Using PlanSnapper · 6 min read

How to Measure a Duplex for Appraisal

Duplexes sit in an awkward middle ground between single-family and multi-unit measurement. Whether you measure one unit or the whole building depends on your assignment — and getting it wrong will flag a review. Here is what ANSI Z765 requires and how to handle the most common configurations.

Assignment type drives the measurement approach

Before you start measuring, know what you are appraising. There are two fundamentally different scenarios:

Whole-property 2-4 unit appraisal (FNMA 1025 or similar): You report GLA for each individual unit and typically the total building GLA. This is the most common assignment for investment duplexes. Fannie Mae wants unit-level GLA so it can be compared to comparable rentals in the market.

Single-unit appraisal (e.g., a half-duplex sold as a fee-simple or condominium unit): You measure only the unit being appraised. In this case, your comparables are likely other half-duplexes or attached homes, and unit GLA is what matters.

Do not conflate the two. If you are appraising a half-duplex as a single-family residence, do not report total building GLA on the 1004 form — report only the subject unit's GLA and note the property type clearly.

ANSI Z765 still applies

Regardless of property type, the GLA rules under ANSI Z765-2021 do not change for duplexes:

• Measure from the exterior of each unit
• Above-grade only (below-grade area is reported separately)
• Finished, directly accessible, and conditioned space only
• Garages, carports, and utility rooms do not count as GLA

The shared party wall between units does not affect GLA calculation. You measure to the exterior face of each unit's outer walls. The shared wall is interior to both units — it does not get double-counted or excluded. Each unit's exterior perimeter is measured independently.

Side-by-side duplexes

A side-by-side duplex has two units on the same level, sharing one party wall running front-to-back. This is the simplest configuration. Each unit has its own exterior footprint except for the shared wall.

Measure each unit separately using its exterior dimensions. For Unit A: the front wall, outer side wall, rear wall, and the party wall face. Same for Unit B on the other side. The two measurements should add up to roughly the total building footprint minus any overlapping shared wall thickness (which in practice is negligible for GLA reporting).

If the units are asymmetrical — one is larger than the other — that is fine. Report each unit's GLA individually. Do not average them.

Stacked duplexes (one unit above the other)

Stacked duplexes are more complex because the floor/ceiling between units becomes the grade line for GLA purposes — but it is not the same as below-grade.

The lower unit is typically at or near ground level. As long as it meets the ANSI definition of above-grade (the finished floor surface is at or above grade on at least one side), the lower unit counts as GLA. If the lower unit is partially or fully below grade — think of a garden-level unit that sits 3 feet below the exterior ground line — it may not qualify as above-grade GLA under ANSI Z765.

The upper unit is always above grade. Measure its floor area using the exterior perimeter of the building at that level.

Stairwells shared between units: include the stairwell area in the unit that controls access (or, if shared, apportion it proportionally — but most appraisers include it in the upper unit or note the methodology used).

What about the basement?

Many duplexes have a basement that serves both units — laundry, storage, mechanical systems. Under ANSI Z765, this space is never GLA. It is below-grade finished area if it is finished, but it does not contribute to GLA for either unit regardless of how nice it is.

Report below-grade finished area separately in your adjustment grid. If the property has a finished basement that is exclusively accessed from one unit, note that in your report — it has value, just not as GLA.

Garages attached to one unit

Some duplexes have an attached garage serving one or both units. Garage area is never GLA — it is reported as garage area in its own line. If Unit A has a 2-car garage and Unit B has no garage, that disparity is an adjustment item in your comp grid, not a GLA difference.

How to use PlanSnapper for duplex measurement

If you have floor plans for both units — from a CubiCasa scan, Matterport, or an architect drawing — upload each floor plan separately in PlanSnapper. Trace the exterior perimeter of Unit A and get its GLA, then do the same for Unit B.

For a stacked duplex with a multi-story floor plan that shows both levels, trace each floor's perimeter separately using the polygon tool. PlanSnapper supports multiple polygons in a single session — trace the first floor, then add a second polygon for the upper level. The measurements are kept separate so you can report each unit's GLA independently.

If the floor plan is a single PDF with multiple pages, upload it and PlanSnapper renders the first page. For multi-page plans, measure each page individually by uploading each separately or using the page selector.

Reporting on the FNMA 1025

The Small Residential Income Property appraisal form (1025) has a section for each unit's GLA. Report measured GLA per unit. In the additional comments section, note your measurement methodology and the source of your floor plans (e.g., "GLA measured from CubiCasa export via PlanSnapper, ANSI Z765-2021 compliant methodology").

Fannie Mae guidelines do not prohibit using digital measurement tools on floor plan images, but the appraiser is responsible for the accuracy of the input. If you set the scale reference from a known wall dimension on the floor plan, document that reference wall and its source.

Common mistakes on duplex appraisals

• Reporting total building GLA instead of per-unit GLA on a single-unit assignment
• Including the below-grade garden unit as above-grade GLA when it clearly sits below the exterior grade line
• Double-counting the party wall thickness by measuring both units to the centerline rather than each unit to the exterior face
• Including the basement in either unit's GLA because it is "fully finished and accessible"
• Forgetting to note the duplex configuration (stacked vs. side-by-side) in the property description — reviewers need context to evaluate your comparables
• How to measure square footage of a house
• How to measure square footage for a real estate appraisal
• Laser Measure vs Tape Measure for Floor Plans: Which Is More Accurate?
• GLA vs Total Square Footage: What Is the Difference?