Planning and Design
Contact your local municipality to find out what permits and requirements you'll need.
Design your garage by hand (using graph paper, ruler and pencil), computer-aided design (Google SketchUp is a free program you can download), buying plans from a website such as Garage Plans or by hiring an architect. Make sure your plans meet all zoning requirements.
Decide which parts of the construction you'll do yourself. Again, check with your local municipality or zoning board, as you may need to hire certified contractors to do work such as laying the concrete foundation, wiring the garage or doing any plumbing work.
Frame out the foundation. For a 20-by-20-foot garage, lay out a foundation of at least 21 by 21 feet.
Lay down a vapor barrier inside the framed-out area. This is a plastic material that prevents moisture in the ground from seeping up and damaging the foundation.
Add a layer of sand or gravel on top of the vapor barrier. Local regulations likely will stipulate the type of material and depth of the layer.
Pour the concrete for the foundation, placing rebar and wire mesh into position to strengthen the concrete. Building codes will dictate how you should position the mesh and rebar and how deep the concrete needs to be.
Let the concrete cure before you build on top of it. This will usually take about four days.
Building the Walls and Roof
Use concrete screws to attach the sill plates to the foundation (these are boards that are specially treated and attach the walls to the foundation).
Build your walls by framing them out on the ground, then lifting the entire wall frame into place. Local regulations will tell you how far apart to place the studs and how to frame doors and windows. Bring in a few friends to help you raise the walls.
Put the wall frame into position on top of the sill plate, making sure that it's level (horizontally straight) and plumb (vertically straight). Nail the wall into place.
Order prefabricated roof trusses to frame the roof, making sure that they're designed to fit your local building codes. Nail them into position on top of the walls.
Cover the walls and roof of the garage with plywood or other manufactured wood, using a staggered pattern to help make the walls stronger. Nail the boards to the studs.
Add the exterior finish to the walls: brick, siding, shingles or whatever you've decided on.
Apply roofing felt to the boards on the roof. To keep water from leaking through, flatten out the felt and overlap the ends so there are no gaps. Install shingles on top of the roofing felt.
Install the doors and windows.