I’m aware of the limitations of an AI researched answer but this is what I got, and it is in line with my understanding in a practical sense:

The Guardia Civil is correct. The AEAT agent gave them tourist rules. Here’s the actual legal position: The rule: You have one month from when you become a resident to re-register your vehicle with Spanish plates. This is Spanish law, sourced directly from the official Spanish government portal. The 30-day/6-month confusion: The AEAT agent conflated two different rules. The 6-month figure comes from fiscal residency thresholds — but in practice, if the police check your status and it shows “resident,” they are very unlikely to check your fiscal status and will issue a fine for driving a foreign-registered vehicle. Sanctions start at €500. Why it matters: Failure to re-register can result in hefty fines, issues with insurance claims, or even the car being impounded. What “secondary address abroad” changes: Nothing, if they’re registered as residents in Spain. The law is clear that if the owner is deemed to be resident, the vehicle must be re-registered within one month of the person becoming resident.

. France has the same rule. This creates a genuine legal conflict for dual residents with one car. France’s rule: Registration of a foreign vehicle in France is not obligatory unless the owner is resident in France. A resident is someone domiciled in France for more than six months (183 days) per year, or who is employed in France. Once resident, the vehicle must be registered. The conflict: If they are legally resident in both countries — which is possible under certain bilateral arrangements or simply as a practical matter — they have one car that both countries require to be registered locally. You cannot simultaneously hold Spanish and French plates on the same vehicle. What actually determines residency for vehicle purposes is the practical sticking point. Under Article R322-1 of the Code de la route, residency for car-registration purposes in France is linked to your “établissement en France” — not only your immigration paperwork. Spain similarly looks at where you’re actually established, not just your NIE status.

The honest answer: there is no clean legal solution for a single-car owner with genuine dual residency between two countries that both mandate local registration. The practical outcomes people fall into are: 1. Register in the country where they spend more time / where enforcement risk is higher 2. Accept they’re technically non-compliant in one country and manage exposure 3. Buy a second vehicle registered in the other country Since January 2025, French customs and traffic police have stepped up joint operations to spot non-resident drivers who overstay grace periods. The fine for driving without proper registration (“circulation sans immatriculation régulière”) now reaches €750, the car can be immobilised on the spot, and the prefecture may impose storage fees of €6 to €30 per day. Spain’s fine starts at €500 per the earlier search results.