Although I only have only a few examples to show you, I will share my many "cheap or free" ideas for building shelves. The first thing you need to determine is: Do you own or rent? If you rent, I would not put a lot of holes in the walls. That said, here are some of the things I have done.
Using a light weight milk crate under each actually gives you more space, adding a "campy" look to the decor. If what you plan to store on the shelves is heavy, these doors are hollow core, so you may need to upgrade to boards. I prefer pine, as it's attractive and light. Press board is extremely heavy, and if it gets wet, it returns to sawdust. It's also very hard to paint. This is good for shops or garages, but inside the home it's not all that pretty.
If the shelves need to be inside a closet, then metal brackets work great, as you don't have room to spare for supports underneath. If you think you will need to move the shelves at any time, I suggest you don't screw the boards to the top of the bracket, especially if there is no room for them to fall.
Once, I had a bathroom and bedroom door so close together that a triangle shelf sat on them perfectly, needing only one 1" bracket on each of the 90 degree sides to secure them.
Source: I have lived so far off the cuff it's not funny, so I have a lot of things that are make shift but have served me well. These are just some.
By Sandi/Poor But Proud from Salem , OR
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I just posted a tip about doing this in a window so I am just going to share this one. They are almost identical. I have an entry hallway and often put things in there that I want to put in the 'donation station' in our clubhouse.
This is a page about creating display shelves above windows. Finding more space for storage or display often requires that you look at often unused areas around your home or apartment.