Justifiable difference, ECJ

Under the Netherlands social insurance system insured people whose taxable income was derived from employment or home ownership had to pay social security contributions but were entitled to reductions. However the reductions could not be set off in full against the contributions; instead the taxpayer received a national insurance tax credit whereby his income tax was reduced by the non-deductible part. Non-resident taxpayers with investment income only in the Netherlands could not be insured under the Netherlands social security system and were therefore not entitled to national insurance tax credits.
The taxpayer who was a citizen of and resident in Belgium owned a holiday home in the Netherlands and paid tax in the Netherlands on the holiday letting income. He was not allowed any national insurance tax credit. He objected to his assessment and the court asked for a preliminary...