A couple of years ago I decided I needed a shed so I started rough designing a wood framed one to work out pricing. No matter what I did I could never beat the price of a 10x12 Arrow metal shed on sale @ Sears (I think it was $275). The only problem was that the metal shed was 5 feet high, and I'm a bit over 6... The obvious answer is to raise the shed or lower the floor, and since the spot I wanted the shed to sit was sloped, I did a bit of both.
(Another thing to thing about at this point is local building code. My city allows sheds up to 150sq.ft. to be built without a building permit so I had no problems but it is worth a call to your city's planning department).
Sorry about the lack of decent photos throughout this instructable.
Step 1: Flatten Site... or Not
The basic plan is to set four posts in concrete (plus two to support the door opening) and build the shed on top of them. This means if you like you can just dig four holes, set your posts and get started on the shed. I tried to flatten the site completely before even thinking about the shed... but it was hard not to be lazy. In the end I got most of the digging done before I couldn't resist setting the posts (I used pressure treated 4"x4" posts about 5 feet high - I bought a couple of 10 foot ones but if you keep looking, and asking to see the cull lumber at your local lumber yard you're likely to find some4-5 foot straight-enough posts to use). Then I had to finish the digging with the posts in the way... and even then the shed, that was sitting in it's box beside my hole, lured me into putting it together before the "floor" was as flat as I'd like it to be. This was a terrible mistake which cost me several weekend afternoons inside the shed with a pick and shovel covered in dust with no wind to clear it away.
Step 2: Dig Post Holes
Obviously to get the site flattened you have spent a little time thinking about layout. Hopefully some of that time involved a tape measure, if not now is the time. With a simple square or rectangular shed you can easily measure and mark the spots to dig your post holes and double check the diagonals with the Pythagorean theorem.
Then you can dig your holes. I try to get each hole the size of a 80 pound pre-mix cement bag so that I can just buy 6 bags. This makes the holes about 12 inches square and 18 in. In any case the most important thing is that the concrete can support the weight of the shed base, shed, and any snow, pine-needles, or leaves that land on it. Compared to a normal wood shed this isn't much, and it won't get heavier over the years because you can't add shelves or hang much from the ceiling (mostly these metal sheds have no framing).
Step 3: Set Posts
It really isn't hard but this is the critical step, you don't want a platform that is a different shape to your shed. Start with any corner (better to pick the one that is most important to get right), drop in the 4x4 treated post and hold it plumb (straight up & down). I drove in two stakes perpendicular to each other and screwed a piece of 2x4 from each stake to the post to hold it but if you have a better method go for it. Mix up a bag of cement and pour it in the hole using a broom handle or piece of steel rod to tamp it down. It is best to check the post several times during drying to make sure it stays plumb.
I left this post for a day or so. I wanted a solid starting point to measure from.
I then set the other posts one by one, mainly since I only had an hour or so every few days. I tried to get the horizontal dimensions as close to perfect as possible (the height of the posts isn't really important as long as they are close to the same height).
Step 4: Install Beam
So I don't confuse you: I'm calling the 2X10's between the posts, "Beams" and the other 2X4's, "The Platform".
The next important bit is that the platform is level. This is pretty easy (i'm hoping you have a level, I used a 48" one). I bought four 2"x10"x12' as flat, straight, untwisted, unbowed as the lumber store could produce (not treated as these are above the ground) and screwed the first one in so that the top face was just above the top of the post. Then screw in the other end so that the top face is level. continue this all the way around the shed base, trimming the 2x10's to fit as needed. If any of the posts are higher than the top face of the 2x10 it will need to be trimmed, or the 2x10's raised. When all the 2x10's are level (and square if you set the posts well) you can secure the 2x10's with carriage bolts.
Step 5: Add the Platform
To fix any errors I had made so far I bought some 12 foot 2x4's to add some width to the shed base. Some of the shed base was going to be underground so I wanted to add a bit of waterproofing and I had some black plastic poly sheet so I screwed down the 2x4's on top of the plastic then folded it out over the 2x4's and trimming the excess plastic afterwards. I hoped this (and a coat of deck stain I put on the wood) would help the base outlast the shed.
Step 6: Build Shed
Next I followed the manufacturer's instructions (basically) and build the shed on top of the 2x4 platform. This wasn't as hard as I thought but if there is a next time I would make the dirt floor a lot flatter before this step, the ladders were a bit wobbly while we screwed the roof together.
After the whole shed was built I screwed it down to the platform, doing it earlier would have made it very hard to line up the holes in the metal panels.
Step 7: Install Skirting
So now I had a shed 4 feet higher than it was intended which was great but I needed to fill that drafty 4 feet at the bottom. As luck would have it I saw some cheap 4x8 sheets of pretend-wood siding as I was on my way to price the T1-11. T1-11 or another type of plywood would probably have been better but the price was good enough for me to think, "If I have to replace it some time it won't be too hard". In retrospect I wonder if it might be harder to do than I thought...
Anyway I cut the siding to fit and screwed it to the posts trimming it with some stained 1x4's that I ripped off an old pallet.
Step 8: Door
As I installed the skirting I remembered the need for a door so I opened the new shed doors as wide as they'd go and cut the platform and beam flush with the door posts. I then used that wood and some more scrap to frame a couple of lower doors that I covered with the same siding that I had used for skirting.
To cut the platform I first needed to remove the metal sill or transition that was at the bottom of the door. The only modification I had to make to my shed was to trim the metal that the sill was attached to with a hacksaw and trim the sill to easily fit the remaining gap so that I can replace it before closing the shed doors (they are sliding doors and require the channel to close).
Step 9: Other Bits
That basically finished up this project.
The first photo below shows the concrete blocks that I placed to give the skirting a bit of support against the backfilling I needed to do on two sides of the shed. It also shows that after considering every flooring available, I flattened it off with some crushed rock.
You can see in the second photo below that I added some white metal drip edge around the top of the platform (ostensibly to stop rain running into the shed but really just to cover up a mis-measure I'd made at the start)