At the beginning of each fiscal year your company hires a lot of new staff to try to balance things out in case you are understaffed. Depending on the situation, however, you will still be short of manpower and will need to keep making use of that hiring plan. I don't know how the plan works exactly, the only thing I know is that you can buy stock of a company that makes the plan hire more, but in some cases the number of new employees is really super small and you have to settle for low productivity.

As for building material factories, if new homes are built near it, production will drop. The zone where you build it also matters, so if it is a residential zone the production will be lower than in an industrial zone. There is also the synergy thing with other industries that makes the production increase, and also certain things that you build that increase their production, as the "administrative centers", I don't remember exactly the name, Junji comments in broken English that the building serves to increase the productivity of the factories.