How to Build a 12 X 20 Garage


Adding a garage to your home can help increase its value as well as providing you with much needed extra space. While the traditional use for a garage has been as a place to park your car, people are increasingly using garages for things such as workshops, offices, art studios and home gyms. Building a garage is a relatively simple task that a do-it-yourself can accomplish with a little help along the way.

Things You'll Need

  • Design plans
  • Permits (if required)
  • Wooden 2 x 4s
  • Measuring tape
  • Hammer
  • Nails
  • Vapor barrier
  • Sand or gravel
  • Concrete mix
  • Water
  • Rebar
  • Wire mesh
  • Bucket
  • Shovel
  • Wheelbarrow
  • Concrete finishing trowel
  • Level
  • Sill plates
  • Concrete screws
  • Prefabricated roof trusses
  • Plywood or other manufactured wood
  • Siding, brick or other exterior finish for the walls
  • Roofing felt
  • Shingles
  • Doors
  • Windows
  • Certified electrician


  • Go to your local zoning board to learn the requirements for the construction and get any permits that you need to begin construction.

  • Design your garage. You can do this by hand, using a computer-aided-design (CAD) program such as Google SketchUp (which you can download for free). (See Resource 2.) You can also buy plans from a website such as Garage Plans or hire an architect. (See Resource 1.) Make sure your design fits all zoning requirements and that the site on your land where the garage will go can accommodate your design.

  • Decide which parts of the construction (if any) that you'll do yourself and which parts you'll hire a contractor for. Note: some states and municipalities require that concrete work, plumbing or electrical work be done by someone certified in that field. Check with your local municipality as these regulations vary widely.

Laying the foundation

  • Lay the foundation. You'll begin by framing out the foundation. For a 12 x 20 foot garage, lay out a foundation of at least 13 x 21 feet.

  • Once the foundation is framed, lay down a vapor barrier. This is material that resembles a garbage bag and prevents moisture from the ground from damaging the foundation.

  • Add a layer of gravel or sand on top of the vapor barrier. The thickness of this barrier will most likely be dictated by your local zoning laws.

  • Pour the concrete and put rebar and wire mesh into position in the wet concrete. Again, your local building codes will tell you how the rebar and mesh should be put in as well as how thick the concrete slab needs to be.

  • Give the concrete time to cure before you do any more work (usually about four days).

Building the walls and roof

  • Attach the sill plates to the foundation with concrete screws. Sill plates are pieces of pressure-treated wood which are used to attach the walls to the foundation.

  • Build your walls following local zoning regulations for spacing the studs (the vertical pieces of wood that make up the wall), and for framing doors and windows. The best way to build the wall is to assemble it by laying it on the ground.

  • Get a few people to help you raise the walls. Position the wall up on the sill plate, making sure that it's level and in proper position, and nail the wall into place.

  • When it comes to building the roof, your best bet is to order prefabricated roof trusses. With prefabricated trusses, all you need to do is nail them on top of the walls. Review your local building codes to see how the trusses need to be constructed and placed on the roof.

  • Sheath the garage with plywood or other manufactured wood, nailing the boards to the studs and staggering them to help make the building stronger.

  • Add the siding, shingles, brick or whatever you have decided to use for the exterior finish.

  • Put roofing felt on top of the sheathing on the roof, flattening it out and overlapping the layer that's below it. This will help keep water from seeping into the garage. Put the shingles on top of the roofing felt.

  • Install the doors and windows, and you're done.

Tips & Warnings

  • Always make sure that you follow your local building codes. These codes vary by locale and are based on the best way to build a safe structure, taking into account the local geography and climate.

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