Installing a window in your garage can make a huge difference by providing more natural light and allowing you to "air out" the place during nice weather. Professional contractors can do the job quickly for a substantial fee, but tackling the project yourself doesn't have to be complex, time-consuming or expensive. With some common household tools and a little woodworking know-how, you can build your own window frame. Even if you choose not to install the window yourself, having the rough opening ready will eliminate a lot of the labour (and expense) involved in professional installation.
Determine the measurements of the window you want to install. Add 1.2 cm (1/2 inch) around for shims and insulation -- i.e., a total of 2.5 cm (1 inch) to both the vertical and horizontal dimensions -- to get the size of your window frame.
If your garage is finished, remove the drywall or panelling in the spot where the new window will go. Remove everything from the floor to the ceiling within this space, because you will need to place new, full-size studs in the wall frame.
Mark your frame width from step 1 onto the top and bottom boards of the wall frame, and mark the top and bottom of the frame on any studs that are inside or immediately outside the opening. Add a pair of extra marks at 3.7 and 7.5 cm (1.5 and 3 inches) beyond your frame width on either side. These will be where you'll place new studs for your window frame, so if one or more existing studs are already in the right place (or you can adjust your placement a bit), you could save some time and materials.
Trim two 5 by 10 cm (2 by 4 inch) boards to fit between the top and bottom boards of the wall frame. Put each one into one of the outermost slots you marked in Step 3, and toenail them (nail through them at an angle) to the wall frame.
Place marks on the new studs to indicate where the top and bottom of your window frame will be. Then mark two 3.7 cm (1.5 inch) slots below the bottom of the frame for a sill -- a pair of horizontal 5 by 10 cm (2 by 4 inch) boards, and an additional distance above the frame for a header, whose size will be determined by the dimensions of your window and local planning requirements.
If any old wall studs intersect your window frame area, cut out the portions that fall within the window frame, sill or header.
Trim two 5 by 10 cm (2 by 4 inch) boards to fit between the bottom board of the wall frame and the top of your window frame (but not into or above the header space). Nail these to the full-size studs you placed in Step 4.
Create a header by trimming a 5 by 10 cm (2 by 4 inches) board to the width of the window frame, or by gluing plywood between a pair of 5 by 15 cm (2 by 6 inch) boards (all cut to width as well). Place the header on top of the short studs from Step 7 and nail it to them and the adjacent full-size studs.
Trim two 5 by 10 cm (2 by 4 inch) boards to the width of the window frame, and nail them together to create a sill. Place the sill onto the sawed-off studs from Step 6, and nail it to them and the adjacent full-size studs.
Within the rough opening you have created for the window, use a reciprocating saw to cut through any plywood or sheeting covering your wall frame and the siding or other exterior covering on the outside of your garage.
To save money and time, choose a standard window size that most hardware shops or builders will have on hand (or can quickly order). It can be a good idea to postpone Step 10 until just before you are ready to install the new window. This is unlikely to cause significant delays with installation, and it will save you the hassle of having a big, open hole in your garage wall for an extended time.