If I wanted to add an Addition, do I need to acquire and build it at the same time I build the house?

My preference is to build the house first and then to build an addition in place against the house.  This requires a bit more care when cutting a doorway in the house's sidewall to avoid damage to the floor, but it makes handling the sidewall during assembly of the house easier because a pre-cut wall is more fragile.
Cut a doorhole slideshow
Having the addition parts on-hand allows you to use them to lay out where on the house's side you don't have to paint, though.

Other comments:
As a house with additions gets long the importance of a worksurface with no 'shape' to it goes up.  Whatever shape the table has will be transferred to the house/addition and if that is different from where the house/addition will be displayed once the project is done, then it may be unsteady... so check a questionable worksurface by pulling a string across the top and shim or pick a different surface if it looks too far off.

JM401 and JM907: An addition on the side with the stairs gives an overlap between any door placement and the stairs unless the stair hole is enlarged 3/4 to 1" toward the front of the house, which then would have to be blocked in at the back if adding the 6803 Banister and Landing set.  That is why the recommended location for an addition is on the left where no extra customization is needed. Enlarging the stairhole is easy, though, if it is done before construction.

Have fun with your build, and let me know whenever you have a question
Best wishes
Gary