If you want to just do a one-off business transaction in Spain or buy a property, you should apply for an NIE. Even if you do not want to settle down permanently in Spain, this document is essential for lots of official procedures and is always useful to have.
The NIE has been mandatory since 2011 for all foreigners who want to sell or buy property in Spain or start a business.
We need not confuse a non-resident NIE with a standard fiscal NIE which serves as a residence permit. When a foreign citizen wants to engage in business in Spain, such as purchasing a vehicle, the incorporation of a company, opening a bank account etc, if he does not have a residence permit, he will be asked for a non-resident NIE which will then serve as a tax reference.
Beware: the NIE is not an ID document. European citizens must possess, in addition to the NIE, personal documentation for their own country: a passport or ID card. This identification number must appear on all public documents regarding the foreign citizen in Spain and on all formal administration and most personal contracts and those of a professional nature
The non-resident NIE must be requested personally at the Commissariat de Policia or at the regional public treasury. If the individual is not in Spain, he can resort to the Spanish embassy in their country of residence. One third of the application can in fact be completed by the applicant's attorney. Specifically, you will need to make an appointment with the local authorities to file the documents required by the Spanish administration and the originals of ID cards, residence permits and passports and their photocopies.
All foreign citizens taking part in business activity in Spain, European or not, are entitled to a Spanish Identification Number for Foreigners. People from a country in the EU and who are living in Spain for longer than 3 months should apply for a certificate of residence rather than an NIE.