It depends entirely on whether the tax office has written to you first. File late on your own initiative and it is a surcharge: 1 percent, plus 1 percent for each complete month of delay, up to 12 months, then a flat 15 percent plus interest from month 13. That surcharge replaces the penalty that could otherwise have been imposed, and is reduced by 25 percent if you pay within the voluntary period. Once a demand has been issued, none of that applies.
Where this comes from: art. 27 Ley 58/2003 (LGT), consolidated, BOE consolidated text, read 2026-09-07; art. 27 LGT, BOE consolidated text, read 2026-09-07; art. 27.5 LGT, Recargos aplicables, Manual IRPF 2025, AEAT sede electronica, read 2026-09-07.