In Belgium
The notice period (preavis / opzegtermijn) is the minimum time between notifying termination and the actual end of the lease.
Primary residence lease: tenant always 3 months; landlord for personal occupation 6 months; landlord without reason 6 months + compensation.
Start date. First day of the month following receipt of the registered letter.
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Good to know
A notice period shorter than the legal minimum is not void. It is extended to the next valid date.
Practical example
A tenant sends a registered letter on 20 May. The 3-month notice starts 1 June, ends 31 August. She must vacate and pay rent through 31 August.