e-Invoicing and myDATA: What Greek SMEs Must Do Before 1 October 2026

If your business issues invoices to other Greek businesses and you have been treating e-invoicing as someone else’s problem, the calendar has caught up with you. Large businesses crossed this bridge in the spring. From 1 October 2026, mandatory B2B e-invoicing applies to everyone else, with an adjustment period running to the end of December. That is next week, and “we will look at it after the holidays” stopped being a plan some time ago.

Here is what the mandate actually requires, in plain language, and what to do about it in the time remaining.

What changes on 1 October

Until now, most SMEs met their myDATA obligations by transmitting invoice summaries to AADE through their accounting software or their accountant. The new mandate goes further: B2B invoices must be issued as structured electronic invoices and transmitted through the myDATA infrastructure, receiving a unique identifier. A PDF attached to an email does not qualify. A paper invoice typed into the system at month-end does not qualify either.

The first wave, businesses with revenues above one million euros for tax year 2023, went live on 2 March 2026 with a phased rollout into May. The second wave, effective 1 October 2026, covers the rest: which in the Greek economy means nearly everyone, from the three-person engineering office to the twenty-person trading company. The adjustment window to 31 December 2026 is breathing room for teething problems, not an invitation to start thinking about it in December.

The scope, under the current framework: B2B invoices between businesses established in Greece, plus sales to businesses in third countries outside the EU. For intra-EU B2B transactions e-invoicing remains optional, and if a foreign counterparty cannot receive an e-invoice, alternative delivery is provided for.

The extension talk, and why it should not change your plan

At the time of writing, late August 2026, the financial press reports that a postponement of a few months is on the table for the second wave, with 1 March 2027 mentioned as the most likely new date. Market bodies have been asking for more adjustment time, and thousands of small businesses have not completed the technical preparations. Nothing has been announced; until AADE says otherwise, 1 October stands.

Our advice does not depend on the outcome. If the deadline holds, the plan below is cutting it close already. If the extension comes, executing the same plan now buys you a calm winter and a compliant stack months early, instead of a second countdown in February. Deadline-driven IT transitions done twice are more expensive than done once; we see it on projects every year.

What compliance looks like in practice

For most SMEs the practical route is a certified e-invoicing provider or invoicing software that handles issuance, transmission and the myDATA identifier automatically. If you already invoice through a modern platform, compliance may be a settings change and a subscription tier. We wrote about running invoicing, CRM and reporting as one system, and this mandate is one more argument for that consolidation: the businesses with a clean invoicing stack are finding this transition boring, which is the correct experience for a compliance deadline.

If your invoicing lives in Word templates, an old desktop application, or your accountant’s office, you have a decision to make in the next two weeks, not the next two quarters.

A two-week preparation plan

• Days 1 to 3: locate yourself. Confirm with your accountant which wave you belong to and whether anything you issue is out of scope. Retail receipts to consumers follow their own rules; the B2B mandate is about invoices between businesses.
• Days 4 to 7: choose the route. Certified provider, compliant invoicing software, or your accountant’s platform. Ask each candidate two questions: is the myDATA transmission automatic, and what happens when transmission fails? The second answer tells you more about the product than the brochure does.
• Days 8 to 10: test with a friendly customer. Issue a real e-invoice to a cooperative counterparty, confirm it transmits, confirm the identifier comes back, confirm the customer’s accountant can receive and book it.
• Days 11 to 14: clean your master data. Wrong VAT numbers and sloppy customer records that a human would quietly correct will now bounce. An hour of cleanup now saves a December of rejected invoices.

The upside nobody mentions

Compliance projects are joyless, so here is the consolation: structured e-invoicing removes the gap between issuing an invoice and knowing about it. Your revenue picture becomes real-time by force. Businesses that connect that data stream to their reporting, the way we connected our own books to an AI assistant, end up with visibility that used to require a finance department. The state is, in effect, forcing an upgrade many SMEs needed anyway.

If you want help choosing the route, testing the flow, or using the transition to fix your invoicing stack properly, that is exactly the kind of engagement our business consultancy services exist for.

Frequently Asked Questions

Who must issue B2B e-invoices from 1 October 2026?

All businesses established in Greece issuing invoices to other domestic businesses, regardless of size. Larger businesses, above one million euros of 2023 revenue, were already covered by the March 2026 first phase.

Is a PDF invoice sent by email compliant?

No. The mandate requires structured electronic invoices transmitted through the myDATA infrastructure with a unique identifier, issued via a certified provider or compliant software.

What is the adjustment period until 31 December 2026?

A tolerance window for the second wave to resolve implementation problems. The obligation still starts on 1 October; the window is for smoothing the transition, not postponing it.

Will the 1 October 2026 deadline be postponed?

As of late August 2026, an extension of a few months, with 1 March 2027 reported as the most likely new date, is under discussion but has not been announced. Until it is official, the 1 October obligation stands, and preparation now pays off under either outcome.

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Editorial note: This article was researched and drafted with the assistance of Claude (Anthropic), and reviewed and approved by Amazing Projects before publication.