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How Do You Handle Open-to-Below Areas and Two-Story Foyers in GLA?

Open-to-below areas, two-story foyers, and vaulted great rooms are common in residential construction. They also confuse a lot of appraisers when it comes to GLA. Here is the ANSI Z765 rule and how to apply it correctly.

The ANSI Z765 Rule

Under ANSI Z765, the portion of the upper floor that is open to the floor below must be excluded from the second-floor GLA. This applies to two-story foyers, vaulted great room areas that extend through both floors, and any similar void in the upper level.

The reasoning is straightforward: that area of the upper level does not exist as finished, enclosed living space. You cannot stand on it, walk through it, or use it. Counting it would overstate the home's finished square footage.

How to measure it

Measure the floor opening on the upper level: the actual dimensions of the void as seen from above. Subtract that area from the second-floor GLA total.

You do not need to do anything special with the first floor. The first-floor area beneath the opening is already counted as part of first-floor GLA: which is correct. You are only adjusting the second-floor number.

Two-story foyer example

Say you have a home where the foyer is 10 feet wide by 12 feet deep and extends two stories up to the ceiling. On the first floor, those 120 sq ft are correctly counted as GLA. On the second floor, there is a railing where a hallway would otherwise be: a 10 x 12 opening.

Your second-floor GLA calculation should exclude that 120 sq ft opening. If the second floor measures 900 sq ft total (including the area above the foyer), the reported second-floor GLA is 780 sq ft.

Open stairwells

Open stairwells are the exception. Under ANSI Z765 (2021), stairways are included in the square footage of the floor from which they descend, so the stair opening on the upper level is not deducted. Trace the upper floor's full exterior perimeter across the stairwell and measure the lower floor normally.

Vaulted ceilings (no floor void)

A vaulted or cathedral ceiling on the first floor does not require any deduction. The ceiling height may be greater than 7 feet on the upper portion, but as long as the floor below meets the ANSI ceiling height requirements (7 feet for at least half the room), the first-floor area is counted normally. There is no upper-floor void to deduct because there is no upper floor above that space.

Skylights and clerestory windows

Skylights do not create open-to-below deductions. They penetrate the roof, not the floor between levels. The area beneath a skylight is counted normally as long as ceiling height requirements are met.

How to do this in MeasureFloorPlan

When tracing a two-story home in MeasureFloorPlan, trace each floor separately. For the second floor, exclude the open-to-below area by simply not tracing it: draw only the enclosed, usable floor area. If you are working from a floor plan that shows the opening, trace around it.

MeasureFloorPlan calculates GLA per floor based on what you trace, so if you trace only the enclosed second-floor area, the output will correctly reflect the adjusted GLA.