For plaster, one generally adds 2 cm to each wall. Then add skirting boards (the cupboard is normally notched out at the back, but not on the sides) another 1-2 cm. Tolerances are usually higher in masonry than in carpentry. So add another 4 cm for masonry, maybe 2 cm for carpentry. And that is per wall. So 8-20 cm would be reasonable, depending on the circumstances. But the best is to order after plastering; you can then add the skirting board.