Using PlanSnapper · 5 min read

Does an ADU Count as GLA?

Accessory dwelling units have become one of the hottest topics in residential real estate — and one of the trickier measurement questions for appraisers. The short answer: a detached ADU almost never counts as GLA for the main house. An attached one might. Here is how to think through it.

What is an ADU?

An accessory dwelling unit is a secondary living space on a single-family lot. It can be detached (a backyard cottage, converted garage, or prefab studio) or attached (a converted basement apartment, above-garage unit, or addition with a separate entrance).

ADUs typically have their own kitchen, bathroom, and sleeping area — making them self-contained. That self-contained nature is actually part of what disqualifies them from being included in the main home's GLA.

Why detached ADUs do not count as GLA

ANSI Z765 defines GLA as the finished, above-grade, heated living area of a single structure. A detached ADU is a separate structure. It has its own foundation, walls, and roof. Even if it is connected to the main house by a covered walkway, it is not part of the main building and cannot be included in the main home's GLA.

Appraisers report the ADU separately — typically as an outbuilding with a contribution to overall value rather than as part of the GLA count. On a standard URAR form, the ADU square footage would appear in the "other" section of the improvement description, not in the GLA line.

Attached ADUs: it depends

An attached ADU — one that shares a structural wall with the main house — is more nuanced. To count as GLA, the space must meet all of the following:

• Above grade (at or above ground level on all sides)
• Finished living space (permanent flooring, insulated walls, ceiling)
• Heated and cooled to a similar standard as the main home
• Directly accessible from the main living area

That last requirement — direct interior access — is the sticking point for most ADUs. If the unit has a separate exterior entrance only, it functions as a rental unit, not as part of the main living area. In that case, most appraisers would not include it in GLA, even if it is attached.

Local appraiser practice varies. In markets where ADUs are common, there may be established conventions for how to handle them. When in doubt, flag it in your comments and explain your methodology.

How ADUs affect market value

Even when an ADU does not count as GLA, it almost always contributes to value. A well-built detached ADU on a suburban lot can add $100,000 or more in some markets — it is a rentable income stream and an attractive feature for multigenerational buyers.

Appraisers typically find comparable sales that also have ADUs, or apply a line adjustment on the grid for the additional unit. The income approach may also be relevant if the ADU is rented.

Garage conversions and JADUs

A junior accessory dwelling unit (JADU) is a conversion of an existing space — often a garage or large bedroom — into a self-contained unit within the main structure. Because it is within the existing footprint of the house, the square footage was likely already counted in the GLA.

When a garage is converted to a JADU, that space moves from "garage" to "finished living area." If the conversion is permitted and meets quality standards, those square feet can be reclassified and included in GLA. Unpermitted conversions are trickier — lenders often require permits before accepting converted space as GLA.

Measuring an ADU in PlanSnapper

If you need to calculate the square footage of an ADU — for reporting purposes or to provide the owner a reference — you can upload any floor plan of the unit to PlanSnapper and trace its exterior footprint. This works the same way as measuring the main house: upload the plan, set the scale, and trace.

You can measure the main house and the ADU in separate PlanSnapper sessions, or use multiple polygons in the same session if you prefer to keep them together. Either way, the resulting numbers are reported separately in your appraisal.