How to Build a Garage Bump-Out Addition and Increase Your Storage Space

For this project, you'll need to remove the portion of your existing garage wall in the location of your garage bump out addition. To prepare to create the hole, use two-by-fours to build a temporary support structure for the roof area near your work.

Use a pair of two-by-tens and a two-by-four to build a header wide enough to accommodate your addition, plus the king studs necessary to support the header, as noted in your construction plans. Nail the two-by-tens together flat and nail the two-by-four flat onto the bottom of the header.

To install your header, mark the existing garage wall studs at the top and bottom of the header installation location. Use a reciprocating saw and claw hammer to remove the stud sections and place the header in the space. Support the header by adding king studs to each side and nailing them in place. Toenail the leftover top sections of wall studs into the top of the header. Once secure, you can remove the temporary support structure.

The space under the header and between the king studs will need removal next. You can remove the exterior siding before doing this step if you wish. Use your reciprocating saw to cut out the wall section, including the studs, and remove the section.

To create a floor for your bump out, first, attach a two-by-ten or two-by-eight ledger board extending the width of the addition plus its walls to the garage wall's exterior with its top equal to 3/4 inches below the existing garage floor height. You may have to remove a small amount of siding for this part. Support the ledger with extra lumber material or anchor it to the garage's concrete blocks if possible. You'll come back to anchor it more securely later.

Use a post-hole digger and concrete to secure treated support posts according to your construction plan for the far side of your bump out. In many municipalities, a frost-depth footing will be necessary at this step. Once your footings and posts are secure, follow the floor-framing plan to build your structure from treated two-by-eights or two-by-tens. Sheath the floor with 3/4-inch treated plywood and fasten it with 2-inch nails and construction adhesive. 

Build a Concrete Floor

You may also install your garage bump out onto a concrete slab instead of a wooden floor. For that, set your concrete forms equal to the existing garage floor height and fill them with concrete and reinforcing material if necessary for your location. Allow the slab to cure for at least 24 hours before proceeding with the next steps. Check with your municipality for any additional support requirements.

To secure the ledger board and create a smooth walking surface, pour concrete between the ledger board and the garage floor to fill the gap and make them level with each other and smooth with a trowel. Use concrete anchors through the ledger board to firmly secure it to the garage structure after allowing 24 hours for the concrete to cure. 

While waiting for the concrete to set, you're free to start the next building step but use caution not to disturb the wet concrete.

Build each of the three bump out addition walls individually by laying them out and constructing them flat on the new floor before raising them and securing them in place with 3-½-inch nails through the flooring and to each other. Many municipalities will also require fastening your walls with metal clips or fastening plates. Frame your walls to accommodate any windows you choose to install as well.

To frame the roof, cut away the garage siding to accommodate the rafters nearest the existing garage wall and fasten them directly to the structure's sheathing. Build the remaining rafters or install roof trusses to speed up the process. Secure the rafters or trusses with nails and hurricane ties, if necessary where you live.

Complete the roof by securing 1/2-inch plywood to the top of the rafters with 2-inch sheathing nails and roofing spacers. Install roof flashing, underlayment, and roofing material according to the manufacturer's instructions for the products you're using and in accordance with the local building code.

Install the 1/2-inch wall sheathing plywood and fasten it with 2-inch sheathing nails. Next, install the windows, house wrap underlayment, and siding to match the rest of the garage.