Yeah, it's the bevel tool.

I generally use 4 segments, the offset distance is set to 0.2 on this part but it really varies depending on the size of the object that you are bevelling, it's really best to play around with the settings. Click 'Apply' rather than 'Bevel' once you've set the tool that way you just have to hit z and try another setting if you're not happy with the result.

Here's an update after duplicating the chair and doorway a few times....