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How to Read House Plans

An architect tells how to evaluate the true size of your new home

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Blueprint and Ruler and Compass

The house plans you admire may look good, but what do those symbols and measurements mean?

Photo © Robert George Young / Getty Images

It's easy to purchase house plans from a Web site or house plan catalog. But what are you buying? Will the completed house measure up to your expectations? The following hints come from an architect who designs luxury house plans and custom homes.

Size Up Your House Plan

When you compare house plans, one of the more important characteristics you’ll consider is the area of the floor plan – the size of the plan - measured in square feet or square meters.

But I’ll tell you a little secret. Square feet and square meters are not measured the same on every house plan. Any two house plans that appear to be of equal area may not really be.

Does this make much difference when you’re choosing a plan? You bet it does! On on 3,000 square foot plan, a difference of only 10% might unexpectedly cost you tens of thousands of dollars.

Question the Measurements

Builders, Architects, Real Estate Professionals, Bankers, Auditors, and Appraisers often report room sizes differently to better suit their particular needs. House plan services also vary in their area-calculation protocols. In order to compare floor plan areas accurately, you’ve got to be sure that the areas are counted the same.

Generally, builders and real estate professionals want to show that a house is as big as possible. Their goal is to quote a lower cost per square foot or square meter so that the house will appear more valuable.

In contrast, appraisers and county auditors usually measure the perimeter of the house – a typically very rough way to calculate area – and call it a day.

Architects break the size down into components: first floor, second floor, porches, finished lower level, etc.

To arrive at an "apples-to-apples" comparison of house areas you’ve got to know what’s included in the totals. Does the area include only heated and cooled spaces? Does it include everything "under roof"? (I’ve seen garages figured into some plan areas!) Or do the measurements include only "living space"?

Ask How Rooms Are Measured

But even when you’ve discovered exactly what spaces are included in the area calculation you'll need to know how volume is counted, and whether the total reflects the net or the gross square footage (or square meters).

Gross area is the total of everything within the outer edge of the perimeter of the house. Net area is that same total – less the thicknesses of walls. In other words, net square footage is the part of the floor that you can walk on. Gross includes the parts you can’t walk on.

The difference between net and gross can be as much as ten percent – depending on the type of floor plan design. A "traditional" plan (with more distinct rooms and therefore more walls) might have ten percent net-to-gross ratio, while a contemporary plan may have only six or seven percent.

Likewise, larger homes tend to have more walls – because larger homes generally have more rooms, rather than simply larger rooms. You'll probably never see the volume of a house plan listed on a house plan Web site, but the number representing the area of a floor plan often depends on how the volume is counted. Typically, the "upper area" of two-story rooms (foyers, family rooms) isn't counted as part of the floor plan. Likewise, stairs are only counted once. But not always. Check how volume is counted to be sure you know how big the plan really is.

Plan services that design their own plans will have a consistent policy on area (and volume), but services that sell plans on consignment probably don't.

How does the designer or plan service calculate the size of the plan? Sometimes that information is found on the service's website or book, and sometimes you have to call to find out. But you should most definitely find out. Knowing how area and volume are measured can make a very big difference in the cost of the house you ultimately build.

~ By Guest Writer Richard Taylor.

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