Hi,
You can ask the builder to provide photo copies of all the documents related to permissions he got from the competent authorities for that particular apartment.Once you get those documents,you please get them verified by the legal expert.He will guide you whether all the required permissions were there for that building or not.If any shortfalls are there after the thorough verification of all the permissions the builder received from the competent authorities,then it is advisable to not to take that flat.
If at all,the builder is not ready to share all the details about the permissions he got , you can approach the competent authorities who had given required permissions for that building and check whether the builder got the approval for the 5 th floor or not.If it is clear after having check with the authorities that there is no approval for the 5 th floor,you can drop your idea to purchase flat in that apartment.
One more thing you can do is that,you can approach another bank with all the details and request for the loan .They will also do all the research before approving the loan.If the other bank also rejects your loan application,then it might be because of not having the approval for that 5 th floor. Banks will give the reasons for not approving your loan application.
you can go with any of the above ways to find out whether the builder got approval for 5th floor or not.