Q3 2026 Close: What to Review Now So Year-End Holds No Surprises

Tax calendars are usually published as a list of dates, and many businesses use them only to know when to file. But a calendar is above all a planning tool. With the Q3 2026 deadline just behind us, it is a good moment to look back, correct what needs correcting and organise the two and a half months left in the financial year.

What the third quarter leaves behind

In October, the quarterly self-assessments for Q3 fall due: VAT (form 303), withholdings on employees and professionals (forms 111 and 115, as applicable) and, for those taxed under the module or direct estimation systems, the personal income tax instalment (forms 130 and 131). If you have already filed, the work is not over: now is when you should reconcile what you declared with your accounting records.

The most common discrepancies are not big mistakes but small oversights that get carried forward: invoices received that were booked in a different quarter from the one in which they were deductible, withholdings applied to professionals that never made it into form 111, or issued invoices with gaps in the numbering sequence. Spotting them now lets you correct them in the next return as a matter of course. If the error meant paying too little, regularising it voluntarily is usually far cheaper than waiting for a notice from the Spanish Tax Agency (AEAT), because penalties are avoided and surcharges are lower.

One case deserves special attention: VAT to be carried forward. If your form 303 came out negative, that balance is not lost: it carries over to the following returns. Check that you have carried it over correctly and that the amount matches the one in your accounting records.

What lies ahead through December and January

For companies, the last instalment payment of Corporate Income Tax (form 202) is filed in December. It pays to decide well in advance which calculation method works best for you, since a poorly considered choice can leave you with an unnecessary cash burden right in the middle of the year-end close. Remember that, in general, the method is chosen in the census declaration or with the first payment, and that the AEAT takes into account what you did in previous years.

In January the fourth quarter arrives and, with it, the annual summary returns: depending on your case, the VAT summary, the withholdings summary and the one covering transactions with third parties. If you work with suppliers or customers who exceed the reporting thresholds, cleaning up the data on those third parties now (tax ID, address, amounts per quarter) will save you discrepancies when the Agency cross-checks the information.

As for electronic invoicing with traceability guarantees (Verifactu), the mandatory deadline has been postponed to 2027. That is no excuse to wait until the last minute: if you use invoicing software, ask your provider now how and when it will be adapted.

What this means for your business

Turning the calendar into practical decisions:

Do a quick reconciliation. Compare the VAT and withholdings declared in the third quarter with your books. It is an hour’s work that prevents larger corrections later.

Project your result for the year. With nine months closed you can already estimate whether you will make a profit or a loss, and that shapes your year-end decisions: investments, depreciation, instalment payments or pension plan contributions, each with its own limits and conditions.

Set aside cash. December and January payments pile up on top of the usual year-end expenses. Work out in good time how much you will have to pay the tax authorities so you don’t have to resort to emergency financing.

Get your paperwork in order. Invoices still to be received, proof of payment, and rental or recurring service contracts: anything not documented by 31 December will be hard to deduct later.

Check that you still meet the requirements of your tax regime. Changes in turnover, headcount or type of activity can affect your taxation method or your census obligations.

At Zythos Business we work with exactly this calendar mindset: we don’t wait for the deadline to prepare your returns. Instead, we review your books during the quarter, anticipate what you will have to pay and propose year-end decisions with real numbers in front of you. That way, when the dates arrive, there are no surprises.