After much thinking and planning I decided to use a tab and slot technique to assemble the house. This would make it nice and sturdy. Foam board is a dream to cut with an EXACTO knife.
I would highly recommend buying a knife with a soft body and glue gun finger guards. The tips of my fingers became numb and I had a blister.
After cutting tabs into the floor I worked on one wall at a time considering the floor, next wall, ceiling and the same for the next floor up. Once all the walls were finished I cut the ceiling/send story floor and then the third story floor. I was amazed I only had to make a few simple adjustments and it all fit together easily!
I used wood glue for some of the floor tabs and a hot glue gun for the rest.
To secure the walls in the interior I pushed a pin through the second story floor down into the center of the first floor wall.
I have made a house with the tab and slot method from wood and I have to say foam board is so much more fun!
Foam board tips:
Purchase board that are the least warped. I bought the foam board dilly board packs because they were a little thicker and less warped.
When cutting focus on keeping your blade angle straight down to you don’t get beveled edges.
Discard dull blades, keep them sharp.
Glue gun tips;
Be sure there are no thick blobs. Melt them down with the tip of the gun as you go.