In Germany the normal way to rent is without any limitation in the lenght of the lease. This is called an "unbefristeter Mietvertrag" or unlimited contract. The notice period is then three months to the end of a month, by law. That means at any time this lease can be ended with a three months notice from both sides.
In some cases however, both parties want a lease contract which is fixed for a set period or length of lease. For the landlord that can mean that he does not have to go and look for a new tenant after some months again. For the tenant it means he does not have to be afraid to be given notice from the landlord for whatever reason.
In this case the lease would contain the sentence that both parties waive their right to give notice until a specified point in time. That clause is legally binding for both. So be careful when such clause is being offered and you are not certain for how long your assignment in Germany will be. It may mean that in case when you have to move unexpectantly and give notice, you are liable to pay the rent and may lose all of your deposit, and have to yet see that someone else is taking over the apartment.
If on the other hand you want to be sure you are not the one being kicked out for any reason (except not paying the rent or grossly misusing the apartment or misbehaving so as to disturb social life heavily), then you can add such clause. In some cases a so-called diplomate clause is being added which says that in case of removal from job or being reassigned to another City or country the three months notice period would apply anyhow.
Another thing often misinterpreted is the decoration work to be done by tenant or landlord. As this is quite an extensive subject with many ramifications it must be looked at individually. So email us if you have any questions regarding this.
A specialty in Germany is that you have to give three months deposit in most cases (sometimes two, but three is the maximum by law). This deposit can be given either in cash or as a bank bond or a bank book which is in your name and cannot be used without your agreement; however, the landlord would hold it as a security until all liabilities are settled in the end when you move out. By law the landlord can keep this deposit until all final bills - especially utilities like heating and water - are settled; this can be three or six months or longer in rare cases. You have the legal right to pay the deposit in three rates, the first at the beginning of the lease and then one at the beginning of each following month, along with the rent (which in germany must always be paid at the beginning of a month, along with any utilities advance payments).