How do I negotiate benefits when salary is not negotiable?

With collective agreement salaries or exhausted margin: switch to benefit negotiation. What is negotiable: more holiday days (BUrlG Section 3 guarantees 20 days minimum, many companies offer 25–30, the margin rarely exceeds 5 days), more flexible remote work arrangements (Destatis homeoffice data 2024, Q049: 25% of German employees work at least partially from home), professional development budget (2,000–5,000 euros per year is standard at many companies but not automatically in the offer), signing bonus (one-time, more cost-effective for companies than salary increase), earlier salary review (after 6 instead of 12 months). Written confirmation for everything agreed verbally.