What are the Guidelines for Modeling an ADU (Accessory Dwelling Unit)?

When modeling an ADU, there are several circumstances to consider.

In the case of an attached Garage conversion, this is considered an Addition, and the floor space of the Garage is input as New at the Zone (no it is not an Alteration).  See our example file “Existing+Alterations+Addition.bld” for an example.  Note in this case, the code does allow for the “Existing Wall w/Siding” exception to be used, listed under the wall input.

In the case where they are adding square footage to an existing home, it is treated the same as the Garage conversion (New at Zone), and you may be able to claim the “Wall Extension” exception, also given under the wall input.

In the case of a renovation of an existing building on the property (garage, shed), this is also an Addition (New at Zone) as described above, and also qualifies for the “Existing Wall w/Siding” provided the siding remains.  Also, any type of Addition to this same structure is still treated as an Addition to the main house.  You can decide to treat this as Addition Alone or E+A+A.  Existing Floor area, where required, should be for the main house.

In the case of a New building on the site, this is now input as New at both the Zone and the Top level of the Tree, and the ADU designation at the Zone has no relevance.  It is now treated as a new home, and will be documented as such, and the output will not indicate ADU.  This circumstance will also require a PV system be installed, either on the main home or the ADU.  Note an expansion of the existing PV system is an acceptable way to meet this requirement, but you cannot count the existing PV system as having met the requirement.

In ALL cases, an ADU will require an IAQ fan, verified in the field by a HERS rater.


Categories: 4. Lowrise Residential Title 24

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