Because different people use different areas of domestic properties for business purposes, each case is treated individually. However, the Valuation Office Agency provides some examples for guidance:
http://www.voa.gov.uk/council_tax/examples_wor king_from_home.htm
If business rates were payable, they would not be deducted from the council tax. The domestic part of the property would be assessed as if it existed as a completely separate building to the business premises, and a market value
as at 1 April 1991 calculated for that notional property. (That assumes that the property is in England. The relevant date for properties in Wales is 1 April 2003). The property would then be allocated to the appropriate council Tax band:
http://www.voa.gov.uk/council_tax/bands_englan d_wales.htm
That process
might mean that the property drops into a lower Council Tax band than before (with a subsequent drop in the size of the Council Tax bill) but, if not, the Council Tax would remain unchanged.
Chris