How to Measure a Garage Conversion for Square Footage
Garage conversions are one of the most common sources of square footage disputes in real estate. The measurement itself is straightforward: the question is whether the space qualifies as GLA in the first place.
Does the garage conversion count as GLA?
Under ANSI Z765, a garage conversion counts as Gross Living Area only if it meets all of these criteria:
- Finished: Walls, ceiling, and floor are finished to the same standard as the rest of the home (drywall, flooring, not just painted concrete and OSB)
- Permanently heated: Connected to the home's central HVAC or a permanent heating source: not a portable space heater
- Minimum ceiling height: At least 7 feet over at least 50% of the floor area, per ANSI Z765
- Directly accessible: Has a door connecting to the main living area (not a separate entrance only)
- Above grade: The floor must be at or above exterior grade on at least one side
If the conversion is missing any of these: unfinished, no heat, only an exterior door. It does not count as GLA regardless of how much the owner paid for the renovation.
How to measure the space
Assuming the conversion qualifies as GLA, measure it the same way as any other room: from the exterior walls.
- Use a floor plan that shows the conversion. If you have an updated appraisal sketch, CubiCasa export, or permit drawing showing the converted space, use that. If not, measure exterior dimensions from outside the converted walls.
- Include the full exterior footprint. Measure from the outside of the converted walls, just like the rest of the home. Do not deduct for wall thickness.
- Check for ceiling height exclusions. If part of the garage has a lower ceiling (below 7 feet), measure only the area with adequate ceiling height.
- Add to the main perimeter polygon if the conversion is attached and the walls are shared. Draw a separate polygon if the converted garage is a distinct outbuilding.
Using MeasureFloorPlan to measure a garage conversion
In MeasureFloorPlan, you have two options depending on how the floor plan shows the converted space:
Option 1: Include in main polygon: If the converted garage is attached and the floor plan shows it as part of the continuous exterior wall, trace it as part of the main perimeter. The area is automatically included in total GLA.
Option 2: Separate polygon: Use the "Add Separate Area" button to trace the converted garage as its own polygon. This is useful if you want to report it separately or if the ceiling-height exclusion reduces its effective area. MeasureFloorPlan will sum all polygons in the total.
Common mistakes
- Using the original garage dimensions from the assessor: County records often still show the space as a garage. Always verify with a current floor plan or site visit.
- Including the space before permits: An unpermitted conversion may not meet code for ceiling height or heating. Check permit records before including it as GLA.
- Including a partially converted garage: If half the bay is finished and half is still a garage, only the finished half counts, and only if it meets the full GLA criteria above.
- Mixing interior and exterior measurements: Use exterior dimensions consistently throughout the measurement. Switching to interior measurements mid-polygon will undercount wall thickness on two sides.