The canvas can be extended via Image>resize>canvas size
It would help if you post the picture for targeted guidance. Use the camera icon in the reply box.
Do you want to retain the foreground and alter just the background?
You could use part of the background to define a pattern and then use the pattern stamp tool which is nested with the clone tool. Right click on the tool or keep pressing the S key to tab.
Have more information now.
After you have enlarged the canvas sufficiently, make a selection of wall with the rectangular marquee tool. Put the selection on its own layer, use the move tool to position the selection. Do this for other areas of the expanded canvas as well. Use the clone tool only to tidy up, i.e. sparingly, and do the cloning on a blank layer at the top of the stack of layers in the layers palette.That way you can adjust the opacity and blending mode.