The builder will give tall promises now in order to sell the flat and it is common for the builders to tell umpteen lies in order to have such wrongful gains.
It is an illegal construction, totally deviating from the construction approved plan.
The builder who assures you now that he will take care of all the problems in future, will not at all be available and he will vanish once the property is sold.
Especially when the authorities concerned wiil issue notice or take steps to demolish the illegally constructed flat, then the builder will not be reachable.
The bankers do not bother about the deviated construction while granting the home loan because they are in hand and gloves witht the builders.
The bank manager who is available now will not remain the same person in future when there is a problem, and the new manager will resort to legal recovery action if there is a default in repayment owing to said reasons of demolition.
You may take a cautious step and of course there is no necessity for you to invest such a huge amount into an illegally constructed property.
Don't get attracted by the looks or other exciting offers and get cheated, i.e., don't fall into the pit with eys wide open
It is advisable that you obtain a proper legal opinion from an experienced lawyer in the local before venturing into the proposed purchase