I spent nearly a year designing Myrtlewood and researching how to build it. Early on, I knew it would be large (60" long by 31" deep) and would need a very sturdy base. I built the base out of 3/4" birch plywood. This was much more expensive than regular plywood, but it provides a very nice surface on which to work and it's very strong so I don't worry about it bowing in the center from the weight of the house. I worked out the dimensions of each piece of the base before going to my local Lowes to buy the plywood. They cut it there for me (for free!) to my exact specifications. Here you can see how I assembled it.
It's a very simple design: just a 60" x 31" rectangle with 3" wide strips attached perpendicularly on all four sides. I glued and clamped the pieces together and then, after everything had dried, added about 20 screws total. This baby is secure!
As I was working on the base, I spent a lot of time looking for a table on which to set it. I hunted through stores, assorted catalogues and online, but just couldn't find one that fit my specifications. Finally, I had an epiphany. Why not make the base of the house itself into a table? I found a great company online that makes table legs and ordered an adjustable pair. These are absolutely perfect! They adjust up and down 24" to 36", so I can set them high to work on the ground floor and lower when I want to work on the upper floors. And if I ever need to lower the house all the way to the floor, the legs fold up and can be completely contained within the base. I just love its versatility!
Here's a picture of the entire table; you can see I've already started the framing - more about that later!