Extending an image can be done but it's not feasible. The amount of work that's required far outweighs the ease of going back and re-taking the photo!
Looking at your image, if your thinking of extending the image this far.....
....your going to be busy for a very long time!!!
Unless your just an incredible artist, extending an image means yo will have borrow information (pixels) from the existing image.
Use your Rectangular Marquis Tool to make a selection.
Use Cmd/Cntrl + J to make a duplicate of the selection.
Use your Move Tool to move the selection to the left.
Move the the new layer upwards and match the duplicate to the original (blue arrows)
The red arrows show areas that areas out of alignment.
You must use Free Transform to now adjust the the duplicate layer so it's in some alignment.
Next, you will have to make a selection of the duplicate layer so it can be aligned.
Use Free Transform to adjust the section to fit.
When your done you can merge the layers.
Now you need to get started with the Clone Tool to make your newly minted extension look right.
When your done with the Cone Stamp Tool work, repeat the process.
Good luck.