Outpainting is the inverse of inpainting — instead of regenerating a region inside an image, it generates new content beyond the edge. You give the model an image and a "canvas" larger than the original, and it fills the new space with a continuation of the scene.
For interior design, outpainting is used to extend a tightly-cropped photo into a fuller view. A real-estate listing photo that only shows half the kitchen can be outpainted to show what the rest of the room plausibly looks like. This is delicate — the model is guessing rather than observing — but useful for marketing visuals and concept boards where a full-frame image is needed.
Like inpainting, outpainting reuses the diffusion machinery: the new pixels are denoised conditioned on the existing pixels, so the result blends rather than seams.