I made this house so long ago that I actually don't have any photos of it's construction. The box has sitting under the house for at least two years. It was gifted to us by our friends down the street who's husband works in engineering at the airport, so I assume it was initially used for some enormous part of a plane.
Whatever the case, the box itself was huge. So much so, that I didn't want to actually cut into it until I was sure it would get full use. It lived under the house and I would take it out every now and then with the other boxes, but it always went back under. After the extension of our house, I realized that we now had a space for an enormous cardboard house to live for an extended period of time, so I got it out and started working.
Essentially the box is what it always was. I just cut one of the flaps off (the front piece) and used that for the top part of the roof. The sides and the back flap just stood up and connected to it. I used another large box from under the house (this one was a TV box) and cut it to be that front panel and support the roof.
One side wall has two windows and the other side has one window cut into the roof. The back top has a ventilation section, which is still able to open and close, but I feel I could cut it off, because it's too high for the kids to do anything with anyway. I painted the roof door and around the windows with the very last of the red paint that has also been used for the play wall, the sandpit and the cardboard boat. The walls of the house (which the girls helped paint) were done with either leftover from our house renovation or leftover front steps paint, I can't remember to be honest.
Even after my additions to the roof and door, the outside still looked a little plain, so I let the girls paint flowers on the walls too. I didn't let them go full crazy, like when we painted the wall, but they did get to paint, and that's what matters right?
I drew the outlines of the flowers in black pen, and then had them paint in the sections with color. It worked pretty well, and they did a good job of staying in the line and respecting my color choices.
The cardboard house has been out on the deck for a good while now, and is lasting well, if a bit dusty on the roof. The girls sporadically ask if they can sleep in it, but over winter, my answer was a pretty resounding no. Maybe in the summer holidays I'll let them give it a go.