You must have buildings insurance in place, on the day you exchange contracts on your new home. Please note, most mortgage providers will make this a condition of lending, so you’ll need to think about this in advance. For new builds, this is less likely to be needed until the day of completion.
If you’re buying a leasehold property, such as an apartment, buildings insurance may be included within the lease and maintenance costs. Your solicitor will be able to advise if this is the case.