A 12-by-12-foot shed is a good size, large enough to store lawn implements and other tools and supplies but small enough not to dominate a yard. Check local building regulations before starting because most localities require a building permit for a shed larger than 100 square feet. Building codes may affect construction styles and locations; a portable shed, for instance, may be exempt from permit requirements.
Designs and Plans
Choose a design, and get a plan; many websites offer shed plans, and some are free. Building supply stores, lumber yards and libraries carry books of shed designs and plans, or you can make your own with graph paper. Lay out the foundation, floor, walls and roof in scale, locating all framing elements and other components. You'll need to pick a roof style and pitch and wall siding to complement the house as well as select a door size and style.
Get your tools in order. You'll need a rake, shovel, tape measure, carpenter's pencil, framing square, circular saw and hammer at a minimum. You also may need a screw gun, reciprocal saw, gravel compactor and concrete tools .Look over your plan; many include a list of tools and supplies needed. Include plenty of 8d framing nails.
Foundation options vary. For a concrete slab foundation, you will need wood or metal concrete forms, gravel, a compactor, a board or leveling screed and a concrete finishing tool. Get concrete from a supplier or use a rented mixer; a 12-by-12 foundation is too big to mix concrete by hand. If you opt for another foundation style, you will need gravel, concrete piers or blocks and pressure-treated 4-by-4-inch beams unless you opt to set a floor frame directly on blocks.
If you did not pour a concrete slab, which will double as a floor, you will need at least six 12-foot 2-by-4-inch framing boards, preferably pressure-treated lumber, and at least three 4-by-8-foot sheets of 3/4-inch plywood. Always buy a little extra lumber; it is cheap and boards can split, crack or have defects. You may want metal joist hangers to support floor joists.
For walls, you will need a dozen 12-foot 2-by-4s for bottom and top plates and wall caps, plus at least 36 8-foot 2-by-4s for wall studs. You also will need one 2-by-6-inch board for a door header; its size will depend on the width of the door you select. The type of wall cladding can vary. You need oriented strand board (OSB) sheathing and installation strips for vinyl siding, 4-by-8 sheets for plywood with 1-by-2-inch batten boards or clapboard or tongue-and-groove boards. Also get 1-by-4-inch trim boards for corners and roof lines.
Roofs require either six prefabricated trusses or enough 2-by-4 framing lumber to make six pairs of rafters or build six trusses on-site. Roof covering can be corrugated vinyl or metal, or shingles. Corrugated panels install with galvanized or aluminum screws with plastic washer caps. Shingles require OSB decking, roofing paper and metal drip edge flashing to seal the edges of the roof, all installed with shingle nails.