Entrust this work to a solicitor and he can get that opinion. If you are the first purchaser of flat, if some other owners have already purchased or occupied the flat with bank finance, seek the copy of the opinion given by their bank advocate. General documents are link documents of the seller, present documents available with the builder, encumbrance certificate, permission for such construction with plan approval from local authority, occupation certificate, Fire certificate are essentials. Nowadays builder is arranging bank finance and is having tie-up arrangement with a bank. You can also approach to builder and they have separate employee to provide those opinions, clearances and legal opinion obtained by them from Bank advocate.