There's an old joke that stuff expands to fill the available space. That often seems to be the case with a shed. What was originally built to provide ample storage for lawnmowers, garden tools and bicycles can over time become jammed. Homeowner interests also change. A gardener might want a greenhouse. Or a woodworker might need a larger work area. That leads to expansion or modification of the original shed. There are many options, depending on the need.
Make a Lean-To
Make a dirt-floor lean-to as the simplest addition to a shed. Set 4-by-4-inch posts at each end of the shed and as far out as you want the lean-to to extend; 6-to-8 feet is best. Set center posts if the shed is 8 feet long or two intervening posts if it is longer. Dig holes as deep as a third of the final height of the post; for an 8-foot post that is 2-1/2 feet. Put in a layer of gravel, then pour concrete to set the posts. Set the outer posts 1 inch shorter than the shed side, to create a drainage slope.
Nail 2-by-4-inch boards across the posts the length of the shed and across the width from post to post. Cut a door opening, if desired, from the inside of the shed; use a reciprocal saw to cut on the outside edges of 2-by-4 studs where the door will be. Make a frame with a horizontal header board atop the door opening and short studs on each side from the header to the floor.
Add wood or metal siding as desired. Use 4-by-8-foot pressure-treated plywood panels, wood planks nailed horizontally or vertically or corrugated metal. If metal, nail 2-by-4 horizontal braces between posts for provide nailing points at 24-inch intervals for the panel. Put siding on the ends or leave open for easy access. Roof the lean-to by nailing plywood or metal over the rafter boards from the shed wall to the outer beam. Nail metal flashing under the shed siding and down the lean-to roof. Finish a wood roof by laying down roofing paper and nailing on shingles.
Add a Greenhouse
Add a greenhouse by setting wood, metal or polyvinyl chloride (PVC) posts in concrete at corners and in the center of the outside edge. Excavate the interior space 8 inches deep and frame it with 2-by-4 boards staked upright around the perimeter. Lay down 4 inches of gravel compacted with a hand tamper and pour 4 inches of concrete floor. Level the concrete with a long board and smooth it with a trowel.
Make a frame by adding top rails all around the posts with horizontal braces about halfway up and with rafters across the width to hold roof panels. Nail, screw or cement these in place, depending on the material. Cut a door in the shed wall with a reciprocal saw and build framing with a horizontal header at the top and extra studs between the header and the floor. Cut existing studs as needed with the reciprocal saw.
Screw clear Plexiglas panels to the frame on all sides and on the roof. Use special screws with a rubber grommet at the head to seal out moisture. An alternative is to use clear plastic sheeting. Stretch it tight around the frame and nail or staple it in place. Seal seams and edges with heavy-duty clear plastic tape. Use clear caulking to seal it to the shed side.
Build a Real Addition
Build a real addition by removing the siding from the shed and building new walls. Frame a wide doorway by bracing the shed roof temporarily and cutting studs with a reciprocal saw. Nail a 2-by-6-inch header board between studs over that doorway and secure it with studs from the header to the floor. Leave this doorway open.
Stake out a floor perimeter and excavate it 8 inches deep. Build a 2-by-4 frame around it, securing the forms securely with stakes. Put down 4 inches of gravel, compact it with a hand tamper and pour 4 inches of concrete. Level the concrete with a long board and smooth it with a trowel.
Frame the addition by building walls, with 2-by-4 base and top plates and upright studs between at 16-inch intervals. Erect the walls and fasten the bottom plates to the floor with concrete nails or concrete screws. Tie the walls together with cap boards that overlap the wall joints. Put a cap between the ends on the shed side; use double 2-by-4s or a 2-by-4 with a 1-by-4 over it to provide a slope for drainage. Nail the end walls and the cap board to studs in the existing shed. Nail 2-by-4 rafters across the addition; just put these flat, wide side down, spaced 24 inches apart.
Finish the addition by adding siding to match; use some of the removed siding if possible. Nail new corner trim boards at the corners and put metal flashing on the roof connection. Slide flashing at least 4 inches under shingles on the existing roof and 4 inches down the new roof. Install roofing to match the shed, corrugated metal or oriented strand board (OSB) sheathing and shingles.