From La Razón
: Felipe VI promotes the enlargement of the Pantheon of Kings
Felipe VI impulsa la ampliación del panteón de Reyes
Apparently,Patrimonio Nacional (the institution that takes care of the Crown's monuments), commisioned 10 years ago a group of experts to make a project of enlargement for the actual cript.
They came to the solution of building a new room, with same size and characteristics as the original one, united to the first one through a gallery.
The project has yet to be approved but according to La Razon it will be in the next months.