double check with your city's building codes in regards to residential property add-ons. including your home owners association.
Even for something as small as that. Because you may strongly be restricted to what type of foundation you put down for it to sit on.
Like the others I'd recommend doing concrete.
Preferably a mix of sand and gravel. 2 parts sand, 1 part gravel, 1/2 part cement. (That's the formula for concrete if doing it home made, cheaper to do it home made, but professionally, they will add chemicals to improve the conretes longevity)
But you NEED to check with your city hall for buildinrg additional structures on a residential lot. Same with your home owners association.
Reason why, is because you are construction a foundation to a structure that will be seperate from your home. Most would think a small shed wouldn't be a big deal, but in actuality, your city can flip out insanely over it.
Like in my home town, you can only build sheds off of gravel foundations. (make a wooden form, add supports to it to keep it in place, pour sand and gravel down, build shed on top) because they are not allowed to be permanently placed structures outside of the home facility itself. And homeowners association dictates how much cubic feet you're allowed to have in a seperate structure. and how much square feet a seperate structure can have.
Unless it's an addition to a back yard patio or something that adds onto a structure of a house, which will not become additonal square footage of the house itself, you don't need a permit to do it, and it's fine via home owners.
BUT if it's seperate from the house, you cannot pour a concrete foundation for anything. That is building code in my home town.
Another thing you need to know about pouring a concrete foundation, even if it will be seperate from your home itself, and allotted its own position on your yard, is building code concerning the use of concrete rebar, how much you have to have of it, if at all, where they have to be placed, etc. in my home town when adding on anything concrete, you have to have every square foot of it (edges of every square foot) to be framed with concrete rebar, and tied together with iron or copper wire. (just to hold it in place while the concrete sets around it).
And if you're building it within 5 feet of your house itself, or if any concrete you plan to pour will be in contact with your home for any reason, you may have to get a permit for it, because legally it may be considered additional square footage to your home itself per your city building codes. which if that happens, that means you have a wholenother mess to clean up, in regards to having physical blue prints of the structure, ensure proper electrical work (even if none planned, city may require a supporting structure such as a shed if it considers it additonal square footage of a home to have electrical fittings, etc.) proper amount of lighting and window placement, etc. etc.
Take the 10 minutes to call your local city hall, and the other 10 minutes to consult your home owners association as well.
it sounds trivial, and stupid, but I'm not kidding. When my neighbor built his shed 4 feet from his home, and had the concrete slab it sits on clearly 3 feet away on all sides from his house, he got slapped a $750 fine for making the slab itself. another $300 fine for not having blue prints on hand/filed away. another $100 fine for not supplying a copy of said blueprints to the city hall. because the shed is considered a permanent structure thanks to the conrete slab its on. another $250 fine for having improper placement of the windows and improper lighting in the shed. and another $75 fine for not having proper electrical feeding into the shed, (nothing was installed to provide power to it) and another $150 for not having the schematics of the shed AGAIN because he couldn't tell the city where wires were supposed to be installed etc.
Next month once drawings are made up of his shed, and he pays an Electrician $75 to install a light fixture, light switch, and power routed from his house to his shed, he has to pay a building inspector to check out his shed.
Everything could have been prevented, if he had taken 20 minutes of his time to check with his city and home owners.
It sounds stupid, and I agree heavily, but TALK TO YOUR CITY HALL ABOUT BUILDING CODES!!! The instant you pour concrete, and a structure goes on top of it, your city, or even your state can consider it a permanent structure of the property.
They can even throw on fines for not having proper permit. If your city requires a permit for building a new structure on your property, you'd best get it. regardless of how small it is.
Take the 20 minutes.
And another thing, almost forgot to mention, is the $300 fine for his shed not meeting building code overall, because he did not have insulation, or drywall installed. Take the 20 minutes.