Cost of a Home Addition

Costing out a Home Addition Requires Attention to Detail and Time

By Mark J. Donovan

The cost of building a home addition or adding a room onto your home continues to shoot upwards. Today you can expect to pay anywhere from $25K to $70K for the average home addition. However, regardless of the cost of a home addition, selling a home and buying a larger one is usually a much more expensive endeavor. Thus, many homeowners take the plunge and elect to build a home addition rather than uproot their home and family.

Key to a successful and less painful home addition build project, is upfront planning and budgeting. 

This includes coming up with solid and detailed building plans, and estimating accurately and completely home addition costs. It is this latter topic that the rest of this article focuses on.

Estimating home addition costs accurately is critical for getting financing, if required, as well as for ensuring your project stays on budget and doesn’t wipe out your savings account.

Calculating the cost of a home addition can be a little complicated, but with accurate home addition plans and a little time and effort you can cost out your own home addition fairly accurately. The cost of a home addition, however, has many variable factors. Home addition costs are a function of the size and type of the home addition, the materials used in its construction, housing market conditions and local contractor labor rates.

As a result, there is no simple formula that can accurately project home addition costs. There is, however, a simple process that will allow you to calculate the cost of your home addition.

Over the years I have built my fair share of home additions and in each case I developed my own set of construction plans and calculated the costs of the home addition.

Decide on What Type of Home Addition Your Building

There are many types of home additions to build and you need to choose upfront which is right for you. There are bump-out additions, second story additions, detached additions and modular additions.

Each one has its own pros and cons, and comes with its own set of unique set of circumstances that impact the estimated home addition price tag. Also, the size of the addition, and the materials and features incorporated into it impact the home addition costs.

Create Building Material List

The first step in the process of costing out a home addition requires creating a building material list from the home addition plans.

The more thorough your building material list, the more accurate you will be in determining the cost of your home addition. Every board, door, window and nail should be included in your material list.

I find by breaking the material list up into the major home addition construction tasks makes creating the building material list easier and more thorough. 

Home Addition Bid Sheets

For example, start from the ground up. This means start at the excavation and foundation phases. From there move on to the rough framing of the home addition.

Next, move on to the siding and roofing of the home addition. Once the home addition is buttoned up with walls, roof, doors and windows, move on to the inside of the home addition and address electric, plumbing, insulation, drywall, trim work, cabinetry, and flooring. For each phase specify the exact set of materials that will be used.

For tasks such as electrical and plumbing where most likely you will subcontract out the work, use the subcontractor quotes for each of these cost items. Likewise use subcontractor quotes for other tasks that you are not specifically performing. If the subcontractor quotes do not include specific costs for certain key items, include these items separately in your home addition cost estimates. For example, if the plumbing subcontractor quote does not include the cost of the Jacuzzi tub you want to install in the home addition then make sure you account for it in your building material list.

Cost of Home Additions and Use of Contractor Allowances

When it comes to items like flooring, lighting and cabinetry contractors frequently like to give you a cost allowance for these items. Do not use cost allowances when attempting to accurately cost out a home addition. Visit your local home improvement store or building supply store to get specific costs for each of these items.

Do not be surprised to see significant variations in contractor quotes for the same task. The variations can be due to many reasons, including how accurate the quote is, and the labor rates charged for performing the work. Make sure you carefully study each quote to understand what you are and are not getting for your money.

The state of the home building industry in your local area can also affect the cost of the quotes you receive. If the home building industry is booming, expect higher quotes. It’s just the simple law of supply and demand, as well as a little greed. If the home building industry is slow the quotes may be a little more reasonable.

In either case, don’t hesitate to question the quotes and ask the subcontractors to sharpen their pencils a little. Sometimes it works and other times it does not.

Estimating the cost of a home addition

Once you have created a comprehensive building material list, and know what items you are contracting out you can then seek subcontractor quotes to fill in the costs for those particular items.

Obtaining Home Addition Costs for Building Materials

For the items on your home addition building material list that you plan on tackling yourself, you need to next obtain material costs for each one of these items.

If for example you plan on rough framing the home addition yourself, take the portion of your building material list that relates to lumber, doors and windows, siding, and roofing to a few local home improvement centers. Visit their contractor desk and ask them to provide you with costs for each line item in your material list. Alternatively you can also walk down the aisles of the building supply store and obtain your own material costs, however I find using the contractor desk much more efficient. The other advantage of going to the contractor desk is that you can occasionally convince them to give you a discount.

With your building material list complete, and costs determined for each phase of the home addition project you can now accurately calculate the cost of your home addition.

Building Permits and Home Addition Finance Charges

When calculating the cost of a home addition do not forget to include building permits costs and financing charges. Building permit costs are relatively low, however they are real costs.

Likewise, if you are using a home equity loan you should also include any costs for obtaining the loan and the finance charges that come with it.

So with a solid set of home addition plans and a building material list you should be able to accurately calculate the cost of your home addition. It just takes attention to detail and some time.

For more help on calculating the cost of a home addition, see’s Home Addition Bid Sheets. Our Home Addition Bid Sheets provide you with the knowledge and information on how to plan a home addition project, and what to look for when hiring contractors. They also include detailed cost breakdown tables and spreadsheets for estimating your own new home addition building costs.

Related Information

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