Supplementary vs replacement tax returns: 2026 guide

A supplementary return corrects an existing self-assessment when the change favours the tax authority: more tax to pay, or less to refund or carry forward. A replacement return supersedes an earlier information return in full — for example, Form 347 or 390 — and is not used for self-assessments that calculate tax payable. The amended self-assessment replaces the old dual system (supplementary return plus request for rectification) only where it is already in place: form 303 from September 2024 / Q3 2024 and the IRPF return from tax year 2024. It does not replace the replacement return. This guide explains when each route applies, the surcharges for correcting after the deadline and how to file the correction.

VeriFactu timeline (updated 6 October 2026). The rule in force, Royal Decree-law 15/2025 of 2 December, sets the adaptation deadline before 1 January 2027 for Corporate Income Tax payers and before 1 July 2027 for everyone else. On 5 October 2026 Spain’s Ministry of Finance announced it will postpone the obligation to October 2028, to align it with mandatory B2B e-invoicing. That postponement has not yet been published in the BOE (Spain’s official gazette)

What is a supplementary declaration and when does it apply

The supplementary return is governed by Article 122 of General Tax Law 58/2003 (in force in 2026). It completes or changes a self-assessment already filed, and may only be used when the correction is against your interests and in favour of the tax authority. In other words:

• It results in a higher amount to pay than the previous self-assessment, or
• It results in a lower amount to refund or compensate than declared.

The typical case: you submitted the model 303 of the second quarter and days later discovered three unrecorded invoices. The actual VAT charged is higher than declared, so you owe more than you paid. This is corrected with a supplementary declaration (or, since the third quarter of 2024, with the amended self-assessment — we’ll see it below).

The same applies to the Form 130 IRPF instalment payment: if you omitted quarterly income and the true net income is higher, you correct that period with a supplementary return. Watch for knock-on effects: Form 130 is cumulative from January, so an error in one quarter may carry into the next and into the annual return. Check whether more than one filing needs correction.

The supplementary declaration does not replace: it adds

A nuance that confuses many people: the supplementary declaration does not erase the previous declaration. The new self-assessment includes all the data (the correct ones already declared plus the ones that were missing) and from the result, it deducts what was already paid in the first one. You only pay the difference. That’s why when submitting it, it asks for the justification number of the original declaration.

What happens if the error is in your favour

If the error harmed you — you paid more, forgot a deductible expense, didn’t apply a reduction — the supplementary declaration is not the channel. Historically, you had to submit a request for rectification of self-assessment (art. 120.3 LGT) and wait for the Treasury to process it. In VAT (form 303, for the periods where it is already in place) and in IRPF from tax year 2024, this path is replaced by the amended self-assessment.

What is a replacement declaration and how it differs

The replacement declaration appears in the same article 122 LGT, but applies to another type of obligations: declarations and communications of data — the informative ones, which do not involve a payment. Its logic is different: instead of adding data to what was already submitted, it completely replaces the previous declaration, which is annulled in all its effects.

Common examples:

• Model 347 (operations with third parties): you declared operations with a client for an incorrect amount and prefer to submit the entire model corrected again.
• Model 390 (annual summary of VAT): the summary does not match the four quarterly 303, and you need to redo it entirely.

In the forms of informative declarations, you will find two separate boxes: “supplementary declaration” (adds records that were missing, without touching those already declared) and “replacement declaration” (annuls and completely replaces). The practical rule: if only data is missing, supplementary; if there are erroneous data to correct, replacement.

Comparative table: supplementary, replacement, and amended self-assessment

The amended self-assessment: the system that unifies corrections

The Law 13/2023, of 24 May, amended the General Tax Law to create the figure of the amended self-assessment: a single channel to rectify, complete or modify a self-assessment submitted, regardless of the nature of the error. It replaces the previous dual system (supplementary return if you owed more, request for rectification if you were owed) in taxes where it is already deployed.

For VAT (form 303)

The Order HAC/819/2024, of 30 July (BOE of 5 August 2024), adapted form 303. The amended self-assessment is used for monthly periods from September 2024 and quarterly periods from the third quarter of 2024. For an earlier period the previous system is kept: a supplementary return if the error harmed the Treasury, or a request for rectification if it harmed the taxpayer. For the covered periods, you submit the complete 303 again, marking that it rectifies the previous one and indicating its voucher number.

Some cases remain outside, which continue to follow the traditional request for rectification channel — notably, VAT charges incorrectly invoiced to other taxpayers and certain special regime operations —, and in rectifications based on the applied rule violating a higher-ranking rule, you can choose between both channels.

For Personal Income Tax (Income)

From the 2024 Income Tax campaign (submitted in 2025), Renta Web incorporates the amended self-assessment through the “Modify submitted declaration” option. The system retrieves your declaration, you apply the changes — in any direction — and submit the amended self-assessment without separately filing the former request for rectification.

Surcharges and interest: the cost of correcting outside the deadline

Correcting within the voluntary filing period costs nothing: the accurate return simply replaces or completes the earlier one. A cost arises when the deadline has passed and the correction increases the tax due. In that case, Article 27 LGT (the version in force in 2026, amended by Law 11/2021) applies, provided that you file before the tax authority contacts you:

Example: you owed EUR 1,000 from the 303 of 1T and submit the correction 4 months and a half after 20 April. Surcharge: 1% + 4% (four complete months) = 5%, i.e., EUR 50. No interest or fine.

Two important reliefs:

• 25% reduction of the surcharge (art. 27.5 LGT) if you pay the surcharge within the deadline of art. 62.2 LGT and the debt of the late declaration was paid when submitted (or is deferred with a guarantee). The example’s surcharge becomes EUR 37.50.
• No fine: voluntary regularisation excludes fines for the same fact. This is the great incentive to correct yourself before being caught.

How to submit a supplementary or replacement return step by step

The procedure on the AEAT’s electronic office is very similar for all forms:

For informative returns (347, 390), the flow is the same but choosing between the “supplementary” or “replacement” boxes, depending on whether you want to add records or replace the entire declaration.

Common errors when correcting declarations

• Using a supplementary return for an error in your favour. It does not apply: if you overpaid, the correct route is an amended self-assessment for VAT or IRPF, or a rectification request where that system is not yet available. A supplementary return showing a lower amount will be rejected.
• Forgetting to tick the supplementary/amended self-assessment box. Without it, the AEAT interprets that you submit a new declaration for the same period and warnings of duplication will appear.
• Declaring only the difference. The new declaration carries the complete data for the period; the system calculates the difference for you based on the previous voucher.
• Confusing the correction of the declaration with that of the invoice. If the error arises from a wrongly issued invoice, first issue a rectification invoice and then, if the period has already been declared, correct the self-assessment. These are two separate acts, both leaving a trail.
• Letting months pass. The surcharge grows by 1% per complete month. Correcting in month 1 costs 1%; in month 13, 15% plus interest.

Correcting less starts with better recording

The vast majority of supplementary returns do not arise from complex legal doubts but from something much more mundane: invoices not recorded when due. A supplier’s invoice that arrived late, a sale recorded in an Excel spreadsheet that nobody uploaded, an expense that appeared when reviewing the bank statement a month later.

This is where a management system does its silent work: in Frihet, each invoice issued and each expense is recorded at the moment, and the fiscal preview of the quarter updates with what has been recorded. When the deadline for the 303 or 130 approaches, the amounts already match what was actually invoiced — and the free plan covers up to 999 invoices per month, so starting to organise your records costs nothing. The best supplementary return is the one that never needs to be submitted.

Correcting a tax error is not a drama: the law rewards those who act early with moderate surcharges and no fines. But the entire system — supplementary, replacement or amended self-assessment — exists to compensate for imperfect records. Tackling the root cause is cheaper than mastering the remedy.