Learn · Floor Plan Measurement · 5 min read

Part of: Square Footage by Property Type: What Counts and What Doesn't

Open Floor Plan Square Footage: How to Measure Combined Living Spaces

Open floor plans are the dominant layout in homes built after 2000, and they're everywhere in remodeled older homes too. The good news: an open floor plan doesn't change the measurement method. The challenge is that the exterior perimeter often hides irregular bump-outs, angles, and offsets that are easy to miss.

Why open floor plans aren't actually harder to measure

The ANSI Z765-2021 standard measures Gross Living Area (GLA) using exterior dimensions, you trace the outside of the house, not the inside. Whether a home has walls between every room or no walls at all, the exterior perimeter method produces the same result. Interior layout doesn't affect GLA.

The misconception is that open floor plans require room-by-room interior measurement. That's the interior method, which some agents and buyers use as a quick estimate. Appraisers on UAD assignments must use exterior dimensions under ANSI Z765-2021. The interior method consistently underestimates GLA because it misses wall thickness (typically 4–6 inches per wall), which adds up quickly in a large home.

The exterior method on an open-plan home

For a simple rectangle, measuring an open floor plan home is no different from any other: measure four exterior walls, multiply length by width. Most open-plan homes aren't simple rectangles, though, they have:

Pop-outs, A breakfast nook that extends past the main wall line
Garage connections, Attached garages share a wall but are excluded from GLA
Covered porches, May or may not meet the ANSI definition of finished, conditioned space
Angled walls, Bay windows and diagonal corners require triangulation
Step-downs, A sunken living room that drops below the main grade line

Each of these requires a separate measurement. The exterior perimeter method handles all of them: trace the outside of every finished, above-grade, conditioned area and calculate the total enclosed area.

Measuring from a floor plan

If you have a to-scale floor plan, from CubiCasa, Matterport, an architect, or a permit drawing, you can calculate the GLA without setting foot on the property. The plan shows the exterior perimeter. You trace it, set the scale, and get square footage.

For open-plan homes, the floor plan is often more reliable than field measurement precisely because it shows the full perimeter in one view. Field measurement requires walking the exterior and sketching as you go. A floor plan lets you see the whole shape at once and verify that all bump-outs and offsets are captured.

Using PlanSnapper on an open floor plan

Upload the floor plan, then trace the exterior perimeter of the above-grade finished area as a single polygon. If the garage shares an exterior wall with the living space, trace only the living portion, exclude the garage footprint. If there's a covered, unheated porch, leave it out. The polygon should represent only finished, conditioned, above-grade space.

For multi-story open-plan homes, trace each level as a separate polygon. PlanSnapper sums them automatically. This is the standard approach for homes where the main floor has a great room that opens to a loft above, the great room area is measured once on the main floor plan, and the loft mezzanine is measured as a separate upper-level polygon.

Common mistakes when measuring open floor plan square footage

Mistake	How It Happens	Fix
Including the garage	Open-plan flow blurs the garage boundary in field measurement	Subtract garage footprint; note garage wall start on sketch
Missing bump-outs	Breakfast nook or kitchen pop-out overlooked on a long wall	Measure pop-out depth + width separately; add to sketch
Double-counting stairwell airspace	Two-story great room measured twice	Measure each level once; airspace above great room is not floor area
Using interior room-by-room sums	Misses hallways, closets, wall thickness	Always measure exterior perimeter — open plan or not

Including the garage

Open-plan homes often flow directly from the great room to the attached garage with minimal visual break. It's easy to accidentally include the garage in an exterior measurement. Always subtract the garage footprint from the total. A clean floor plan makes this obvious; field measurement requires careful noting of where the garage wall begins.

Forgetting the breakfast nook pop-out

Bump-outs that extend the kitchen or dining area are part of GLA. They're easy to miss when measuring a long wall because the bump-out might only add a few feet. On a floor plan they're immediately visible; in the field you may need to measure the pop-out depth and width separately and add it to your sketch.

Double-counting the stairwell

In a two-story open-plan home with a dramatic two-story great room, the stairwell opening is not double-counted. You measure the exterior footprint of each level once. The main floor great room is measured at the main floor level. The upper floor hallway/loft is measured at the upper level. The airspace above the great room is not a floor area, it doesn't add to GLA.

Using interior measurements instead of exterior

Many buyers and agents measure room by room, length times width for each space and add them up. This produces a number that is reliably lower than the ANSI exterior method, often by 3–7% depending on wall construction. For a 2,500 sq ft home, that's 75–175 sq ft of difference. Always use exterior dimensions for GLA.

What open floor plans mean for appraisal comparables

Open floor plans command a premium in most markets, buyers perceive them as larger and more functional even when the square footage is identical to a compartmentalized layout. Appraisers need to be careful not to conflate "feels bigger" with "is bigger." GLA is GLA regardless of how it's divided internally.

When selecting comparables, the open vs. closed floor plan distinction is a quality and condition factor, not a GLA factor. A 2,000 sq ft open-plan home and a 2,000 sq ft traditional-layout home have the same GLA. Any value difference between them is reflected in your quality adjustments, not in the square footage line.

If you're adjusting for floor plan layout (functional utility), make sure you're using paired sales that isolate that variable, same GLA, similar location and age, different floor plan configuration. Market-extracted adjustments for open vs. closed layouts vary widely by price point and market area.

Measuring an open floor plan for a listing vs. for an appraisal

Listing agents sometimes use interior measurement or a quick digital tool that traces interior walls. That's fine for marketing purposes in markets where the MLS doesn't require ANSI compliance. But buyers who rely on listing square footage for an investment decision, and appraisers who are tempted to adopt the listing figure without verification, both run the same risk: the number may not reflect ANSI-compliant GLA.

If you're buying a home and want to verify the square footage before closing, the best approach is a floor plan from a third-party provider (CubiCasa, iGUIDE) or a licensed appraiser's sketch. Either gives you a to-scale exterior-method measurement you can trust.

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Bottom line

Open floor plans don't change how you measure, they change what you see. The ANSI exterior method applies the same way it does to any floor plan. The main hazards are bump-outs that get missed in the field and garage footprints that accidentally get included. A good to-scale floor plan eliminates most of those risks by showing you the full perimeter clearly.

Related Resources

Frequently Asked Questions

Does an open floor plan have more or less square footage than a closed plan?

Open floor plans have the same square footage as equivalent closed plans — the exterior dimensions do not change based on interior wall placement. However, open plans often feel larger because the space flows continuously rather than being divided into separate rooms.

Do interior walls count toward square footage?

Interior walls are included in GLA when measuring from the exterior under ANSI Z765, because the exterior measurement captures the full footprint including all wall thickness. Interior room-by-room measurement would exclude wall material between rooms.

How do appraisers value open floor plans vs. traditional layouts?

Appraisers account for layout appeal through condition, quality, and design ratings, not directly through square footage. An open plan may appeal to more buyers in some markets, reflected in higher sale prices for otherwise comparable homes — but this is a market preference adjustment, not a measurement difference.

Does removing walls increase a home's square footage?

Removing interior walls does not change the home's square footage. GLA is measured from exterior walls, so the total footprint stays the same. However, opening up the floor plan can make the space feel larger and may affect how buyers perceive value — which can influence comparable adjustments.

Does an open floor plan affect a home's appraised value?

Open floor plans can affect appraised value indirectly through market preference. Appraisers use paired sales to determine whether buyers in a specific market pay more for open vs. closed layouts. In many suburban markets, open plans command a premium; in others, buyers prefer defined rooms.

How is an open great room measured for square footage?

An open great room is measured the same way as any other above-grade finished space — from the exterior walls. There is no special treatment for open vs. closed layouts under ANSI Z765. The absence of interior walls does not affect how the footprint is calculated.

Can an open floor plan make a home harder to appraise?

Open floor plans can make comparable selection harder because room counts differ from traditional layouts. A 2,000 sq ft open-plan home may have fewer rooms than a 2,000 sq ft traditional home. Appraisers typically note layout as a qualitative feature and may make a small adjustment if the market supports it.

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