Spanish succession is doubly variable: the EU rules decide which national law applies, and within Spain the region can change both the reserve and the tax dramatically.
Which law applies
Under the EU Succession Regulation, habitual residence governs by default, with a choice of national law available — often decisive for foreigners who own a holiday home in Spain.
The legítima and regional variation
Common (civil-code) Spanish law reserves a substantial share for children — the legítima, broadly two-thirds. But several regions apply their own foral law: some (for example parts of the north-east) give far more testamentary freedom, so the result can differ greatly depending on which regional law applies.
Tax is regional too
Inheritance tax (impuesto de sucesiones) is set largely by the autonomous communities, so liability on the same estate can vary widely by region.
For foreign families
A verified Spanish colleague can confirm which regional law and tax apply, and how a choice of national law interacts with them.
Regional rules and reliefs change — confirm with admitted Spanish counsel.