Bangalore Water Supply & Sewerage Board [Bwssb] — Excess Billing

I have a legitimate BWSSB Water connection with RR No - N-348332 taken five years back. We extended our first floor about three years back.That time meter reader has put the bill under Partial ND & never reverted back to Domestic tariff even after completion of the work.The bill should be on the quantity of water consumed under various slabs & that is fair.But, why are we getting levied an additional amount of Rs 750 every month which is unreasonable & no one gives any clarity on this even after several requests.