You'll have to ask an architect or builder for the figure. It's handy if you know one, then they probably won't charge you. If not, you'll probably have to pay, but it shouldn't be much. Basically, they just look at the house and come up with a figure. It doesn't have to be accurate to the �, just a figure that would cover the rebuilding, so it's usually rounded up quite a bit.
Or, if there are similar houses nearby, you could ask them what figure they have.
Or if building plots have been sold nearby recently, deduct that cost from the value of your house.