Once you’ve exchanged contracts, you take over the legal responsibility for your new property and you’ll need buildings insurance from that day. In fact, in a lot of cases, your mortgage provider will insist that you have buildings cover in place. Be aware that when you move house in the UK, you usually exchange contracts a few weeks in advance of the actual completion day, so you’ll probably still be living in your old house when you exchange (and have building insurance in place for it). Your insurance provider will be familiar with this overlap of policies and it shouldn’t be a problem.
Insurance of your old property is your buyer’s responsibility once they’ve exchanged contracts. However, your buildings insurance may provide cover for your buyer for any damage to the property that occurs between exchange of contracts and completion of the sale.
Your insurance provider will want to know the rebuild value of your new house. This isn’t the market value; it’s the amount it would cost to rebuild your home at the same location if it were completely destroyed by fire, flood or any other disaster.