Does a Half Story Count as GLA?

Half stories -- the finished space under a sloped roof on the upper level of a 1.5-story or Cape Cod style home -- are one of the most commonly misunderstood GLA calculations. The short answer: yes, a half story can count as GLA, but only part of it qualifies, and how you measure it matters.

The ANSI Z765 ceiling height rule

Under ANSI Z765, finished space qualifies as GLA only if the ceiling height is at least 7 feet over at least 50% of the finished floor area. For sloped ceilings, the area where the ceiling drops below 5 feet is always excluded.

This is the two-part test that determines how much of a half story counts:

Step 1: Does the ceiling reach at least 7 feet over at least 50% of the floor area? If yes, the space qualifies as GLA.
Step 2: Measure the full floor area, then subtract any area where the ceiling is below 5 feet. The remainder counts as GLA.

If less than 50% of the floor area has a 7-foot ceiling, the entire space is excluded from GLA -- even the areas that do meet the height requirement.

Why Cape Cod and 1.5-story homes are complex

In a typical Cape Cod or 1.5-story home, the upper level has a knee wall along the eaves (often 4-5 feet high), a sloped ceiling rising toward the center, and a ridge that may or may not reach 7 feet. The usable center area may have full standing height while the edges slope down to knee wall height.

For an appraiser, the practical question is: does the ridge height create enough 7-foot ceiling area to clear the 50% threshold? If the room is wide and the slope is shallow, yes. If the room is narrow and the slope is steep, the answer may be no.

See our guide to measuring Cape Cod homes specifically.

How to measure a half story for GLA

Measure the full floor area of the finished upper level (using exterior dimensions at that level).
Identify the 5-foot ceiling line. Measure from the exterior wall inward to where the ceiling reaches 5 feet. Exclude the area beyond that line (under the knee wall and eave).
Check the 7-foot threshold. Confirm that the remaining area (above the 5-foot line) has at least 7 feet of ceiling over at least 50% of that area. If yes, the entire floor area above the 5-foot line counts as GLA.
If the 7-foot threshold is not met, the entire upper level is excluded from GLA.

Common mistakes

Measuring only under the peak. Some appraisers count only the area directly under the highest part of the roof. ANSI Z765 counts the full floor area above the 5-foot knee wall, not just the area under the ridge.
Ignoring the 50% rule. A room with a 7-foot ceiling over only 30% of its area does not qualify as GLA -- even though part of it is tall enough.
Using interior vs exterior dimensions. ANSI Z765 uses exterior dimensions for above-grade GLA. Some appraisers accidentally use interior dimensions for upper-level spaces, which understates the GLA.
Treating the whole second floor as full story GLA. A half story is not the same as a full second floor. The area under the knee walls does not count.

Half story vs finished attic

The distinction between a "half story" and a "finished attic" is largely descriptive. Both apply the same ANSI Z765 ceiling height rules. What matters is not the label but the actual geometry: if the space is finished, above grade, and meets the height thresholds, it counts as GLA regardless of what it is called.

See our guide to finished attic GLA rules for more on this.

Using PlanSnapper for half-story measurement

When measuring a half story in PlanSnapper, upload the floor plan for the upper level. Trace only the perimeter that corresponds to the area above the 5-foot knee wall line. Set the scale from a known dimension, and PlanSnapper will calculate the GLA for that level. Combine with the first-floor GLA for the total above-grade figure.

If you have a CubiCasa scan or Matterport export that includes the upper level, those services typically show the measured floor plan with the knee wall boundary already indicated. PlanSnapper can measure directly from those exports.

Measuring a half-story home?

Upload your floor plan to PlanSnapper and trace the qualifying perimeter. ANSI Z765-compliant GLA in minutes.

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