The once-only principle: ask once, reuse safely

The once-only principle means that people and businesses give a piece of information to public administrations once, and administrations then reuse it, within the law and data protection rules, instead of asking for it again. The European Interoperability Framework (EIF) recommends it: as far as legislation allows, users should be asked for information once only, and only for relevant information.³ For services across EU borders, the Single Digital Gateway Regulation turns the idea into a working system, the Once-Only Technical System (OOTS), through which authorities in different Member States exchange official evidence at the user's request.¹

What the principle means in practice

Once-only is about reuse, not about collecting more. An authority that needs proof of a birth, a residence or a qualification asks the authority that already holds it, rather than asking the person to fetch a certificate and hand it over. It is not a licence for every body to read every register. In the EU cross-border system, an exchange needs the user's explicit request unless Union or national law provides otherwise, and the evidence may be used only for the procedure it was requested for.¹

The EIF pairs "once only" with "relevant only".³ The first removes repeat requests; the second stops services asking for information they don't need in the first place. Both reduce the burden on users, but only the first depends on public bodies being able to exchange data with each other.

The Single Digital Gateway Regulation

Regulation (EU) 2018/1724, adopted on 2 October 2018, establishes a single digital gateway to information, procedures and assistance services for people and businesses using their rights in the single market. Its common user interface is integrated into the Commission's Your Europe portal.¹ The Regulation also requires Member States to offer the procedures listed in its Annex II fully online, where they exist nationally, and to let cross-border users complete national online procedures on the same terms as everyone else.¹

Article 14 is the once-only part. It requires the Commission, working with the Member States, to set up a technical system for the automated exchange of evidence between competent authorities in different Member States. Where an authority lawfully issues relevant evidence in an electronic format that allows automated exchange, it must make that evidence available to requesting authorities in other Member States too.¹ The detailed rules are in Commission Implementing Regulation (EU) 2022/1463.²

Which procedures are covered

The system covers the online procedures listed in Annex II of the Regulation, plus procedures under four directives: on recognising professional qualifications (2005/36/EC), on services in the internal market (2006/123/EC) and on procurement, including by utilities (2014/24/EU and 2014/25/EU).¹ Annex II lists 21 procedures, grouped by life event, an approach explained in life-event services:¹

• Birth and residence: requesting proof of registration of a birth, and proof of residence.
• Studying: applying for study financing, applying for admission to public tertiary education and requesting academic recognition of diplomas.
• Working: deciding which country's social security legislation applies, reporting changes relevant to benefits, applying for a European Health Insurance Card and submitting an income tax declaration.
• Moving: registering a change of address, registering a vehicle, and obtaining road-use and emission stickers.
• Retiring: claiming a pension and requesting information on pension data.
• Running a business: notifying business activity, registering as an employer, registering employees with pension and insurance schemes, submitting a corporate tax declaration, notifying the end of an employee's contract and paying social contributions.

How an evidence exchange works

The OOTS links national components rather than holding data centrally. It connects the procedure portals of authorities that need evidence (evidence requesters) with the data services of authorities that issue it (evidence providers), using eIDAS nodes for user authentication and identity matching and eDelivery access points to carry the messages; eIDAS is the EU framework for electronic identification and trust services.² The Commission provides three common services: a directory of evidence providers and the evidence types they issue, an evidence broker that shows which evidence from another country meets a national requirement, and a semantic repository of shared data models and definitions.² For the user, an exchange runs roughly like this:²

1. They start an online procedure in another Member State and are told that using the OOTS is voluntary and that they can preview the evidence first.
2. They sign in with an electronic identification means notified under eIDAS.
3. They choose the evidence to be fetched, see which authority will provide it and make an explicit request.
4. They are redirected to the evidence provider, which matches their identity against its records, and they preview the evidence.
5. They decide whether to use it. Only then is it returned to the requesting authority.

Users can still submit evidence in other ways, and a preview is not required where Union or national law already allows automated exchange without one.¹ Evidence received through the system is deemed authentic by the requesting authority, and each exchange is logged, including the user's decision after the preview.¹² For more on sign-in, see digital identity and eIDAS.

The timeline as officially stated

The legal date and the working reality are not the same. The Commission states that the OOTS infrastructure, including its common services, has been live since December 2023, while Member States are still implementing their national parts and connecting their authorities.⁴ It tracks their technical readiness on a seven-level "Acceleratormeter", where the top level means a country is live to exchange evidence.⁴ The State of the Digital Decade 2026 report, published on 17 June 2026, calls for sustained investment in cross-border infrastructure, including the OOTS, and names completion of the OOTS among the priorities for closing funding gaps.⁶

What gets in the way

• Evidence that isn't digital yet. The duty to share applies only where an authority already issues evidence electronically, in a format that allows automated exchange.¹ The implementing rules even provide for a reply saying that requested evidence is still being converted.²
• Data quality. An automated exchange moves whatever the register holds. If a residence record is out of date, the system delivers the wrong answer faster, so the state of base registries matters as much as the connection.
• Legal bases and evidence mapping. National law decides what counts as proof in each procedure. Authorities have to agree what a document from one country proves for a procedure in another, which is what the evidence broker and semantic repository support, and why semantic interoperability matters here.²
• Trust. Requesting authorities must treat evidence received through the system as authentic, and users must trust that only what they asked for is sent and used for that procedure.¹ Both depend on security, logging and clear explanations on procedure portals.²
• Matching identities. The evidence provider must match the identity attributes from the user's electronic ID, plus any extra attributes it asks for, against its own records. If there is no match, or more than one, the user cannot preview the evidence and nothing is exchanged.²