It may surprise you to learn that you become liable for all risks associated with owning a property when exchange of contracts takes place and that your liability is both date and time sensitive.
Most people would assume the date and time aspect because that applies to other insurances, car, travel, etc, but many people expect property insurance to start on completion of the purchase, when you collect the keys, when you have access, where you are 'in possession'.
But that is not the case, even though you may not be 'in possession', or have a right of access on exchange of contracts you become liable for all risks connected with the property and should insure.
When the hammer falls on your bid at auction, in that instant, you have exchanged contracts.
Since the inception of buy to let in 1996, more and more specialist insurance policies have been released to cover the additional risks associated with developing, fixing up and letting property.