How to Measure a Cape Cod or 1.5-Story Home for GLA

Cape Cod and 1.5-story homes are among the most common residential styles in the United States — and among the trickiest to measure correctly. The upper floor has finished living space, but sloped ceilings mean only part of that floor qualifies as GLA under ANSI Z765. Here is how to get it right.

What makes Cape Cods and 1.5-story homes different

A Cape Cod typically has a full first floor plus a finished upper level tucked under the roofline. The upper floor has usable space in the center of the house, but the sides taper to knee walls where the roof pitch drops the ceiling height below the livable threshold.

A 1.5-story home is essentially the same configuration: a main living floor plus a partial upper floor with sloped ceilings. The “0.5” in the name acknowledges that the upper floor is not full-height throughout.

The ANSI Z765 ceiling height rule

Under ANSI Z765-2021, finished space counts as GLA only when the ceiling height is at least 7 feet. For sloped ceilings (like those on Cape Cod upper floors), at least half the ceiling area must be 7 feet or higher.

This means the knee-wall areas on a Cape Cod upper floor — where the ceiling is only 4 or 5 feet high — do not count toward GLA, even if they are finished and used for storage or closet space.

How to measure the upper floor in PlanSnapper

If your floor plan shows the full footprint of the upper level (including knee-wall areas), you cannot simply trace the entire outline. You need to trace only the qualifying area.

The practical approach:

Upload the upper floor plan into PlanSnapper.
Identify where the knee walls are on the plan. These are typically marked as a dashed line, a change in wall type, or labeled as “knee wall” on architect drawings. On CubiCasa or iGUIDE outputs, the non-qualifying areas may be shaded differently.
Trace a polygon that follows the outer walls of the qualifying area only — stop at the knee-wall line rather than the exterior wall.
Set your scale using a known dimension on the same floor plan.
Record the upper floor GLA separately from the first floor. Then add them together for total above-grade GLA.

If the floor plan does not show the knee-wall line

Many floor plans — especially MLS photos or older architectural drawings — do not clearly mark the 7-foot ceiling line. In that case, you have a few options:

Field measurement: Measure on-site where the ceiling reaches 7 feet and mark that line on your floor plan sketch before uploading.
Roof pitch calculation: If you know the roof pitch and the ridge height, you can calculate where 7 feet is reached from the exterior wall. A 6/12 pitch drops roughly 6 inches per foot horizontally.
Conservative approach: Some appraisers measure to the exterior knee wall and note that the result may overstate GLA if the knee-wall areas are below 7 feet.

Common mistakes to avoid

Tracing the full footprint: Including knee-wall areas inflates GLA. Always stop at the 7-foot ceiling line.
Combining floors before checking ceiling heights: Measure the upper floor separately, verify qualifying area, then add to first floor.
Treating finished knee-wall space as GLA: Finished does not mean it qualifies. Ceiling height is what matters under ANSI Z765.
Using MLS square footage as your baseline: MLS listings frequently include knee-wall areas in the square footage, overstating GLA. Always measure independently.

Measuring in PlanSnapper: step by step

Upload the first floor plan. Trace the exterior perimeter. Set scale. Record GLA.
Start a new polygon (or a new session) for the upper floor.
Upload the upper floor plan. Trace only the qualifying area inside the knee walls.
Use the same scale reference (or re-set if you are using a different plan view).
Add both numbers together for total above-grade GLA.

Related resources

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