You Do Not Know Who Has a Key
Moving into a new property is one of those milestones where the excitement tends to outpace the practical considerations. You have negotiated the sale or tenancy, arranged the removals van, and spent weeks thinking about what goes where. Changing the locks is usually not on the priority list, but it should be close to the top. The straightforward reason is this: you have no way of knowing how many copies of the existing keys are out there.
The previous owner may have given spares to family members, neighbours, cleaners, or tradespeople, some of whom may not even be aware they still hold them. In a tenanted property, the list extends further still. A previous tenant, their partner, their landlord’s agent, or a contractor may all have had access at some point. Most of those people are entirely trustworthy. The problem is you do not know which ones are.

