Learn to Interpret the Listed Dimensions of Modular Plans

There are a few things you should know about reading and interpreting modular plans from dealers and manufacturers. In this blog I’ll discuss the exterior dimensions, interior dimensions, and square footage of modular plans. Keep in mind that these points apply equally to the house plans marketed by stick, log, and panelized builders.

Modular Plans – Exterior Dimensions

Modular plans typically list the exterior length and width as well as square footage of the home. The dimensions are almost always rounded-off, especially the widths. It is very common, for example, to list modular plans that are 27’ 6” wide as 28’. When modular plans are not an exact rectangle, manufacturers usually indicate the maximum width and length of the structures as if the modular plans were a true rectangle. For example, when a modular plan includes a 13′ wide bedroom that projects out from a section of a 28′ wide home, the manufacturer may list the width as 41′ even though the plan is only that wide where the bedroom is located. See the attached plan.

The main section of the home is 28' wide. But the third Bedroom, Foyer, and Covered Porch add another 13' to the width in the middle of the plan. A manufacturer might list the width of this plan as 41', but keep in mind that it is only 41' in the middle. — Here’s an example of what you need to know about the exterior dimensions of modular plans. The main section of this plan is 28′ wide. But the third Bedroom, Foyer, and Covered Porch add another 13′ to the width in the middle of the plan. A manufacturer might list the width of this plan as 41′, but keep in mind that it is only 41′ in the middle.

Modular Plans – Interior Dimensions

The interior dimensions of modular plans are always less than the exterior dimensions because of the thickness of the walls. To calculate the usable space you have to take these exterior dimensions into account. A room that is contained entirely within a 13’ 9” wide module will have an interior dimension of 13’ if the exterior wall is 6” thick and the marriage wall (where the modules join) is 3” thick. A great room that is created across two modules that are both 13’ 9” wide will have 26’ 6” of interior space because both 3” marriage walls will be eliminated.
Modular plans typically label the dimensions of irregular shaped rooms the same way they label the exterior dimensions of non-rectangular plans. They use the maximum width and length of the rooms as if they were true rectangles. This tends to exaggerate the size of rooms with jogged entries or closets that jut out from a wall.

Bedroom 3 is a true rectangle that is listed as 12' 10" deep. Bedroom 2 is also labeled as 12' 10" deep. But it is not a true rectangle and it is only 12' 10" deep when you count the space in front of the entry door. The actual useable depth is about 9' 6". — Here’s an example of what you need to know about the interior dimensions of modular plans. Bedroom 3 in this plan is a true rectangle that is listed as 12′ 10″ deep. Bedroom 2 is also labeled as 12′ 10″ deep. But it is not a true rectangle and it is only 12′ 10″ deep when you count the space in front of the entry door. The actual useable depth is about 9′ 6″.

Modular Plans – Square Feet

Many manufacturers use rounded-off dimensions to calculate the square footage of their modular plans. This will slightly inflate the size of these designs. However, modular manufacturers give accurate dimensions when they complete the drawings they use to build a modular plan.

Modular Plans – Verify What You’re Getting

The marketing literature for modular plans offered by manufacturers and dealers are is helpful when starting your search for house plans. But given the conventions for labeling dimensions and square footage, you need to take extra care to ensure you know what you’ll actually be getting before you authorize your home to be built.
For more information about calculating the exterior and interior dimensions of modular plans, see Designing a Modular Home and Modular Home Specifications and Features in my book The Modular Home.