Do you need extra storage or a second storage location? One easy and relatively inexpensive solution is to purchase a shipping container or conex. They come in lengths of 10, 20 and 40 ft. Most are approximately 8 ft high and 8 ft wide. I have had one for years and have never had a problem with rodents or bugs damaging anything I have stored in it. Mine has never leaked and was exposed to the elements for years, it is now covered.
Basically, they are a large metal box that has been used to carry cargo on container ships. Some have wood floors and may be lined with wood. When you purchase a box there are a few things you need to look for. Rust is a big one; some outlets will just paint over the rust and try to hide it. A second thing to look for is damage to the doors, do they swing freely and are the rubber seals in good condition.
I have seen them used for everything from houses to mini storages. Since they are a box you can place them side by side and connect them or stack then to make an upstairs. You are only limited by your imagination.
The biggest disadvantage I have found is that because they are a metal box they get hot in the summer and cold in the winter. Now where I live cold is not the problem, but heat is. I have seen several solutions to this problem. One answer was to line the container with 2 inch styrofoam insulation. A second was to cover the container so that it is not exposed to direct sunlight. A third is to cut a hole and install a window air conditioner. I have seen a combination of these ideas used. For cold weather, I would leave it exposed to the sun and insulate it if I stored anything in there that would be damaged by the cold.
I like to keep them out of the dirt so mine is setting on railroad ties. It is been there close to twenty years and the ties are still good.
I know that some of these containers have been buried. Personally, I do not recommend it. The main reason is that they are not engineered for that type of weight load. They are build on a strong frame work that is designed to carry the weight of another container, but the areas of roof and walls between the frame work are not designed to carry weight.
If you are interested in purchasing a container, try a locate trucking company that does long distance hauling, also check for companies the sell them for storage. You should be able to find one for between 1 to 2 thousand dollars, maybe cheaper if you live near a port. The twenty-foot containers can be loaded and moved easily on a flat bed wrecker.