How to verify building sanction plan copy genuinity?

Hi, I bought an apartment in kaggadasapura Bangalore in 2006 which is in ground floor (above car parking). Construction had started in 2005. It is a A katha property. Builder had the home loans approved from IDBI and SBT. I took home loan from IDBI and closed it in 2011. Now I am planning to sell the apartment and my buyer tried to process the loan with IDBI. The bank suspects that the building sanction plan copy we provided could be bogus and they asked me to get the copy verified and sealed from BBMP office. I approached BBMP Krishnarajapuram and they asked me to contact BBMP Mahadevapura office for building plan related queries. When i approached BBMP Mahadevapura, they looked at the plan and told it is bogus as the plan shows car parking + ground + 3 floors + 4 floor pent house. They said they do not sanction Ground + 4 buildings. One more person in the same office told since the building was constructed 2006, the sanctioning authority was not BBMP Mahadevapura (but main office - it is not clear to me). My questions are: 1. Can IDBI and SBT approve loans without building sanction plan? 2. Is it possible that the builder got an approval but he gave us something different? The original sanctioned plan is not found with any of the owners or the soceity. 3. How to find out which office was responsible for giving the sanction in 2005-2006-2007 period? 4. Assuming there is no copy i have, what is the process to find out if there is a plan sanction for this property and how to get a copy of this? One of the owners in our apartment had taken from SBT and he has also checked for legal with Deutsch bank by paying legal fees (he was working with Deutsch bank that time). The copy I have is a photocopy which has a True copy seal (the text in the seal is partially readable and partially blurred). The signature says "Assistant director of town planning East zone Bangalore Mahanagara Palike" Kindly help me with these questions. I am stuck without knowing what to do next. with warm mregards, Sridhar