Photo by Scott Tennant

by Dan Fritschen

Update: Working out a suitable cost estimate for your home addition project is the first step to creating the home you’ve always wanted! There are some general rules you can follow when it comes to initial cost estimates, but there are a number of factors that will affect the outcome depending on type of addition and what you intend to use the space for: the size, location, number of rooms, cost of fixtures, fittings and materials, as well as electrical and plumbing costs all need to be considered.

So what would be a reasonable estimate to add a 400 square-foot ground-floor addition with a gable roof?  For budgeting purposes, a quick estimate looks something like this.

I am assuming a labor cost of $50 an hour.  Individual workers would chargeless than that typically.  A general contractor would likely charge more to cover overhead, like benefits and management costs.

So starting at the bottom and working up.  Demolition and excavation would take several workers several days to do the job for a cost of $3,000.  Disposing of the debris could cost $1500 and equipment costs maybe an additional $500.  Total $5000.

The foundation – a stem wall 12″ below grade (which is OK for warmer climates) would cost approximately $5000 for labor and materials.  Floor joists, girders and subfloor would be approximately $2,500.  Framing the walls and roof would take several days for 2 or 3 workers, for a cost of labor and materials of $4500.  Next comes rough plumbing and electrical.  In this example, there is no plumbing, but electrical is a bit more complicated because breaker box needs to be moved – fortunately in the direction of the power line so the wires are cut shorter instead of having to be rerun. Rough electrical would cost $4,000 for materials and labor.  (Typically electricians charge more than the $50 average we are using for this estimate).  Duct work for heating and cooling won’t cost too much if added on to an existing system.  A day and a half of labor and $200 materials cost should cover it, for a total of $1000.  Good quality windows of average size are approximately $300 each and a patio slider is approximately $800, so $2500 for windows and doors, and another $50o to install them, for a total of $3000.

With the electrical complete, the walls can be insulated for $1,000.  Siding or stucco can be installed on the exterior walls for anywhere from $1000 to $3000, depending on the type selected.  Roofing will be about $4,000.  Drywall is next and will cost about $3500.  Flooring can vary tremendously, but for typical hardwood floors $10 to $12 per square foot is a safe estimate.  Lighting fixtures, outlets, switches, and painting will finish up the project and will cost about $700.

Clean up will take another day, plus another trip to the dump for a total cost of $1000.

So the total for this project is $35,200 for the basic costs – labor and material.  If you hire a general contractor who would coordinate and manage this project, count on the following additional costs:  $10,000 for markup on the materials, $20,000 for supervision and management the project, $15,000 for overhead, $10,000 for profit, and $10,000 for “overages”.  In this example, a general contractor could add $100,000 to the cost of this project.

To use our free cost calculators for your home addition, click here.