How to Read House Plans

An Architect Tells How to Evaluate the True Size of Your New Home

Cropped photo of male architect's arms and hands on a large blueprint, man dressed in black holding a pen, large silver watch
The house plans you admire may look good, but what do those symbols and measurements mean?. Hisham Ibrahim/Getty Images (cropped)

It's easy to purchase house plans from a website or house plan catalog, but they hardly ever come with directions for reading floor plans. 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. He wants you to know about measuring. — ed.

Key Facts About Measuring

area: measured in square feet (or square meters), the rectangular length times the width; the area of a triangle is one-half the base times the height

volume: the length times the width times the height

area of a composite: for an irregularly shaped room, divide the room into regular shapes (rectangles and triangles) and sum the areas

gross area: measured from the exterior wall foundation, so the area includes wall thickness

net area: measured from interior walls; area of the living space

architect scale: a three-sided measuring device with six measuring edges (described as "prism-shaped"), similar to a ruler, but used to interpret the true size of a line drawn to scale on a floor plan or blueprint

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.

Here's 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 a 3,000 square foot plan, a difference of only 10 percent 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, assessors, 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"? Even garages? What about closets? 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 10 percent — depending on the type of floor plan design. A "traditional" plan (with more distinct rooms and therefore more walls) might have 10 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 website, 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.

Conclusions

Building workers looking at plans, all you see are their fingers pointing
Leave the Building to Builders. Damian Gillie/Construction Photography/Avalon/Getty Images

The guest writer, Richard Taylor of RTA Studio, is an Ohio-based residential architect who creates luxury house plans and designs custom homes and interiors. Taylor spent eight years designing and renovating homes in German Village, an historic district in Columbus, Ohio. He has also designed custom homes in North Carolina, Virginia, and Arizona. He holds a B.Arch. (1983) from Miami University and is an active blog writer on social media. Taylor says: I believe that above all, a home should create a quality living experience as unique as the people that live in it, shaped by the owner's heart, and by his image of home that's the essence of custom design.

Construction designs can get complicated, so let your building crew decipher the symbols the way they are trained to do. A couple of things for the homeowner to keep an eye on include the orientation of the building on the lot (where is south and the sun? where are the doors and windows?), the HVAC symbols (where is the ductwork?), and for future reference it's good to know where your load-bearing walls will be located.

And how big will your new house measure out to be? According to the U.S. Census Bureau Survey of Construction, the average new single-family American house was 2,392 square feet in 2010 and in 1973 it was 1,660 square feet. A small home is considered 1,000 to 1,500 square feet. And tiny homes? Could you live in less than 500 square feet? That's the plan!