What does buildings insurance (opstalverzekering) do?

The rule of thumb: turn the house upside down, and everything that stays in place falls under the buildings and everything that falls out under the contents. Walls, roof, fitted kitchen, bathroom and boiler are buildings. Furniture, clothing, appliances and curtains are contents. Borderline cases such as a free-standing garden house, laminate or sun blinds are classified differently under each policy; check that in the conditions.

Not by law, but almost every mortgage lender sets it as a condition and records that in the mortgage deed. Without financing it remains voluntary. With a flat, the VvE insures the whole building collectively. You then do not take out buildings insurance yourself, but you do check whether the VvE's sum insured and excesses are adequate.

No. Settlement of the foundations, pile rot and damage from a falling groundwater level are excluded by almost every insurer. It concerns a gradual process rather than a sudden event. Cracking resulting from it is also outside the cover. There are local and national schemes for repairing foundations. Those are separate from your insurance.

Extended cover applies to a list of named events: fire, storm, burglary, water, lightning and impact. Comprehensive cover adds sudden and unforeseen damage from outside, such as a drill hole through a water pipe or a ladder falling through the roof light. The exclusions remain the same under both forms; comprehensive is therefore wider, but not unlimited.

Usually yes. Insurers index the rebuild value each year on the basis of building cost figures, so the premium rises with it. That indexation only corrects for price rises, not for your building work. If you have added an extension, fitted a dormer window or turned the loft into living space, you have to report that separately; otherwise the sum insured no longer matches the property.