My wife works for one of the local school districts in an early childhood program. One of her co-workers in a children's room had an old refrigerator box playhouse that was wearing out and my wife wondered if we could do better than another box. The main requirements were that it was safe for little monkeys, had to fit through a 34" door and had plenty of light. This is the result.
Step 1: Framing
I decided to build the frame out of 2x3 lumber which had recently become available at home depot. I'd used 2x2 for some previous projects and it just wasn't big enough to keep from splitting, even with predrilling of everything. So I laid out my basic frame, built in such a way that I could add hinges at each corner to allow it to be broken down easily by just removing the 12 hinge pins - this became a lot more complicated later during the trim phase.
Step 2: Sheathing
Because the playhouse would always be used inside I used some thin MDF as a sheathing for cost and weight. If you are building something that might get wet then glorified cardboard isn't a good choice, use some thin plywood instead.
Step 3: Siding
I had a bunch of vinyl siding leftover from when I re-sided the house a few years ago, so that became the siding for the project. Just had to make sure that the screws hit the studs since I was not going to finish the interior walls and didn't want any sharp screws for the kids to cut themselves on.
Step 4: Trim
This is where stuff got complicated. I used pre painted mdf trim, a whole lot of it. The complication was securing it in such a way that it did not defeat the ability to separate the walls. This took a whole lot of thinking. All the screws were drilled, countersunk and then filled.
Step 5: Lets See If It Comes Apart
Thankfully when I pulled all the hinge pins it actually slid apart.
Step 6: Final Assembly
One thing I didn't mention was the roof. Basically just a piece of 3/4" plywood with a couple of 2x3s cut and affixed to the bottom of it so that it pressure fits into the top. Then it was trimmed out on the sides. Could have done something fancy on the roof but it is over 6ft tall at the front and kids are too short to see the top ;) Added a couple flower boxes and a mailbox to finish it off.