A commercial lessee who wishes the renewal of his lease must request this by registered letter or via process server´s writ. Furthermore, in accordance with article 14 of the Commercial Lease Act, the request must "contain a mention that the lessor shall be deemed to have consented to the renewal of the lease under the proposed conditions if he does not, within three months, give notice in the same manner of his refusal of renewal (with specification of the grounds for such refusal), or of other conditions or of the offer of a third party".
However, taking this legislative text over verbatim is not sufficient for a valid request. Already in a decision of 2 March 2006, the Court of Cassation noted that the lessee must expressly state in what way the lessor may respond, i.e. either by registered letter or by process server´s writ. The simple mention “in the same manner” is not enough – as the Court reconfirmed in a recent decision of 16 February 2018.
Although this is a reiteration of a somewhat older decision, there was a bit of an open question as to whether the Court would retain this strict and formal interpretation, after it had specified in more recent decisions (concerning other parts of the request) that the renewal application did not have to take place in the consecrated terms. However, as far as this particular aspect is concerned, the Court of Cassation evidently still hews to a rather strict formalism.
For more information on this topic, you can consult Ewoud Willaert.