I don’t think there are any "official" rule as such. That said, for companies or administration sending letters, it is very common to write recipient name and address in top right corner so that once folded (commonly in 3 parts, accordeon style) this info can show through the transparent window of the envelopes commonly used in that case. Hence you don’t need to print it on the envelope and don’t take the risk of putting letter for A in the envelope aimed for B.

Other practices are more common sense or generally used, like putting the signature(s) at the bottom or the date (+city) on top left. There could also be internal rules like "at company X we always place the signature on the left".

Consider the following remarks and spacing suggestions. See the following topics and sample letters. Look into specific associtated topics, such as letter components like the appel, salutation or objet etc. Generally:

[…] l’important étant d’atteindre un équilibre visuel entre les
divers éléments. Pour rendre la présentation de la lettre harmonieuse,
on peut laisser plus de lignes blanches ou moins de lignes blanches
entre deux parties. Le texte doit être disposé de façon aérée; le
texte d’une lettre courte doit, dans la mesure du possible, être
distribué sur toute la page. […] (BDL, espacement de la lettre en résumé)