Where by this or the special Act the Occupiers of any Lands or Buildings are made liable to the Payment of any Expences which are directed to be recoverable as private Improvement Expences, the Commissioners may charge the Occupiers of such Lands and Buildings respectively with special Rates, over and above any other Rates to which such Persons may be liable under this and the special Act, after the yearly Rate of Six Pounds Ten Shillings in the Hundred Pounds on the Cost of such private Improvements respectively, such special Rates to be payable during Thirty Years next after such Expences have been incurred.