If you run a small business in Greece and still invoice your business customers with a PDF attached to an email, that habit expires in less than two weeks. Starting October 1, 2026, every remaining Greek-established business — including sole proprietors and the smallest companies — must issue structured electronic invoices through the tax authority's myDATA platform. A handsome PDF will no longer count as an invoice at all: the tax authority treats a non-compliant invoice exactly as if you never issued one, and the penalty is half the VAT on the transaction.
There is also a tax break in this story, and honesty requires saying it first: the early-adopter incentive — a doubled deduction for e-invoicing costs plus immediate write-off of the related equipment — expired on August 3. If you integrated early, make sure you claim it. If you did not, stop mourning it and focus on the deadline you can still hit. This guide covers who is in scope, what "compliant" actually looks like, the declaration you must file before October 1, what happens if you miss it, and a concrete action plan for the days remaining.
What Changes on October 1
Greece has been building toward this moment for years. Its tax authority, the Independent Authority for Public Revenue (AADE), runs myDATA, a real-time platform that already collects transaction data from Greek businesses. Government contracting went to mandatory e-invoicing first, starting in 2023. Business-to-business invoicing followed in two waves:
- Phase A — March 2, 2026. Mandatory for Greek-established businesses with 2023 gross revenue above 1 million euros, with a transition window through May 3, 2026.
- Phase B — October 1, 2026. Mandatory for everyone else, with a gradual adjustment period running through December 31, 2026. Full enforcement begins January 1, 2027.
The legal backbone is Law 5222/2025, the National Customs Code adopted in July 2025, which amended the accounting rules in Law 4308/2014. The implementation details sit in two joint ministerial decisions: A.1128/2025 (scope, dates, methods) and A.1129/2025 (declarations and incentives). The EU Council authorized the whole scheme as a derogation from the VAT Directive running from July 2025 through the end of 2027.
Who is in scope
The obligation applies to all businesses established in Greece — meaning a real local presence, not just a VAT number — that invoice other domestic businesses, for both goods and services. It also covers your sales to businesses outside the EU. Three carve-outs matter:
- Intra-EU sales stay optional for now. E-invoicing a customer in another EU member state remains your choice until the EU's broader ViDA digital-reporting framework takes effect around 2030.
- Consumer sales are out of scope. Retail receipts and B2C transactions follow separate reporting rules, not this mandate.
- Receiving matters too. If you are a domestic business customer, accepting e-invoices from your suppliers becomes mandatory alongside issuing your own.
If you are a freelancer, a sole proprietor, a two-person agency, or a neighborhood wholesaler, Phase B means you. There is no revenue floor and no exemption for the smallest traders.
The Tax Break You (Probably) Just Missed
To pull businesses onto the platform early, Greece offered genuine tax incentives for adopting e-invoicing at least two months before your mandatory date:
- A 100 percent increased tax deduction for e-invoicing software and services — effectively deducting double what you spent on producing, transmitting, and archiving e-invoices during the first twelve months.
- Immediate, 100 percent depreciation of the related IT equipment and software in the first year, instead of spreading the write-off over several years.
The cutoff was December 1, 2025 for large enterprises and August 3, 2026 for everyone else. Qualifying also meant actually running e-invoicing as your exclusive issuance method from that point and filing the prescribed declaration — not merely signing up for a tool.
That date is six weeks behind us. Two practical consequences follow. First, if you did integrate and declare by August 3, flag it to your accountant now so the super-deduction and accelerated depreciation land on the right return; incentives left unclaimed are just trivia. Second, if you missed it, do not confuse it with the older incentive rounds from 2023 and 2024, which have their own closed windows — and do not let a missed carrot distract you from an approaching stick. The October 1 obligation applies whether or not you got the bonus, and the adjustment period through December 31 is for ironing out errors, not for starting from zero.
What "Compliant" Actually Looks Like
Greece runs a centralized clearance model: every in-scope invoice must be validated by AADE before it counts as fiscally valid. In practice, issuing a compliant invoice means three things.
Pick one of three transmission channels. You can contract with an AADE-certified e-invoicing provider, use AADE's free Timologio web application, or use the free myDATAapp on your phone. Most micro-businesses with simple invoicing will find Timologio perfectly adequate; busier operations with an ERP system generally route through a certified provider that integrates with software they already use. All three are equally valid in the eyes of the law.
Produce structured data, not a prettier PDF. The invoice must follow the myDATA specification — a Greek localization of the European EN 16931 standard — carrying machine-readable fields for the parties, line items, VAT breakdown, and classifications. Your provider or the Timologio app generates this format for you; your job is feeding it complete, correct inputs.
Get the stamp before delivery. Once validated, the platform returns a unique MARK registration number and a verification URL, rendered as a QR code that must appear on the invoice before you send it to the buyer. No MARK, no invoice — that is the clearance model in one sentence.
Note the difference from what you may already do: transmitting summary data to myDATA has been mandatory since 2021. This mandate adds the second half — the invoice itself must now be born digital, validated in near real time, and only then delivered.
The Declaration You Must File Before October 1
Here is the step small businesses most often overlook, because it feels like paperwork about paperwork: you must formally declare your chosen transmission method to AADE before you start issuing e-invoices. That means filing either a Declaration of Commencement of Electronic Issuance of Documents (if you use a certified provider or your own ERP connection) or a Declaration of Use of the Timologio Application.
Phase A is the cautionary tale. In late January 2026, AADE emailed large businesses that had not filed the declaration, set a final deadline of February 12, and warned that invoices issued without an approved declared method would be deemed never issued. Businesses that were technically ready — software installed, provider contracted — still faced penalties if the declaration itself was missing. The form is the compliance, not just the tooling.
File yours now, in September, not on September 30. AADE's portals slow down under deadline-week load in every country that has ever run one of these mandates, and a declaration timestamped after your first October invoice is an argument you do not want to have during an audit.
What Happens If You Miss It
The penalty scheme sits in the Tax Procedure Code, and it is deliberately asymmetric — cheap to comply with, expensive to ignore:
- VATable transactions: a fine equal to 50 percent of the VAT on each non-compliant invoice discovered during an audit.
- Exempt or non-VAT transactions: fixed fines of 500 to 1,000 euros per tax audit, depending on whether you keep single-entry or double-entry books.
Two multiplier effects make this worse than the headline numbers. First, non-compliant issuance equals non-issuance, so there is no partial credit for trying — a PDF sent in good faith after the mandate is, fiscally, nothing. Second, your customers have skin in your compliance: a Greek buyer generally cannot deduct input VAT on a purchase whose invoice was never properly transmitted. Expect business customers to start rejecting non-compliant invoices outright, because accepting one means paying VAT they can never recover. The market will enforce this mandate faster than any auditor.
The October-to-December adjustment period softens the landing — it exists so businesses can fix classification mistakes and workflow bugs without penalties raining down — but it is not a postponement. AADE has already postponed once, pushing large businesses from February to March; nothing suggests Phase B moves.
Your 13-Day Action Plan
Starting today, work through this checklist in order. Most sole proprietors can finish it in a long afternoon; businesses with an ERP system should start the provider conversation immediately, since integration lead times are the one thing you cannot compress.
- Confirm you are in scope. Greek establishment plus domestic B2B sales (or non-EU B2B sales) means yes. Pure B2C and intra-EU-only sellers can stand down on issuance — but double-check before assuming.
- Choose your channel. Low volume and simple invoices: register for the free Timologio app. Higher volume or ERP-driven billing: shortlist a certified provider this week and confirm it supports the Greek CIUS format and MARK/QR handling.
- File the declaration. Submit your Declaration of Commencement or Declaration of Use of Timologio to AADE now, and save the confirmation. This is the step that blindsided Phase A stragglers.
- Issue a test invoice end to end. Create a real invoice, transmit it, confirm the MARK and QR code come back, and deliver it to the buyer. Fix whatever breaks while the stakes are zero.
- Brief your customers and suppliers. Tell business customers that compliant e-invoices are coming and what to expect; ask your own suppliers for theirs, since your input-VAT deductions now ride on their transmissions.
- Reconcile myDATA against your VAT return. AADE cross-matches transmitted data against filed returns automatically, and discrepancies flag you for review. Build a monthly habit of comparing the two before you file.
- Set up archiving for six years. The statutory retention rule runs five years plus one; confirm your provider or your own backups keep every validated invoice retrievable for the full period, and that AADE can access records remotely if you archive elsewhere in the EU.
One more date for goods businesses to diary: the second phase of e-delivery (digital consignment and movement documents) turns mandatory from October 12, 2026, with item-level classification following in January 2027. If you move physical goods, treat October as a double deadline and coordinate both workstreams with your accountant.
The Bookkeeping Upside Nobody Advertises
Deadlines and penalties dominate the conversation, but the steady state is genuinely better than the paper world it replaces. Validated invoices flow straight into myDATA, which means pre-filled VAT return lines, faster VAT refunds with fewer document requests, earlier detection of classification errors, and an audit trail your accountant can actually follow. Businesses that reconcile monthly instead of reconstructing quarterly spend measurably less time on compliance.
Use the transition to tighten one habit: track every euro of e-invoicing cost — provider subscriptions, software, equipment, setup fees — in its own ledger accounts from day one. Clean cost tracking is what lets your accountant defend deductions, document any incentive claim, and show an auditor exactly what changed and when. If you want to see how structured, transparent books make this kind of compliance work routine rather than frantic, explore the live dashboard tour under /fava/ and the plain-text accounting guides in /docs/.
Keep Your Books Ready for Whatever Compliance Throws at You
As Greece moves tax compliance onto real-time digital rails, the businesses that adapt fastest are the ones whose records are already clean, complete, and under their own control. Beancount.io provides plain-text accounting that gives you complete transparency and control over your financial data — no black boxes, no vendor lock-in. Get started for free and see why developers and finance professionals are switching to plain-text accounting.
