The rules for taxing petrol company cars apply equally to diesel cars, but the latter are then subject to further adjustment.
The taxable benefit arising on a car is, broadly, calculated using the car's full manufacturer's published UK list price, including the full value of any accessories. This figure is multiplied by the 'appropriate percentage', which can be found by reference to the car's CO2 emissions level using HMRC's ready reckoner. This will give the taxable value of the car benefit.
For cars that are 'propelled solely by diesel', a supplement of 3% is normally added to the 'appropriate percentage', although, from 2015/16 onwards, the maximum cannot rise above 37% (previously 35%).
The diesel supplement was due to have been abolished from 2016/17. However, the 2015 Autumn Statement announced that the abolition would be deferred, meaning that the result that the 3% diesel supplement continued to apply to all diesel cars registered on or after 1 January 1998.
At the time of the Autumn Statement, it was announced that the diesel supplement would remain until 2020/21, although, since the legislation currently contains no provision for its withdrawal, the date remains unconfirmed.
However, legislation contained in the most recent Finance Act (Finance Act 2018, s. 9) increases the diesel supplement to 4% (from 3%), with effect from 6 April 2018. This is still subject to the overall upper limit of 37%.
The increase applies to all diesel cars first registered from 1 January 1998 to 31 August 2017.The increase also applies to diesel cars first registered from 1 September 2017 that either do not meet, or are not certified against new standards for nitrogen oxide (NOx) emissions under the Real Driving Emissions (RDE2) regime (known as 'Euro 6d'), or which do not have a certified NOx emissions figure under RDE2 standards.
The diesel supplement does not apply to cars registered after 1 September 2017 if their certified NOx emissions meet the new standard. However, diesel cars registered on or after 1 September 2017 will be subject to the 3% diesel supplement for the period 1 September 2017 to 5 April 2018 irrespective of their NOx emissions value.