How to join Spain's SII: who must use it and how to register

The Immediate Supply of Information is a way of keeping your VAT record books through the AEAT's online portal. Instead of keeping them in your software and summarising them at the end of the year, you send the details of each invoice, issued or received, almost in real time.

It isn't a new tax and doesn't change how much VAT you pay. It changes when and how you tell the tax agency about your transactions.

Under article 62.6 of the Spanish VAT Regulation, it is mandatory for businesses that file VAT every month and are also:

• Large companies: turnover above €6,010,121.04 the previous year.
• Registered in REDEME, the monthly VAT refund register.
• VAT groups.

For a small or medium e-commerce business, the usual route into the SII is the second one: asking for monthly VAT refunds brings the SII with it. I explain it in the REDEME guide.

If you aren't required to, you can still opt in:

1. File form 036, ticking boxes 143 and 532.
2. You can do it at any time of year. It takes effect from the first tax period that starts afterwards. The AEAT's own example: opt in on 10 April and you start on 1 July.
3. Voluntary members keep their quarterly form 303. The SII doesn't move you to monthly returns; monthly filing is for those who were already required to use it.
4. You stay in at least for the calendar year you opt in for. If you don't withdraw, the option renews automatically every year.
5. To leave, you file form 036 in November, effective from 1 January.

Before ticking the box, check that your invoicing software or your accountant can send the records on time. Without that, joining is a problem, not an improvement.

The general deadline is 4 days, not counting Saturdays, Sundays or national holidays:

• Issued invoices: 4 days from issue (8 if the invoice is issued by the customer or a third party). In any case, before the 16th of the month after the tax point.
• Received invoices: 4 days from when you record them in your accounts. In any case, before the 16th of the month after the period in which you deduct the VAT.
• Intra-EU transactions (for example, sending your own stock to a warehouse in another country): 4 days from the start of the shipment or the receipt of the goods.
• Capital goods: within the deadline of the last period of the year, by 30 January.

Late submission carries its own penalty: 0.5% of the invoice amount, with a minimum of €300 and a maximum of €6,000 per quarter (article 200 of the General Tax Law).

• You don't file form 347 (transactions with third parties).
• You don't file form 390, the annual VAT summary. Not even if you join part-way through the year.
• If you are in the SII because you're required to be, you file form 303 every month: from the 1st to the 30th of the following month. January's return is due by the last day of February.
• If you leave the SII, you go back to filing forms 347 and 390.

There are two ways:

• Web service with XML messages. This is the normal route: your invoicing or accounting software does it without you having to touch anything.
• Web form on the AEAT portal, meant for businesses with few transactions or for sending the odd invoice.

With e-commerce volumes, the form isn't a realistic option. If you sell on Amazon or in your own shop, you need invoices to go from your system to the SII without passing through a person.

Businesses keeping their records through the SII, whether required or voluntary, are outside Verifactu, Spain's verifiable invoicing system. It's one of the reasons some companies join the SII voluntarily.

The Verifactu dates in force today are 1 January 2027 for corporate income tax payers and 1 July 2027 for everyone else (Royal Decree-law 15/2025). On 5 October 2026 the Ministry of Finance announced that it plans to postpone them to October 2028, but at the date of this guide that change has not yet been published in the Official State Gazette (BOE).

It depends on what you want it for:

• If you're going to apply for REDEME, there's no choice: it comes with it.
• If you want to stay out of Verifactu, it's one way, but in exchange you send every invoice within 4 days for at least a year.
• If you have neither reason, voluntary SII is mostly more admin.

The right decision doesn't come from looking at the SII on its own, but from looking at cash, software and invoice volume together.