Not sure if this is known, just something I noticed and wanted to make it known - placing furnishings then re-arranging them adds the number of available slots each time you move it. So the holiday furnishings give you 4 and 4, moving them each once gets you 16 total slots, then again gets you 24, and so on.

It resets back to 8 total once you leave and re-enter your house, but I just figured I'd let you know in case (especially if people started losing items, thinking they were getting extra space).