Separate emergency access from permission to visit. Explain when someone may use the key, whether they should contact you first and what you will do if the arrangement stops working. A useful boundary describes an action you can take.
Define the purpose of access
List the situations the key is for: a lockout, an agreed delivery or a specific emergency. 'For emergencies' leaves room for different interpretations. Decide whether routine visits, cleaning and letting another relative in require separate permission.
Give a clear request
Try: 'Please ask before coming over and wait for us to confirm. The spare key is for the emergency arrangement we agreed.' Explain the practical reason once; a long defence can turn a household decision into a debate about whether you deserve privacy.
Agree what happens next
Consider a trial period, returning the key or another access arrangement. Check tenancy, ownership and local requirements before making physical changes. A co-resident's access and a visiting relative's spare key may involve different rights.
Use the discussion carefully
A similar story can help you find words, but it cannot establish another person's motives or your legal rights. If setting a boundary could expose you to threats or coercion, seek appropriate local support rather than relying on a confrontation script.
Sources and further reading
These are general conversation aids. Related scenarios illustrate different perspectives; they do not establish facts about your circumstances.