Estimate your room addition like a contractor

Adding a room is no small undertaking, especially when you plan on being your own general contractor or doing most of the work yourself. Before you even get started with something as big as a room addition, you need an accurate cost estimate, and if you're like most homeowners, you probably have only a certain amount of funds earmarked for home improvements. Calculating costs at the outset is essential to staying within budget.

The good news is that there are quite a few resources -- some of which professional remodeling contractors use on a daily basis -- that make it possible for you to accurately estimate remodeling costs.

Remodeling cost guidebooks: estimate like the professionals

David Foster, president of Foster Remodeling Solutions Inc., has been remodeling homes in Northern Virginia and the surrounding area since 1983. He says that many of his customers base their potential costs for an addition on square foot averages.

Unfortunately, they often turn out to be less than accurate -- at times the price the customer calculated has been less than half what the project would actually cost. The reason: estimating by the square foot may not take into account pricing variations due to the size of the addition, specific job variables, its intended use and regional construction price differences.

In his experience adding a bathroom can cost much more than a study or bedroom of the same size, and smaller additions usually have higher square foot prices than larger rooms. Foster estimates that a typical room addition in Northern Virginia might cost about $200 to $300 per square foot, but the price range could be vastly different in other parts of the country.

He suggests that a more accurate method of arriving at the cost is to use estimating guidebooks. The same manuals can be found on many professional remodeling contractors' bookshelves. Here are a few of the most helpful:

• Residential Cost Data 2012 Book -- This R.S. Means guidebook is divided into the various construction stages and includes just about everything that might be encountered during a home improvement project.
• National Repair & Remodeling Estimator -- Craftsman Book Company updates their manuals on an annual basis to ensure all pricing is current.
• Remodeling & Renovation Cost Estimator -- HomeTech Publishing puts out over 200 editions of their estimating guidebooks every year with each being dedicated to the construction costs of a particular region of the country.

Many of Foster's customers have found Remodeling Magazine's annual Cost vs. Value report helpful as well. The survey is broken down by project type and region of the country and even includes some major metropolitan areas. However, he cautions that while the guidebooks can be a great resource, for an accurate room addition price it's hard to beat an experienced remodeling contractor.

Room addition costs: details make a difference

Any home improvement cost estimate is basically just a total of the labor and materials required to complete the project. Whether you've included everything needed for the job determines the accuracy of the estimate. While it might difficult to overlook windows, roofing, or floor coverings when compiling potential costs, smaller details such as termite proofing and building permits may be another matter.

Estimating guidebooks often provide a complete breakdown of all the various labor and materials that might be involved in a typical room addition. Using their data as a checklist can be a good method for ensuring you're not leaving anything out of your total job cost.

Many remodeling contractors also use comprehensive lists that cover all possible aspects of a project -- everything from excavation to drawing plans, attic insulation, and trash disposal are shown to make sure no item is missed in their pricing.

Ask a local contractor if it would be possible to use one of their estimating line item lists as a template when calculating your room addition costs. Overlooking just a few small details can make a big difference in how close the final job cost is to your estimate.

Hiring contractors: experience and proper credentials matter

Almost every major remodeling project involves hiring a few contractors -- installing wiring, plumbing, HVAC systems, and even roofing shouldn't be attempted by DIYers.

Jeff Easter, president of Easter Remodeling, has been remodeling high-end homes in Charlottesville, Va., since 1980. Even in his exclusive niche he still hears horror stories about homeowners lured by low prices into using contractors without proper licensing or insurance.

Easter says that when contractors without insurance are hired, homeowners are opening themselves up to possible lawsuits if someone happens to get hurt on the project. He added that "unlicensed contractors may not have enough knowledge to accurately bid a project and could find themselves in over their heads when the job starts." When a contractor informs you that they need to increase their price in the middle of their phase of work and you haven't approved any changes, you may be looking at a budgeting problem.

Easter believes the most important thing to remember when hiring a contractor is to check their references -- especially from the past year. Verify with recent customers that they were happy with the contractor's pricing, schedule and the quality of their work.

So, what's the secret to calculating accurate costs if you're not a professional contractor? Use industry-standard estimating manuals, and -- for the parts of the project you won't be doing yourself -- get pricing from experienced contractors with proper credentials.