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Staircases in ADUs

When I was designing my ADU with an architect, the stairs presented the most challenging aspect of the whole design.  Stairs and their associated components (guardrails, handrails, ceiling, stair treads) are specified in considerable detail in the building code Portland uses.  Since Portland’s code is likely to be similar to many other places in the country, in this post I’ll talk about the stair issues in ADUs in general.

The problem

Complying with stair codes is sometime difficult in small and stout houses, such as ADUs, because stairs take up a lot of volume and floor space on the entry level.

Portland’s code requires the ADU roof must be no taller than 18ft at its mid-point.   If the roof is pitched like most roofs are, the side of the ADU will necessarily be less than 18ft tall, and in most cases, the sides will be significantly lower than 18ft. Roofs are about one foot thick, effectively dropping the interior ceiling height by one ft; so a 14ft exterior roof means than you may have a interior ceiling which is 13ft high.

Portland’s stair code requires that each stair tread must have 6’ 8” of vertical head room. So, as you ascend your stair case to a floor that is ~9ft off ground level, your top stair treads must still maintain 6’8” of head room. Above the top step, your ceiling must be at least 14’ 8”. If the ceiling must be at least 14’ 8” high and the ceiling is only 13ft, there’s a problem. The rise and run requirements (eg. each step can only be 8” high and must be at least 9” deep) also insure that stairways take up a lot of volume and can’t be very steep.

A diagram of rise and run requirements from the City of Portland

A diagram of rise and run requirements from the City of Portland

Of course this is a generic case I’m considering, so this exact conundrum may not apply to you.  But clearly it’s easy to run out of room, and you may be tempted to use non-code-compliant stairs.  Non compliant stairs still might be permit-able with an appeal or a variance but that can be a hard row to hoe.  Here are some options for both the “compliant” and the “non-compliant” routes:

To Comply…

If you want or need a code compliant stair and have a height constraint, one of the simplest solutions is to build a straight, simple staircase in the middle of the house (presuming this is the line of the highest roof height).  With shorter structures (15ft at their apex), this may be your code compliant option. However, such a stair may use up to 75 valuable sq ft of prime real estate on the first floor, so consider how to use the space under the stairs effectively. See this great post for creative ways to use the space under stairs.

The other main strategy is to place stairs along the side of the house, and turn the top of stairs toward the center of the house before you reach the 2nd floor landing to avoid hitting your head on the ceiling. The turning strategy is usually executed as an L shape (two connected straight runs), but winder stairs are also possible.  It’s more complicated to design and build winder stairs, but it’s less invasive in the first floor layout since only the top stairs turn into the center of the house, cutting into your main floor head room. You can read about my winder stair solution in on my personal ADU blog.  Putting a straight staircase in the center of the 1st and 2nd floor will impact your 1st floor layout.

A diagram of winder stairs

A diagram of winder stairs

Either of these general ideas could be augmented with “headroom additions.”  You might-

  • Raise a portion of the roof using a shed or gable dormer. This will probably add $1-2K to the total project cost. This would be a good addition if the dormer adds other utility to your ADU. For example, perhaps you can expand the dormer in order to add more space to more of the 2nd floor.
  • Add a skylight to the ceiling at the top of the stairs, which would add some natural light to the stairs but would also add headroom. A skylight may add $2-3K to the project.  The biggest Velux skylights are 4’ x 4’, so this approach may only work for a small spot.
  • Alternatively, you might build a multilevel interior to take advantage of the vertical space and vertically shorten the stair runs. I haven’t seen this in a small house, but I imagine that such a design could work well if the space beneath the landings was used well.

Or Not To Comply…

A very common approach for small (and stout) homes such as tiny houses, garage conversions, house boats, tree houses, and mobile homes, is to build non code compliant access to a non code compliant lofted sleeping area. This is typically accomplished via:

1)      A ladder. There are some great, crafty ladder designs out there.
2)      An alternating tread staircase. If you’re trying to build an alternating tread stair, it may be a losing battle to attempt to build them to be code compliant, even if they are “safe” to use. Read this post if you’re considering alternating tread stairs to access a bedroom.

With a non code compliant access to the sleeping area, you will not be able to call the lofted sleeping area a “bedroom”. If it’s a “bedroom”, you need to have code compliant access to it. Instead, the space will have to be identified as “storage” space on your permit drawings. Similarly, you should not include non code compliant access (ie. ladder) on your drawings. Rather, once you’ve passed your final inspection, you can build the access to your “storage” space however you deem appropriate.

Here are some creative staircase design posts that will inspire you to think outside the box in designing your code compliant stairs:

Or your non-code compliant alternating tread staircase:

Feel free to chime in with other ideas if you have experience with code-compliant stairs in small houses.

About Kol Peterson

Kol is an ADU consultant, advocate and author of Backdoor Revolution: The Definitive Guide to ADU Development. Read more here: and learn about building your own at Email at

4 comments on “Staircases in ADUs

  1. Pingback: Ten ADU Aspirations for 2015. Let the countdown begin! | Accessory Dwellings

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  3. Jenny
    January 8, 2016

    If given the option in a new ADU with a second floor entry, would you use an exterior staircase or build an interior staircase with a 1st floor entry door?

    • Kol Peterson
      January 8, 2016

      1st floor, definitely, all things being equal. More comfortable to walk up inside, shake off the wet, not feel cold, etc.

      But, I could imagine all sorts of situations where an outside entry on the 2nd floor is better, if the first floor was being used for other valuable uses (workshop etc).

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This entry was posted on September 27, 2012 by in Design & Build and tagged , , , .
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