We have a pretty high end house insurance policy (for the simple reason it offered the best price/cover ratio when the broker went to market) and one of the benefits of it is that the insurers pay the cost of having professional quantity surveyors inspect the house and calculate accurate rebuild costs. The policy is based on that figure and there is then no limit on rebuild costs if it turns out to be more.
Our policy last year was based partly on what we paid for the house increased by a bit.
This year it was valued as per the policy by their retained surveyors. The rebuild cost was almost double what we had it insured for.
I’ve not spoken to our brokers yet. However, I mention it here because it proved to me that rebuild costs have literally nothing whatsoever to do with market value. We paid barely half of the rebuild cost valuation when we bought the house a year or so ago.
You may want to check your insurance.