The space under your deck or porch is one of the most overlooked places to store items. While it is possible to simply go with the "out of sight, out of mind" mantra when utilizing this space, chances are that things are going to get dirty, broken or rusty without the proper storage climate. While it does seems like a daunting task, building a shed under the deck or house is a fun activity, that will pay dividends and reduce clutter and stress in your life.
Clean out the entire area. Take this time to sort through your stuff, and decide what gets saved, what gets trashed, and what is sold or given away. The goal isn't just to add storage, it's to add organized storage for the items you really need, so keep your eyes on the prize.
Measure the area where you plan on building. Here, you have to decide if you want to build a deck-like floor, or use a concrete slab for the floor. If the area is level, either option works, but if there is an incline, a deck-like floor is the best, and best option.
Construct the frame. If you're pouring cement, make a frame for the floor using 2x4s. If you're building a floor, build a frame with 2x4s, and use 4x4s to secure it and attach it to the house and posts. Make sure the frame is strong enough to handle the weight of what you're going to subject it to.
Lay the flooring, or pour the cement. If you're laying flooring, use weatherproof decking material. Don't skimp on the screws, use rustproof deck screws. Keep things neat; lay the floor starting from the front, and work your way backwards. That way any rough edges are far out of sight. Here is where your level and tape measure come in handy.
Install the sides, if desired. Make sure the area is obscured from sight, with lattice, if possible. If you're planning on putting walls on your storage area, you must cover them with siding that matches your house. If the shed is hidden by latticework, then you don't have to shingle the walls, but it should be painted to match your house.
Install a door at one end, and construct a ramp, if necessary. This will assist in wheeling lawn mowers, leaf blowers and wheelbarrows into the storage area.
Put a protective roof over the area, if it's under a deck that is exposed to the elements. This doesn't have to be fancy; simply attaching 4x8' sheets of plywood to the ceiling of the shed will do the trick. Just make sure that you've painted the tops with weather-resistant paint, and that you attach them sloped to allow water to run off.