If we were to support our terrace with L-bricks, the L-bricks alone would cost €12,000. In our case, however, we are talking about a total of 18m at heights of 1.5m, 2m, and 3m. The rest (setting, filling, etc.) would be 'only' about €2,000 (depending on how fillable our excavated soil would be). That would be the offer from the civil engineer of our general contractor. In conversations with acquaintances, many found that realistic. The €900 for the excavator would then be added for you? I would find that quite expensive! However, we ultimately decided to cellar our terrace.