In general, liabilities are deducted from the total assets in box 3. However, the exception is tax payable. Only inheritance tax payable can be included as a liability in box 3. By paying taxes before the reference date you can avoid a higher taxable amount in box 3.
If you expect to have tax payable upon the final tax assessment then you can request a provisional assessment from the tax authorities or file the tax return and pay the tax payable before the reference date. If you made the request on time but the tax authorities have reacted either too late or not at all, then you may take the unpaid amount into account as a liability. A request for a provisional assessment is on time if it is submitted 8 weeks before the end of the calendar year. A tax return is on time if it is submitted thirteen weeks before the end of the calendar year.