September Tax Planning: The Numbers Your SMB Must Fix Before the Final Quarter

Every September article says the same thing: "review your figures, anticipate provisions, bring purchases forward". Here is what they leave out, and it is the part that costs money.

If your business files in Spain, the date that actually constrains you is not 20 October. It is 15 October. The general filing window for the Q3 returns (modelo 303 for VAT, 111 for withholdings, 130 for the personal income tax instalment, 202 for the corporate tax instalment) runs from 1 to 20 October — but if you want the payment direct-debited, the return has to be in by the 15th. That is the AEAT rule for domiciliación, and it silently removes a third of the window. After a half-empty August, your real margin is around ten working days.

And the second omission: an unrecorded supplier invoice does not make you lose the VAT — Spanish VAT law (art. 99 LIVA) lets you deduct within four years of the right arising. But it absolutely costs you cash in October, because your instalment payment is calculated on profit, and profit inflated by missing expenses is paid now and recovered much later.

The arithmetic nobody puts in writing

Take a sole trader who ended the quarter with 40 supplier invoices sitting in a WhatsApp thread, an email folder and a shoebox — an average base of €300 each:

Expenses missing from the books: 40 × €300 = €12,000
Modelo 130 impact: the instalment is 20% of net income accumulated in the year. Missing €12,000 of expenses means paying €2,400 more in October than you owe.
Input VAT deferred: 21% of €12,000 = €2,520 that sits outside your Q3 return.

That is roughly €4,900 of cash leaving the business in a single October, for documents that already exist and are already yours. The VAT comes back in a later quarter. The €2,400 of instalment comes back when the annual return is filed — in the middle of the following year.

That is the actual argument for a September pre-close. Not "being organised". A five-figure cash timing difference on a small business.

The decision you can no longer take in September

Here is a criterion most September checklists skip entirely, because it only becomes visible when the year goes badly.

Spanish companies calculate the corporate tax instalment (modelo 202) in one of two ways:

Article 40.2 LIS — a percentage of the tax liability from the *last filed* corporate tax return. Simple, but completely blind to how the current year is going.
Article 40.3 LIS — calculated on the *actual taxable base* of the running period (3, 9 or 11 months). More accounting work, far more accurate.

If your 2026 is going worse than 2025, method 40.2 makes you pay in October on the basis of a good year you are not repeating. And you cannot change it now: opting into 40.3 must be communicated on form 036 during February (or the first two months of the financial year). Above a turnover of €6,010,121.04 the 40.3 method is mandatory anyway.

So the September decision is not "which method do I use". It is: *does my running result justify committing, in February, to the method that tracks reality?* Answer that in September, while you still remember the year, and write it down. Nobody remembers in February.

The four-item September pre-close

1. Close the document gap, not the accounts

The goal is not a full accounting close. It is that every invoice issued and received up to 30 September is recorded and readable before the first week of October. Chase the three usual sources: recurring suppliers who email PDFs, field staff who photograph tickets, and the subscriptions nobody books because they are €19/month.

If you have never measured what that chase costs you, the breakdown in what it actually costs to process one invoice by hand is a useful reality check before you decide it is "just admin".

2. Hunt duplicates before they hit the return

The classic Q3 error is the same invoice entered twice — once from the emailed PDF in July, once from the paper copy that turned up in September. It inflates deductible VAT, and it is exactly the kind of discrepancy that surfaces later in a cross-check. Sorting by supplier tax ID plus amount takes minutes; there is a practical routine in how to catch duplicate invoices before you book them.

3. Recover what earlier quarters left behind

Because of the four-year window, VAT you failed to deduct in Q1 or Q2 is not lost. September is the moment to sweep old folders and bring those into the Q3 return rather than writing them off. The deductible input VAT you lose every quarter covers where it typically hides.

4. Pre-check the return itself

Before filing, run the same checks you would run in a review: totals against your ledger, reverse-charge entries, intra-EU operations, and the reconciliation between what you declared for withholdings and what you actually paid. The modelo 303 checklist works line by line for any quarter, not just the one it was written for.

One thing to plan in September because 2027 is closer than it looks

Spain's Verifactu invoicing-software rules were pushed back a second time by Royal Decree-Law 15/2025: corporate taxpayers are obliged from 1 January 2027, sole traders and professionals from 1 July 2027. Software vendors have had to ship compliant systems since 29 July 2025.

The practical reading: you are not late, but the last comfortable window to change or upgrade your invoicing system is the coming autumn and the first half of 2027. Doing it in the middle of a quarterly close is how migrations go wrong.

Different country, same September logic

The dates change; the pre-close does not.

Italy: 30 November concentrates the second income-tax instalment (IRPEF/IRES/IRAP) and the Q3 LIPE. September is the month to get the books to a state where the acconto is a calculation, not a guess.
Portugal: the second *pagamento por conta* of IRC falls on 30 September, the Q3 VAT return is due by 20 November with payment by 25 November — and the obligations that would expire in August are legally shifted into September, which is why September feels crowded.

How to make October boring

The reason the September pre-close fails is never willpower. It is that the documents are scattered across channels nobody controls: a supplier's email, a technician's phone, a paper receipt in a van.

The fix that actually holds is to give everyone a single place to send documents — the one channel they already use all day. With WhappScan, invoices, receipts and IDs sent to a WhatsApp number come back as structured data in Excel or through the API: supplier, tax ID, date, base, VAT rate, total, line items. No app to install for the person sending, and no re-typing for the person recording. That turns "chase 40 invoices in the last week of September" into a file that is already there.

If you want to see what that produces before changing anything, try it free — no signup: upload one of your supplier invoices and get the extracted fields back in seconds.