I added the two story addition to the Newport but to the right side of the house instead of the left (it is made to fit on the left side).  Real Good Toys sells the nosing needed to fit this kit to the right side, but in my experience it wasn't necessary.  Perhaps this was because I added the addition while the original house was still unassembled.  If anything, I had to cut one of the original nosing pieces from the kit down to fit properly.  Some might need the additional nosing, but I didn't end up needing it after all.

I had a couple of issues fitting the addition.  First, although the first floor of the original house kit and that of the addition are 1/4" thick, one is mdf and one is wood.  This made for a slight difference in height in the materials.

Since there was a bit of the house floor that extended into the addition, I used some 1/16" shims on the addition foundation to make the floor flush.

Second, there wasn't enough room to fit a door adjacent to the front door under the stairs as they were originally intended to be installed.  Instead, I cut the door opening toward the back of the house.

As a result, the stairs needed to be flipped in the opposite direction.  I traced the outline of the new stair opening and used a coping saw to make the cuts.

The excess opening will be filled when I get to the ceiling/flooring finishing.

I actually like this layout better since it makes sense to me that when you enter through the front door, you would either go directly upstairs or into the main part of the house.