Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to ask permission to sublet a room, use this factual base: record the property, room, payment, repair, behavior, and relevant lease or household agreement.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Ask for one remedy with reasonable access and timing, then document what was agreed. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not enter private space, withhold essential services, threaten eviction, or change locks without lawful authority. Use emergency, housing, tenant, health, or legal support for unsafe conditions, violence, or unlawful conduct. You do not need the other person to agree with a boundary for you to follow it consistently.