What happens if a self-assessment is filed on time but remains unpaid?

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If a self-assessment shows an amount due but is not paid, the payment deadline is the one set by the rules for that tax (section 62.1 of the Spanish General Tax Law, LGT). It does not necessarily coincide with the filing deadline.

If the debt remains unpaid when that payment deadline expires, the enforcement period generally begins on the following day (section 161.1(b) LGT). From then on, the Tax Administration may initiate enforcement proceedings. The enforcement surcharge is 5% if the entire debt is paid before the enforcement order is notified; it is 10% if the debt and the reduced surcharge are paid within the payment period opened by that order; in all other cases, the ordinary surcharge is 20% and is compatible with late-payment interest. These surcharges are mutually exclusive (section 28 LGT).

The taxpayer may apply for deferral, payment by instalments or offsetting during the voluntary payment period, provided the debt is eligible and the legal requirements are met. An application made in time generally prevents the enforcement period from starting while it is being processed, subject to statutory exceptions. If the application is refused, the payment deadline stated in the decision must be observed (section 161.2 LGT).

Therefore, non-payment does not always mean that the ordinary 20% surcharge and all interest are immediately due. The applicable surcharge depends on when payment is made and on the collection steps taken. Only the ordinary surcharge is compatible with interest from the start of the enforcement period (section 28.5 LGT).