Back to the Tax Calendar: What Self-Employed Workers and SMEs Should Check Before October

Summer is winding down, and with it the relative calm that many self-employed workers and small businesses use to catch up on their bookkeeping. August may not bring major quarterly deadlines, but it’s the ideal month to take stock of your tax obligations before the third-quarter filings arrive in quick succession: VAT (Modelo 303), personal income tax withholdings (111 and 115), and the installment payment for self-employed workers under direct estimation (130) or modules (131). Anyone who puts this review off until the last minute usually ends up finding unrecorded invoices, poorly documented deductible expenses, or gaps between what’s been declared and what the income and expense ledger actually shows.

Alongside this routine quarterly close, 2026 brings a deeper shift that’s no longer a looming threat but an operating reality: the requirement to use verifiable invoicing systems, in line with Verifactu. Most self-employed workers and SMEs must now operate with software that generates tamper-proof invoicing records, complete with a digital fingerprint and, where applicable, automatic submission to the Spanish Tax Agency (AEAT). The rollout has been gradual and varies depending on the type of taxpayer and how they file, so it’s worth confirming your specific situation with your software provider or your accountant rather than assuming a generic deadline applies. What matters isn’t just ticking the technical box — it’s understanding that an improperly issued invoice, or one outside this system, can jeopardize VAT deductibility and expense deductions for corporate tax or personal income tax purposes.

At the same time, Social Security continues to fine-tune the system that ties self-employed contributions to actual income, which means many will receive adjustments throughout the year — either refunds or additional charges — reflecting the gap between provisional contributions and the net income eventually declared. Ignoring these adjustment notices, or failing to set money aside for a possible extra payment, is one of the costliest oversights we see repeated year after year.

What this means for your business

In practical terms, this translates into a handful of concrete actions. First, bring your third-quarter bookkeeping close forward to the last weeks of September rather than waiting until just before the Modelo 303 or 130 deadline, so you have room to react if something doesn’t add up. Second, check with whoever handles your invoicing — your own software, a POS system, or your accountant — that it meets the verifiable invoicing requirements, and that the invoices you issue, as well as those you receive from suppliers, hold up for deduction purposes; a poorly formatted invoice can end up triggering an adjustment with a surcharge. Third, review your Social Security contribution forecast against your actual income to date, so the annual adjustment doesn’t come as a surprise — and if the business is doing better or worse than expected, update your contribution bracket sooner rather than later. Fourth, use these lower-pressure months to revisit deductions that often get overlooked: home-office utility costs, properly documented travel and mileage expenses, or depreciation on investments made during the year.

None of this requires drastic action, but it does call for method: reviewing the numbers on a regular basis, not just when a filing deadline is looming. Experience shows that cases which end up facing an AEAT inquiry or penalty rarely stem from bad intent — they’re usually the result of small oversights piling up: a missed deadline, an incorrectly formatted invoice, a contribution bracket left unadjusted. Caught in time, these issues resolve themselves at no extra cost.

At Zythos Business, this is exactly what we work to prevent leaving to memory or luck: we track each client’s tax calendar, cross-check what’s been declared against the actual accounts, and flag issues early — whether something doesn’t add up or a regulatory change, like verifiable invoicing or the new contribution rules, calls for a specific adjustment in the business. The goal isn’t just filing on time, but making sure every tax decision is made with the right information in hand.