It is very easy to make nice corner shelves and nooks in a cob house. For this corner shelf we had to stab in some wooden steaks whilst the cob was wet. The steaks are pushed downwards in to the wall at a 45 degree angle. These steaks then hold the cob mixture up. Try to merge the new shelf cob in to the wall with your thumbs and fingers, and maybe use a stick also to integrate the wall and shelf.
You can also make a more artesanal wooden frame to hold the shelf, it doesn’t really matter, as long as it is sufficient to hold up the cob from the shelf whilst it dries. Just make sure to really integrate the cob from the wall with the cob of the shelf and when it dries it should be really strong, maybe even strong enough to stand on.
Making little nooks whilst you are building is a bit easier than adding them on after. The little candle nooks are simply made by sculpting the cob with your hands.
As you do your wall finishes, just continue over the shelves as with the rest of the wall. You can either start to build up the corners with a rounded shape, or have slightly sharper edges. Just be aware the right angle corners are not recommended with a natural finish as they are much more likely to chip and break over time.
If you want to create an alcove with a protruding roof you can use a different technique to create the overhanging roof feature. The photos below are from a plug fitting but the technique is the same. Just take a bundle of straw that you would have been using in your cob, and make sure it is all lined up in a bundle.
Then mix it well with cob, whilst retaining the straw in the bundle. and place over the area that needs to be spanned. The straw lined up in this way creates a strong bridge over your gap.
Then all you need to do is merge in the edges and integrate the bridge to the rest of your wall
Here are some nice cob nooks and shelves I found online. Remember you can always use wood for the frame or the base of the shelf.