The best answer I can come up with, is that different people view "work" on a building (and "construction" of a building) in different ways.
One who uses the article views the work process as collective. In other words, we are all working together on this building. The construction of this building requires a single collective work effort. The phrase, "the work", best describes this.
On the other hand, one who chooses to use no article is using "work" as a mass noun (In other words, "the glass is filled with water", vs. "the glass is filled with the water"... the latter sounds awkward, unless you are referring to a specific source of water), and they are trying to encompass not only the greater collective "work" that is being done, but the individual "work" being performed by each individual member of a crew.
In this particular case, in the first example, they want to make sure that nobody is doing anything to work on that building, and so, rather than viewing all work on the building as a single "work", they are viewing it as a plural, with masons, bricklayers, carpenters, plumbers, etc. providing individual work.
The same principal applies in the second example, with it viewed as a process of a collaborative effort made up of everyone's individual efforts, rather than as a singular collective effort.
Really, there is no real reason why it is written this way, except, perhaps, as a "politically correct" way of appeasing workers who are putting a great deal of effort into a project. Whether or not anyone actually picks up on this is debatable; however, you will find the article-less "work" used far more uniformly in announcements made to workers than in announcements made to those for whom the building is being worked on and in literature.