FAQ · 5 min read

Bay Window and Bump-Out GLA: Does It Count as Square Footage?

It depends on whether the bump-out has an actual floor. A cantilevered bay window with a full floor at living-room height counts as GLA. A decorative bay window that is simply an angled glass assembly — with no floor and a bench seat at window level — typically does not.

The key test: is there a floor?

ANSI Z765-2021 measures GLA from the exterior perimeter of finished, above-grade living area. For a projection like a bay window or bump-out to count, it needs to:

• Have a floor that is continuous with the adjacent living area (not a built-in window seat hovering above the exterior grade).
• Have a ceiling height of at least 7 feet for the majority of the projected area.
• Be heated and cooled as part of the conditioned living space.

If all three conditions are met, the exterior footprint of the bump-out is included in the GLA measurement.

Cantilevered bay windows (floor present)

A cantilevered bay window — one that extends the floor plane outward from the main wall — clearly qualifies. You can walk into it. It has floor space. Appraisers measure the exterior including the projection, which adds a few square feet to total GLA.

Common in craftsman and Victorian homes, these projections typically add 8–20 sq ft depending on depth. Small, but it's counted.

Decorative bay windows (no floor)

A decorative bay window that sits above a foundation — where the exterior wall angles outward but there is no added floor area — does not add to GLA. The interior is typically a built-in bench or window seat. The floor inside the home is flat and flush with the rest of the room; only the walls angle out to create the bay effect.

In this case, appraisers trace the exterior of the home's foundation line, not the glass angles at window height. The bay does not contribute to GLA.

Room additions and bump-outs

A bump-out addition — a small room extension added to a kitchen, master bedroom, or family room — almost always qualifies for GLA. It has a foundation (or cantilevered floor framing), continuous floor space, and is part of the heated living area.

These are measured just like any other room extension: trace the exterior perimeter including the bump-out, and the area is included.

How to handle bay windows in PlanSnapper

When tracing a floor plan in PlanSnapper:

• If the floor plan shows the bay as part of the floor area (the room extends into it): trace the exterior outline including the bay projection. This is typical for cantilevered bays.
• If the floor plan shows the bay as a wall detail only (the interior floor plan does not extend into it): trace the main exterior wall, not the bay angles. The bay does not add floor area.

When in doubt, look at the floor plan carefully: does the room's usable floor space extend into the projection? If yes, include it in your trace. If the bay is shown as a window detail at the wall line, keep your trace at the main wall.

Curved and angled bay projections

Traditional bay windows often have angled sides at 30° or 45° rather than a simple rectangle. For GLA calculation, appraisers typically treat these as rectangles (the bounding rectangle of the projection) or use the exterior perimeter method.

PlanSnapper traces the exterior perimeter, so you simply follow the outline of the projection on your floor plan — including the angled walls — and the tool calculates the area automatically.

Quick reference

• Cantilevered bay (floor extends out): ✅ Counts as GLA
• Decorative bay (window seat, no floor extension): ❌ Does not count
• Bump-out addition with foundation: ✅ Counts as GLA
• Greenhouse window / garden window: ❌ Does not count — no floor area
• Oriel window (projects from upper floor, no first-floor equivalent): ✅ Counts if at the floor level being measured with adequate ceiling height
• What counts as square footage in a house

Related: Measuring irregular-shaped homes · Do covered porches count as GLA? · Sunroom and screened porch GLA rules

Compare: GLA vs Total Square Footage: What Is the Difference? · ANSI Z765 vs BOMA: Square Footage Standards Compared