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Distance between a house and an ADU??

C

codehelpneeded

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I'm trying to figure out distance between the primary dwelling and an accessory dwelling on a single residential lot. I'm assuming one would just use the FSD of IRC302. If there is 10 feet between the house and detached ADU there would be no required fire rating or opening limitations. If they were 8 feet apart would you use the imaginary lot line? For example the lot line could be 5 feet from the existing house and 3 from the ADU. Therefore only the ADU wall would need a rating. Thoughts?
 
arwat23 said:
Imaginary? Correct me if I'm mistaken, but there either is or isn't a lot line, right? What's the imaginary line for?
Click to expand...

THE IBC uses the concept of an "imaginary lot line" when discussing the fire-resistance rating of exterior walls of two (or more) buildings on the same lot (parcel). Unfortunately, the exact term "imaginary lot line" seems to occur only in the Commentary:

1760139240134.png

However, the definitions include the term "imaginary line" under the definition of Fire Separation Distance. In my corner of the world, we generally refer to the "imaginary lot line" as the "fire separation line."

1760139328614.png
 
Last edited:
Back to the original poster’s question, the imaginary line is for the purposes of treating each dwelling independently for purposes of allowable area calcs. Washington building code allows you the option of treating them as one building:

503.1.2 Buildings on Same Lot​

Diagram
Two or more buildings on the same lot shall be regulated as separate buildings or shall be considered as portions of one building where the building height, number of stories of each building and the aggregate building area of the buildings are within the limitations specified in Sections 504 and 506. The provisions of this code applicable to the aggregate building shall be applicable to each building.

Otherwise, just follow Residential code R302.1(1) or (2) as anpplicable, and don’t worry about imaginary property lines.
 
If an ADU is an "accessory structure" (as seems only reasonable) then I take R302.1 Exception 2 to mean that you don't have to consider the distance between the structures in determining the fire separation distance for exterior walls of either structure. You just consider each structure separately with respect to the lot lines and street centerlines (per the definition of Fire Separation Distance above).

Whereas if you have a residential lot with two main dwelling units on it, no ADUs, then the "imaginary line" portion of the Fire Separation Distance definition comes into play. I infer you can put the imaginary line where you want so that both main dwelling units comply with R302.1.

Cheers, Wayne
 
codehelpneeded said:
I'm trying to figure out distance between the primary dwelling and an accessory dwelling on a single residential lot. I'm assuming one would just use the FSD of IRC302. If there is 10 feet between the house and detached ADU there would be no required fire rating or opening limitations. If they were 8 feet apart would you use the imaginary lot line? For example the lot line could be 5 feet from the existing house and 3 from the ADU. Therefore only the ADU wall would need a rating. Thoughts?
Click to expand...
Interesting question. I don't think it's clear in the code.
Yankee Chronicler said:
THE IBC uses the concept of an "imaginary lot line" when discussing the fire-resistance rating of exterior walls of two (or more) buildings on the same lot (parcel). Unfortunately, the exact term "imaginary lot line" seems to occur only in the Commentary:

View attachment 16786

However, the definitions include the term "imaginary line" under the definition of Fire Separation Distance. In my corner of the world, we generally refer to the "imaginary lot line" as the "fire separation line."

View attachment 16787
Click to expand...
See 705.3
 
wwhitney said:
If an ADU is an "accessory structure" (as seems only reasonable) then I take R302.1 Exception 2 to mean that you don't have to consider the distance between the structures in determining the fire separation distance for exterior walls of either structure. You just consider each structure separately with respect to the lot lines and street centerlines (per the definition of Fire Separation Distance above).

Whereas if you have a residential lot with two main dwelling units on it, no ADUs, then the "imaginary line" portion of the Fire Separation Distance definition comes into play. I infer you can put the imaginary line where you want so that both main dwelling units comply with R302.1.

Cheers, Wayne
Click to expand...

I haven't had to deal with ADUs yet, but based on R302.1 Exception #2, that's the way I understand it. An ADU is an accessory structure to a single-family dwelling, so Exception #2 says we don't have to be concerned with the fire separation and the imaginary lot line.

From the 2021 IRC Commentary:

1760215435653.png
 
wwhitney said:
If an ADU is an "accessory structure" (as seems only reasonable)
Click to expand...
Perhaps in name only. I am inclined to ascribe more import to an ADU than an accessory.

Adjective - Supplementary or Additional:
  • As an adjective, "accessory" describes something that is supplementary or additional to a main thing. For instance, an "accessory building" might be a structure like a garage or shed that is secondary to the main house.
As the definition implies, an accessory is not a replication of the main thing but rather an enhancement to the main thing. If an ADU, which is by definition a dwelling unit, why would another house that does not fall under the constraints of an ADU not be just another accessory.
 
I can only speak from how it is in CA. The term "ADU" comes from the planning and land use side. It's intended to allow more housing on lots that are zoned for a "Single-Family" home. So in from that perspective, it's "Accessory" but that's doesn't mean it's an "Accessory Structure" in the eyes of the building code.

ACCESSORY DWELLING UNIT. [HCD 1 & HCD 2] An
attached or detached residential dwelling unit that provides
complete independent living facilities for one or more persons
and is located on a lot with a proposed or existing primary
residence. Accessory dwelling units shall include
permanent provisions for living, sleeping, eating, cooking
and sanitation on the same parcel as the single-family or
multifamily dwelling is or will be situated. (See Government
Code Section 65852.2)

ACCESSORY STRUCTURE. A structure that is
accessory to and incidental to that of the dwelling(s) and that
is located on the same lot.

1760371462439.png
 
Yankee Chronicler said:
THE IBC uses the concept of an "imaginary lot line" when discussing the fire-resistance rating of exterior walls of two (or more) buildings on the same lot (parcel). Unfortunately, the exact term "imaginary lot line" seems to occur only in the Commentary:

View attachment 16786

However, the definitions include the term "imaginary line" under the definition of Fire Separation Distance. In my corner of the world, we generally refer to the "imaginary lot line" as the "fire separation line."

View attachment 16787
Click to expand...
Thanks for adding this. I should have done the leg work and posted it.
 
The AHJ might have their own requirements. We just adopted an ADU ordinance and took the fire chief's advice of min 20ft and max 100 ft from primary dwelling.
 
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