Better not. The WOZ value is a model-based valuation as at a reference date that lags behind, and it includes the value of the land. When rebuilding, the land does not have to be bought again, but the building costs are current. In areas with expensive land, WOZ leads to substantial over-insurance; where building costs have risen, to underinsurance. Use a rebuild value calculator or a valuation.

The insurer applies a proportionate reduction: the payment is reduced in the same proportion as the sum insured bears to the actual rebuild value. That applies not only to a total loss but to every claim. It is almost always discovered only after a claim, when the loss adjuster measures up the home. It can only be prevented in advance.

At every substantial change to the home, and otherwise periodically, because building costs shift. Many policies have indexation that moves the sum insured each year, but that does not absorb an extension or a more luxurious finish. A guarantee period against underinsurance also expires; have the details filled in again then.

No. The building is the structure with everything fixed to it, such as the kitchen, the sanitary ware and a fixed floor. The contents are everything you take with you when you move. Each has its own sum insured and its own method of valuation. For contents, replacement value as new applies, unless the item has fallen in value to below a certain proportion of that replacement value.