De or het detailplanning?

The correct Dutch article is:

de detailplanning

Rule Words ending in -ing: almost always de. See the rule

How to use detailplanning

the detailplanningde detailplanning
this detailplanningdeze detailplanning
that detailplanningdie detailplanning
our detailplanningonze detailplanning
your detailplanningjouw detailplanning
every detailplanningelke detailplanning
which detailplanning?welke detailplanning?
a big detailplanning / the big detailplanningeen grote detailplanning, de grote detailplanning