surtax
noun [ U ]
uk/ˈsɜːtæks/ us/ˈsɝː-/ TAX
an extra tax on income above a particular high level, or an extra tax on something that is already taxed:
In addition, a 10% surtax applies to certain high-income taxpayers.
He'd prefer to see an increase to the surtax rather than the base tax rate.