דברי הימים ב, פרק א׳, פסוק י״ח

Once Solomon's rule was firmly established, the time arrived to fulfill his father David's legacy and begin a massive construction effort. A firm decision was made to build two monumental structures: a sanctuary dedicated to God, known as the Temple, and a royal palace [ביאור שטיינזלץ]. This palace was designed to be built in a manner truly fitting and proper for his royal status [מצודת דוד].

The simultaneous launch of these two massive projects explains the immense logistical operations that followed. Had the construction been limited to the Temple alone, there would have been no need to recruit additional laborers or request cedar wood from Hiram, the King of Tyre. David had already gathered a sufficient workforce and stockpiled enough cedar wood from within the borders of the Land of Israel. However, because Solomon decided to build his own royal palace at the exact same time, the original preparations and materials left behind by his father were simply not enough. This dual undertaking is what ultimately required him to expand the workforce significantly and import extra raw materials from foreign lands [מלבי״ם].