Habitable Attic Calculation

[MKolchins]

I'm an architect trying to determine if an attic with habitable space constitutes a habitable attic. The IRC limits habitable attics to having ceiling heights of 7 ft. or more totaling not more than 1/3 of the area of the floor below; however, it's not clear if the calculation should be based on just the ceilings of the actual habitable spaces under the roof or if it should also include actual non habitable attic spaces (space between bottom of unfinished rafters and attic floor?).

 

[mtlogcabin]

or more totaling not more than 1/3 of the area of the floor below;

I do not see that requirement in the 2018 edition. What code are you using? perhaps a local or state amendment.
2018 IRC
[RB] ATTIC, HABITABLE. A finished or unfinished habitable space within an attic.
[RB] HABITABLE SPACE. A space in a building for living, sleeping, eating or cooking. Bathrooms, toilet rooms, closets, halls, storage or utility spaces and similar areas are not considered habitable spaces.

R325.6 Habitable attic.
A habitable attic shall not be considered a story where complying with all of the following requirements:

1. The occupiable floor area is not less than 70 square feet (17 m2), in accordance with Section R304.

2. The occupiable floor area has a ceiling height in accordance with Section R305.

3. The occupiable space is enclosed by the roof assembly above, knee walls (if applicable) on the sides and the floor-ceiling assembly below.

4. The floor of the occupiable space shall not extend beyond the exterior walls of the floor below.

 

[MKolchins]

I do not see that requirement in the 2018 edition. What code are you using? perhaps a local or state amendment.
2018 IRC
[RB] ATTIC, HABITABLE. A finished or unfinished habitable space within an attic.
[RB] HABITABLE SPACE. A space in a building for living, sleeping, eating or cooking. Bathrooms, toilet rooms, closets, halls, storage or utility spaces and similar areas are not considered habitable spaces.

R325.6 Habitable attic.
A habitable attic shall not be considered a story where complying with all of the following requirements:

1. The occupiable floor area is not less than 70 square feet (17 m2), in accordance with Section R304.

2. The occupiable floor area has a ceiling height in accordance with Section R305.

3. The occupiable space is enclosed by the roof assembly above, knee walls (if applicable) on the sides and the floor-ceiling assembly below.

4. The floor of the occupiable space shall not extend beyond the exterior walls of the floor below.

You are correct it's the New Jersey edition of the 2018 IRC and they did modify the definitions. I contacted the Code Interpretation unit of NJ DCA and was pointed to a subtle difference in the definition of attic vs. habitable attic that clarified the intent that the 7 ft ceiling area restriction only applies to the habitable attics portion of the entire space under roof.