Is a national patent useful for a subsequent international application?

The short answer is yes, but there is a time limit. When you file a patent app in a first country, you can file a PCT application within one year of that first filing which then opens the door to other European countries as well as countries outside of Europe, but the PCT filing must be filed within one year from the date of your first application filing.

If you believe you will file in other countries, a PCT application is your best option rather than filing in just one European country. Although it is more expensive than a national filing, a PCT application will grant protection in up to 148 countries. However, you will need to eventually file for specific country/region protection no later than 30 months after the earliest filing date of your initial application.