Most invoice disputes are commercial, but a surprising number are administrative: the document was fine as a request for payment and defective as a tax record. Those are two different jobs, and only one of them has a statutory checklist attached.
The checklist exists because invoices do double duty. Your customer uses one to reclaim input tax; the tax authority uses it to verify that the tax you charged was declared. When a required field is missing, the risk usually lands on the buyer — their deduction can be challenged — which is why finance departments reject invoices over details that look trivial from the sending side.
The fields VAT law asks for
In the European Union, Article 226 of the VAT Directive sets out the mandatory content of an invoice — around fifteen items, applied through each member state’s own legislation. The list is the closest thing to a universal answer, and most non-EU VAT and GST systems ask for materially similar information.
The recurring core, in practice, is this:
- The date the invoice was issued
- A sequential number that uniquely identifies the invoice
- Your full name and address, and your VAT identification number
- The customer’s full name and address, and their VAT number where they are liable for the tax
- The quantity and nature of the goods, or the extent and nature of the services
- The date of supply, where it differs from the invoice date
- The taxable amount per rate, the unit price excluding tax, and any discounts not already deducted
- The VAT rate applied and the VAT amount payable
- An explicit note such as "Reverse charge" where the customer accounts for the tax, or a reference to the provision where an exemption applies
Why sequential numbering is treated so seriously
Of all the requirements, invoice numbering is the one small businesses most often treat casually and auditors most often scrutinise. The rule is that numbers must uniquely identify each invoice and run in a sequence — one series, or several defined series, without gaps.
The reason is straightforward: a gapless sequence makes suppressed sales visible. If invoices 101 and 103 exist and 102 does not, an auditor has a question, and "I deleted a draft" is a weaker answer than most people expect. This is also why cancelling an invoice properly means issuing a credit note rather than deleting the original and reusing its number.
The practical rules that follow are simple. Never reuse a number. Never delete an issued invoice — correct it with a credit note. If you restart numbering annually or run separate series per entity or currency, make the scheme deliberate and document it, because a system an auditor can understand is a system an auditor stops asking about.
Simplified invoices and other relaxations
Not every transaction needs the full field set. Many jurisdictions permit a simplified invoice below a monetary threshold, typically dropping the customer’s details and some tax breakdown. Retail receipts are the everyday example — nobody expects a coffee shop to capture your VAT number.
The thresholds and the permitted omissions are national, and they matter mainly to businesses issuing high volumes of small invoices. If that is you, it is worth confirming the local threshold rather than defaulting to the full format on every transaction. If it is not you, the full format is the safer habit.
The United States works differently
There is no federal law prescribing what a US invoice must contain. The United States has no VAT, and sales tax is administered at state level and generally collected on retail sales rather than reclaimed through a deduction chain. The result is that the elaborate invoice-content rules familiar in Europe simply have no US counterpart.
That does not make the document optional. An invoice remains the evidence behind a deduction if you are the buyer and behind declared income if you are the seller, and the IRS expects records adequate to substantiate both. Where you have nexus and charge state sales tax, state rules on what the document must show apply. And where you are paid as a contractor, information reporting — the 1099 family — draws on the same underlying records.
The working conclusion for a US freelancer is that the European checklist is a good template even though it is not the law: dates, a unique number, both parties identified, a clear description, amounts and tax stated separately. It satisfies clients, survives audits, and means nothing has to change if you later invoice into a VAT jurisdiction.
Cross-border invoicing
Invoicing a business in another country introduces the question of who accounts for the tax. Within the EU, business-to-business supplies of services are commonly subject to the reverse charge: you invoice without VAT, the customer accounts for it in their own return, and the invoice must say so explicitly and carry the customer’s VAT number. Omitting the wording or the number is the classic reason such an invoice comes back.
Exports outside a VAT area are typically zero-rated or outside scope, but the evidence requirements are real — you may need to show the goods left, or that the customer is genuinely in business abroad. Verify the customer’s VAT number rather than trusting what was typed into a form; the EU operates a public validation service for exactly this purpose.
Common Mistakes
Deleting an invoice instead of crediting it
A gap in the sequence is an audit question. Cancel by issuing a credit note that references the original, then issue a replacement with a new number. The trail should show what happened, not hide it.
Omitting the reverse-charge wording
On a cross-border B2B supply where the customer accounts for the tax, the invoice must say so and carry their VAT number. Without both, the customer’s finance team will reject it — and they are right to.
Treating the invoice date and supply date as interchangeable
Where they differ, both are required. The supply date often determines which tax period the transaction falls into, which is precisely why the rules ask for it separately.
Assuming a US template works everywhere
US invoices carry no VAT identification because there is no VAT. Send the same layout to an EU business customer and it is missing fields they need to reclaim tax.
Frequently Asked Questions
Does an invoice need a signature to be valid?
In most jurisdictions, no. VAT rules across the EU do not require a signature on an invoice, and the United States has no such requirement either. A few countries do impose signature or seal requirements in specific circumstances, which is why it is worth checking locally rather than assuming.
Can I start my invoice numbers at 1000 to look established?
Yes. Nothing requires you to start at 1. What matters is that the sequence is unique, gapless from wherever it starts, and applied consistently. Starting at 1000 and then jumping arbitrarily to 5000 is the part that causes problems.
Is a handwritten invoice legal?
Where no e-invoicing mandate applies, yes — the law cares about content and retention, not about how the document was produced. Note that this is exactly what the mandates covered elsewhere in this cluster are changing for domestic B2B trade in a growing list of countries.
What if I forgot a required field on an invoice I already sent?
Issue a corrected invoice or a credit note plus a replacement, depending on what is wrong and on local rules. Correcting the record openly is routine; quietly editing a document the customer has already booked is what creates the mismatch an auditor eventually finds.
Sources & Further Reading
- VAT Directive 2006/112/EC, Article 226 — content of invoices — EUR-Lex, European Union
- Recordkeeping for businesses — Internal Revenue Service
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