.

However, the change of ownership only becomes legally valid after it is registered in the "Grundbuch" - typically months after you sign the purchasing contract and pay the agreed price to the previous owner. Until then, unless otherwise agreed on in the purchasing contract (check it!), the previous owner remains landlord, receives rental payments and keeps the deposit (if any was agreed on and paid by the tenant - check the rental contract and ask the previous owner or tenant!).

After the official change of ownership, you can claim the deposit (if any was paid) from the previous owner and also infom the tenant to pay the rent to you. You also become liable for returning the deposit to the tenant when he moves out - regardless of whether it was transferred to you from the previous owner or not (that is a problem between you and the previous owner, not the tenant's).

If the previous owner refuses to give you the deposit (in full) - and you can prove that one was paid to him - then you can sue him for it. Please note that the cost and risk of this might outweigh the amount in question, so in many cases a mutual agreement or even writing it off as "lessons learned" is a better option.

Hello @beppi

Thank you for your reply. Yes the transfer of ownership is complete and the tenants are paying rent since September. The previous owner just gave half the deposit and he said the rest half he will give later but it's January now and he still isn't giving it. He is always delaying. What can I do in this case?

@paritaorama I don't know what else to add, in addition to the last paragraph of my reply above. Consult a lawyer!