Why your floor plan and 3D model disagree on square footage

Conceptual model only. Not a survey or construction document.

Listing square footage, gross living area under ANSI Z765 and a model’s floor area count different boundaries. How to compare them and which to trust.

A floor plan, an MLS listing and a 3D model can report three different square footage numbers for the same home, and all three can be honest: each counts a different boundary. Gross living area under ANSI Z765-2021 is measured to the outside of the exterior walls and only above grade, a condo listing is usually measured to the interior perimeter, and a model built from a drawing reports the floor inside the wall faces it traced. This page is for buyers, sellers, agents and renovators holding two numbers that will not reconcile.

Why do a plan, a listing and a model disagree?

Each figure is the area of a polygon, and what separates them is where somebody drew that polygon’s edge: around the outside face of the exterior wall, along the interior perimeter, or around the painted face of each room added up one room at a time. Every time the edge moves inward, square feet that were being counted a moment ago stop being counted. Nothing has moved in the building. The rule for reading two numbers is the same as the rule for measuring: find out which boundary each one used before you compare them, and never compare two homes on figures drawn different ways.

You can watch the gap open on that drawing. The areas printed inside the rooms add to 64.4 m², which is 693 sq ft: 30.0 m² living (323 sq ft), 10.5 m² kitchen (113 sq ft), 8.7 m² hall (94 sq ft), 8.3 m² study (89 sq ft), 6.9 m² WC (74 sq ft). The dimension lines outside them enclose a larger rectangle, and both chains close on their printed totals, which is the first check to run on any plan: 5,420 plus 3,520 plus 3,060 is 12,000 mm across (39 ft 4 in), and 3,330 plus 2,930 is 6,260 mm down (20 ft 6 in). Those lines are set out to the outside face of the wall, so the rectangle they enclose is 75.1 m², or 808 sq ft. The difference is 115 sq ft, about 14 percent of the larger figure, and it is walls, structure and the floor outside the named rooms, including the stair marked UP 16R that carries no area at all.

What does ANSI Z765-2021 actually measure?

ANSI Z765-2021, Square Footage: Method for Calculating, is the residential measuring standard behind the square footage on a US appraisal. Fannie Mae requires appraisers to follow it, and its Standardizing Property Measuring Guidelines FAQ states the rules a homeowner most often trips over. Measurements are taken to the nearest inch or tenth of a foot and reported to the nearest whole square foot. Staircases count on the floor they descend from. Any space partly or wholly below grade is reported as below-grade area, not as finished above-grade area. The opening over a two-story foyer is not counted twice. Finished space needs a ceiling at least 7 ft high, and under a sloping ceiling at least half the finished floor has to reach 7 ft, with no part of it counted below 5 ft.

Two limits on the standard matter for the plan in your hand. It applies to detached and attached single-family houses, including townhomes, rowhouses and manufactured homes, and the same FAQ says it cannot be used for apartment or multifamily buildings. For a condo or co-op unit inside an apartment building, Fannie Mae asks for interior perimeter measurements instead, which is a smaller boundary than the one used on the house next door. That single sentence explains most arguments about why a 1,200 sq ft condo and a 1,200 sq ft townhouse do not contain the same amount of floor.

Why doesn’t the MLS number match the appraisal?

Because nobody promised it would. The same Fannie Mae FAQ is direct about it: square footage in local MLS systems and assessor records may not have been derived using the ANSI standard, and the appraiser often cannot tell which method produced it. A listing can carry a builder’s brochure figure, a previous appraisal, a tax record from before an addition, or a measurement taken by whoever had a tape that morning. When two sources differ, the useful question is not which is bigger but which boundary and which rule each one used.

• Wall thickness. A partition takes floor from the rooms on both sides of it, and the exterior wall takes its whole thickness out of any interior figure. Multiply the wall thickness by the total run of wall on the floor and you have most of the gap before you look at anything else.
• Below-grade finished space. A finished basement is real floor you use and is reported separately from above-grade area, so a listing that adds the two together describes a different house than the appraisal does.
• Garages, covered porches, three-season rooms and unheated space. These sit inside a gross building area and outside a finished-area figure.
• Ceiling height. A finished attic room under a sloping roof can be excluded in whole or in part by the 7 ft and 5 ft rules while still holding your desk.
• Balconies and terraces on a condo plan. They can be counted in full, counted in part, or left out, which moves the headline number without moving a wall.
• Chases, columns and shafts. A duct chase or a column standing inside the unit is floor you pay for and cannot use. Some plans hatch them; some leave a blank square you have to notice.

How does INK TO BRICK compute a floor area?

Your model is built from the floorplan you upload, and the measuring run traces a boundary polygon around each space it can resolve. The floor area is the sum of those polygons, computed from their own coordinates, so it describes the floor inside the mapped boundaries: an interior figure by construction, not gross living area and not an appraised number. The viewer prints that total in its Model details panel beside the room count, the extent as width by depth, and the model height. Where some spaces have no traced boundary, the workspace does not round the total up to cover them. The label changes to Mapped room area, with the number of rooms it covers, so the figure you read is always the total of the rooms the model actually mapped. The app reports meters; the editor carries a units control with Metric and Feet and inches, and its two-point measure tool reports in whichever you pick without changing the model.

Scale is one source of uncertainty you can help investigate. The optional drawing context accepts original units, a scale denominator as 1:n, and up to 30 named reference measurements in meters. The run interprets scale from the drawing and that evidence; one supplied length does not guarantee a resolved or accurate result. Read the scale status and compare a second independent dimension before relying on an area. For a room-by-room handoff, Plus includes all-row CSV and XLSX schedules. Filtering a schedule to rooms, walls, openings, fixtures, or finishes requires Creator, Project Pass, Studio, or Business. These schedules report the geometry in your floor plan to 3D model. Free covers one public project and does not include the export library, and the plans page sets out which tier carries which format.

NoteA floor area computed from a model is not a legal or appraised area. It is not gross living area under ANSI Z765-2021, it is not the interior area in a condo declaration, and it is not a surveyed figure. Conceptual model only. Not a survey or construction document. Use this model for design exploration. Verify dimensions on site before ordering or building.

Which number should you use?

Do not pick a winner. Write both numbers down with the boundary each was drawn around, and ask the listing agent or the builder which method produced the advertised figure, because that answer changes the price per square foot you are comparing. Use the appraisal figure for lending and for comparable sales, the interior figure for flooring, paint and furniture, and the model’s floor area as an inspectable estimate you can check against the source drawing. For the manual route through a drawing you already have, read how to measure a floor plan PDF. For what an upload does with the same sheet, the PDF floor plan to 3D guide walks the conversion, and measuring a plan sets out what arrives and what does not. If the home is not built yet, buying from a floor plan is the same arithmetic before the walls exist.