A number of my clients have gone back to the U.K. and northern Europe and then sold their Spanish cars and or had them registered with U.K. plates. Then, to the surprise of all (or not) a road tax bill has continued to be issued against the car and if it was not paid then monies have been forcibly (embargo) removed from Spanish bank accounts to cover the supposed debt.
It is against European Union ruling that a car can be taxed in two member states but if the car has not been properly decommissioned in Spain then it will continue to be taxed until the proper decommissioning process has been completed. One of my clients after selling her Spanish car in the north of Europe had all the paperwork send to the DGT in Alicante but nothing happened and they continued to take the road tax from her account.
This is what the traffic office requires :-
-A copy of the current road tax bill paid and up to date.
-The decommissioning from which can be downloaded from the DGT website at this link http://www.dgt.es/was6/portal/contenidos/documentos/oficina_virtual/vehiculos/bajas/9.060-Castellano.pdf
-The original car documents & photocopy.
-The original NIE and photocopy
-The original passport and photocopy.
-Pay the 8€ fee.
All these documents must be presented and if you are not allowed to go in person they will need to be accompanied with an authorization with the person you send on your behalf.
If you can do this before you take the car out of the country then you are allowed to drive the car for two weeks in Spain in a decommissioned status (baja).If you have left the country and sold the car already then you might have some difficulties with the paperwork but it is not impossible. You will have to send your passport & NIE to Spain to be able to complete the process.
Once the correct documentation is presented then the process is immediate & you will no longer be charged car tax.