Account for the relationship as it is now
Use a tone that fits the present relationship, not the closeness, authority, or access that existed in the past. For a message about how to decline a request to become roommates, use this factual base: acknowledge what was requested, any real timing issue, and whether your answer applies only now or more broadly.
Ask without assigning an obligation
Explain what you hope for while making decline, delay, or a different format genuinely acceptable. Give a direct no, a limited alternative only if you mean it, and any practical information they still need. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Keep the ending honest
Do not overexplain private details, blame another person, or say maybe when the answer is no. When pressure, coercion, or retaliation is possible, use a safer channel and involve appropriate support. Avoid future promises, guilt, flattery, or nostalgia that imply the recipient owes you access.