I am constructing an ADU in San Jose, CA, and the code says "If the accessory building or structure exceeds 120 sf and is less than 5 feet from the property line, walls parallel to the property line must be built with a 1-hour fire protection."


  1. Do both the ADU wall and the existing property wall be built with a 1-hour fire protection?
  2. How much will it cost and how long will it take to upgrade an existing wall (built in 1952) to provide it with 1-hour fire protection? The wall is about 20 feet long (single story).
  3. Can we provide fire protection to an existing structure from the outside of the house (without having to change anything inside)?

The other option is to move the ADU wall to provide a separation of 5 feet (I'll lose about 18" from the side wall), but that requires a new foundation and new plans to be submitted. Not sure if this is a route I should explore.

  • 1
    Cost is off-topic - too local, too variable with time. You have a wall ON the property line now, I infer? What's that made of? Note that the code language does not assume anything other than a line (which your neighbors might have built up to however close, however flammable was allowed when they built.) A 1-hour wall is not terribly difficult IIRC. – Ecnerwal Jun 1 at 20:43
  • Thanks! My first time posting and I don't know much about construction. To answer your questions: The wall is on the property line now. The wall has stucco on the outside, and not sure what else it has inside other than studs (I am not even sure how the city inspectors will check to see if it is 1-hour fire protected for an existing wall, or if they'll rely on past permits). The two property walls are between my main dwelling and the ADU. – Data Max Jun 1 at 20:50
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    What property line? Are you subdividing the land parcel to potentially sell the ADU separate from the house? If not, then "property line" means the edge of your property. You can't control the fire protection of the other building since it does not belong to you. – Harper - Reinstate Monica Jun 1 at 21:45
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    I take that language to mean "the exterior walls of the ADU must be 1-hour fire rated if they are parallel to and less than 5' from the property line. For that rating, the wall assembly needs to be 1-hour rated; which for stucco on wood I believe means means min. 7/8" of stucco and 5/8" Type-X wallboard (or other equivalent) on interior. – Jimmy Fix-it Jun 1 at 22:25
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    I found this in the 2019 California Residential Code: codes.iccsafe.org/content/CRC2019P2/…. I also found that both walls need to be 1-hour fire rated, and stucco usually does it. And if the separation is > 3 feet, then I am permitted to have 25% of the wall as openings (windows I presume) – max_max_mir Jun 2 at 6:09

You pay for it in taxes, ask your inspector what he would recommend. If you do that you will be safe and it will be approved.

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    The inspector is not recommending anything. They want that GC or the owner to submit a plan, and they'll either approve or deny. – Data Max Jun 2 at 23:45
  • @DataMax -- can you get us information about the existing and proposed windows in the wall(s) in question? – ThreePhaseEel Jun 3 at 1:29
  • Sorry about that, our inspectors are great people and very helpful. Good Luck, I use to live San Jose and it is a great area. I left about 20 years ago, I missed the seasons. Good Luck – Gil Jun 3 at 2:40

@Data Max.
After some research, I have determined that your question about an ADU wall as posted is not valid and cites a requirement for a different structure.

Your question on an ADS wall, "If the accessory building or structure exceeds 120 sf and is less than 5 feet from the property line, walls parallel to the property line must be built with a 1-hour fire protection." is NOT a code requirement for an ADU, but for an Accessory Building and Structure. This requirement is stated in this Bulletin #250

Bulletin #210 ADU Universal Checklist provides the general guidelines and states a requirement for a 6 ft separation from the ADU and the main structure. There is no mention of a fire-rated wall, in Bulletin #210.

What are you building, a detached Accessory Building, i.e., workshop, garage, shed, playhouse, storage unit, etc. or an Accessory Dwelling Unit (ADU) which has different code requirements?

You should work with your general and check out if your plans are meeting the requirements of Bulletin #210 if this is a ADU.

To compile with the format of this forum. Since your questions were directed to an ADU structure.

  1. The existing main would be grandfathered. Of the existing ADUs that I know that have been build in the Bay Area, no one has yet had to make modifications to upgrade to existing code on their main structure. Your fire-rated questions on the two walls is not a requirement for ADUs

  2. Estimate of cost is off-topics and it would depend on how you would do the upgrade if really required.

  3. You probably could, but not reverent to what you are doing in regards to the main structure.


Fire walls need to be “protected” from fire approaching from either side of the wall. Therefore, both buildings (the original house and the ADU) walls must be protected on both sides of the walls.

When there are two buildings on a lot, a property line is assumed to be exactly between the two buildings. So in order to eliminate the one hour fire requirement, the two buildings must be at least 10’-1” away from each other.

In addition, windows and doors are not allowed in one hour walls or in walls within 10’ of a property line, unless they are protected. So, if you have windows or doors between each building the buildings must be 20’ apart.

  • OP stated that the two buildings are on the same lot, the property line is the footprint of the lot. Windows and doors for a firewall must be fire-rated at the same level as the firewall for the wall to be considered a firewall. I believe you are stating code requirements for building on different properties. This question is specific to an ADU, which by SJ definition, is on the same lot. – Programmer66 Jun 2 at 3:48
  • Yes, a property line can be the perimeter, but for ADU’s there is an implied property line between buildings. Also, windows and doors need to be 20 minute rated for one hour fire walls. Doors are somewhat easy to get the rating, but tougher for windows. The only way I know for a window is to use a giant fire damper. Very $$$ – Lee Sam Jun 2 at 3:55
  • @LeeSam -- fire-rated glazing is a thing, but it's not something you can just up and order from Marvin or Anderson...you'd need to talk to a specialty commercial glazier who can get frames and glazing from TGP, SAFTIFirst, or Vetrotech... – ThreePhaseEel Jun 2 at 3:58

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