Storm shelters and root cellar round up
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Classic


Concrete block or poured. Building to code probably requires inspection in most areas.
Alternative materials like sandbags or timber might be used, but may not pass more modern building codes if that applies to your area.
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Hillbilly
The "OMG you built that yourself, are you sure that's legal?" root cellar. It works if there are no building codes and no neighbors... Also a great way to make bases for paintball!
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Steel


Medium cost and structurally sound. Also since many designs use materials that the city uses and require installation by a contractor, they are less likely to conflict with building codes (depends on area).
Roccland used the large can design. And with the locking manhole cover on top, it is large yet subtle in appearances.
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Fiberglass

Good for wet areas. Expensive as a boat, but good quality and likely to be within building codes.
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Plastic (one piece)
Good for wet areas. Also expensive, good quality and likely to be within building codes.
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Miniature root cellars


Barrel root cellars are a classic and subtle way to store apples and root crops without building a more extensive structure. They can be easily added to most gardens and are unlikely to conflict with building codes.
Though they do not provide human shelter, they can be made using many different materials and can be an excellent asset to any remote garden.