Clarification on legal approval for building a house

whether municipality can tell to leave 5mt.space from a water tank before construction of building because already a guard wall had been constructed beside the tank which was approved by the same municipality? I have a plot of 1440 sq.ft area under madhyamgram municipality at west bengal. Beside my plot there is a water tank partially along the southern side. when I bought the piece of land a guard wall was already standing there which was approved by the municipality and seeing that only I have bought the same and on that approved plan of the guard wall it was also written that the plot is fit for building construction.Now, when I applied for a plan for building construction the same municipality is saying that I have to leave exactly 5 mt. space from the tank and they are also saying that they have commited wrong to approve the plan of the already constructed guard wall. My question is when the guard wall is already given whether the municipality can ask me to leave 5 mt space.