How to Design & Build A Cantilever Home Addition

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The cantilever juts out from the main house structure.
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A cantilever addition is a superb way to increase the floor space of a house without encroaching too deeply into the surrounding property. Most cantilever designs require no major excavation, either. The cantilever addition perches atop concrete footings or piers nestled deep into the ground. The wooden framing is attached to the existing sill of the home. Also called a "bump out," the cantilever is identified as a long, narrow box that projects out from the side wall of the house with a sloping roof. Before you begin tearing out the existing wall and start framing your addition you will need to know your local guidelines regulating this type of construction.


Step 1

Consult with your local building codes department. Obtain a building permit or variance if required. The building codes department of your municipality will inform you of any limitations, regulations and necessary inspections for your addition. For example, some municipalities prohibit the construction or expansion of living quarters within 10 feet of property boundaries. Before you start planning the cantilever addition, consult with your local governing authorities to determine what is and is not allowed. Failure to abide by local codes can result in steep fines or forcible removal of the offending structure.


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Step 2

Plan the site well. Consider the layout of your home's existing floor plan and the ecology of your property. If the north side of your home is plagued by pooling water from water runoff, fix the water problems by regrading the property or site the addition at a better location. If you are building the cantilever addition to serve as a breakfast room or a morning family gathering room, consider building the addition on an east wall of the house to take advantage of the warm dawning sunshine.


Step 3

Keep the design small and simple. Many cantilever additions are 5 feet wide or less but can be as long as the existing wall. The National Park Service and well-known contractor Bob Vila recommend that homeowners limit the size of the addition to prevent the addition from overpowering the main style of the house. Even if your house is not a national historic site, maintain the architectural style of your home. If your house is a traditional colonial, build the cantilever in the rear of the house to maintain the colonial design. A contemporary or rustic-style home may be improved with a simple cantilever addition in the front. Keep the addition design smaller and simpler than the original structure to prevent the addition from detracting from the style and symmetry of the house. A well-planned design may also raise the value of your home.


Step 4

Insulate the addition. Many municipalities require insulation for new construction. Recommended insulation R-values for a cantilever addition in cold or very cold climates are R-30 to R-40. Depending on the type of insulation, the cantilever addition may require a minimum of 5 to 10 inches of insulation to meet the recommended guidelines. The underside of the addition as well as the walls and ceiling must be insulated and sealed to prevent heat loss in the winter and cooling loss in the summer. The cantilever with its unique slanted roof design may require special roofing insulation if the interior of the ceiling is open to the roof. In such a case, determine the insulation requirements for this type of construction from your local building code department.


Step 5

Build the addition in steps: footings, framing, electrical wiring, plumbing, wall finishing, painting and furnishing. Expect the preparation work, such as digging the concrete footings or piers and connecting the framing to the existing home, to take several weeks. The addition must be sited properly and the concrete allowed to cure. Once the wall framing is installed, the remainder of the process will rapidly progress. Be prepared for periodic building inspections by codes officers. Most municipalities require several inspections as each step is completed -- inspection of the footings, framing, wiring and plumbing, insulation and the final inspection.


Step 6

Correlate the exterior with the rest of the house. Select identical or corresponding siding, windows and window style to the existing home. Duplicate the pitch of the roof and the roofing materials. The cantilever addition should subtly enhance the house, not the other way around.



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