Customer Service: (253) 677-0290
Customer Service: (253) 677-0290
By Daniel Rotaru Owner and CEO at Decks Restore.
Most of the ADU deck calls I get in Federal Way start the same way. Somebody’s mom is moving in, or an adult kid is coming back from out of state, and the family wants the backyard cottage to feel like a real home, not a granny flat with a 36-inch concrete step out the back door. Then they ask if I can “just throw a small deck on it.” And the conversation gets longer than they expected.
I want to walk through what I actually tell those homeowners, because the ADU rules in Federal Way changed in 2023, the state added more changes in 2024 under HB 1337, and the accessibility piece keeps tripping people up. I’m pulling from jobs we’ve built off 320th, up by Twin Lakes, and a couple over in the Mirror Lake area where the lots slope hard.
You can build a detached ADU up to 1,000 square feet here. You can also build an attached one. The 2023 amendments lowered some of the old barriers, and the state law that followed in 2024 made it so the city has to allow two ADUs per residential lot inside the urban growth area. That part most homeowners already know by the time they call me.
What they don’t know is how much the deck affects the permit timeline.
If your ADU sits more than 30 inches off finished grade, which is common on the sloped lots in the Twin Lakes area, the deck stops being a casual add-on. It becomes part of the egress path. The building department wants to see it on the plans before they issue the permit for the ADU itself. I watched two clients last year delay their ADU framing inspection by six and seven weeks because their deck drawings got submitted as a “later phase.” Submit the deck with the ADU. Same packet. It saves you the second review fee and it saves you the wait.
The ADA technically doesn’t apply to your private residence. It applies to public accommodations. So if your mother-in-law is moving in and uses a wheelchair, you are not legally required to meet ADA standards.
You should still build to them anyway.
Part of the reason is the resale story. Federal Way’s buyer pool over the last three years has skewed older. We’ve been getting a lot of multi-generational households buying in the Lake Geneva and Steel Lake neighborhoods. An accessible ADU deck is the kind of detail a 60-something buyer notices when they’re touring with their realtor. It pushes price.
The other reason is that family situations change. I’ve had two clients in the last 18 months who built standard 4-inch step decks for a parent who could still walk, and within two years we were back tearing it out and putting in a ramp because mobility declined. Building it right the first time saves about $6,400 in our cost data. Tear-out and refit is more expensive than just doing the slope correctly to start.
The slope rule is the part most carpenters get wrong. The federal standard the U.S. Access Board publishes for accessible ramps is 1:12 maximum. That means 12 inches of run for every 1 inch of rise. If your ADU sits 24 inches off grade, you need 24 feet of ramp, plus landings at the top and bottom that are 60 inches long. People hear “24 feet” and start trying to negotiate. Don’t. A steeper ramp in Federal Way rain is a hospital trip.
I pulled our last six projects that included accessibility features. Here’s what they came in at, not what a calculator on some national site would tell you.
A standard 12×16 ADU deck without accessibility features ran us between $11,800 and $14,200 in Federal Way last year. Pressure-treated framing, composite decking, simple rail. The same deck with a wheelchair-accessible ramp added about $4,100 to $5,800 depending on how long the ramp needed to be and whether we could integrate it into the deck footprint or had to extend it out into the yard. The longer ones on sloped lots cost more because of the additional concrete footings and the structural posts.
The biggest hidden cost is the landing requirement. Two landings, each at least 60×60 inches, with a turning radius of 60 inches at the bottom if the ramp changes direction. On a small Federal Way lot, that turning radius is sometimes the thing that forces a redesign of the whole back yard layout. I had to move a propane tank, two raised beds, and an irrigation manifold on one job in Redondo just to make the math work.
Handrails are another line item people forget. Anything with a rise over 6 inches needs them on both sides. Code says 34 to 38 inches above the ramp surface, with a 1.25 to 2 inch grip diameter. We use cedar with a routed top because it’s easier on the hands than the steel powder-coated rails you see in commercial settings, and the inspectors in Federal Way have never given us trouble on it.
I stopped using standard pressure-treated decking for accessibility ramps in Federal Way a few years back.
The big reason is moisture. PT pine gets slick when it’s wet, and we get rain about 156 days a year. Slick ramp plus wheelchair plus 1:12 slope is a problem waiting to happen. We switched to composite with a textured surface around four years ago. The coefficient of friction is roughly double what wet cedar gives you.
There’s also a structural span issue I learned the hard way. A wheelchair plus an adult passenger can hit 450 pounds of point load. Our standard residential deck math assumes 40 pounds per square foot live load and the joists get spaced at 16 inches on center. For ramps that will see wheelchair use, I tighten that to 12 inches on center, and I upsize the joists from 2×8 to 2×10 if the span exceeds 8 feet. It costs maybe $180 in extra lumber. It also means the ramp doesn’t flex underfoot, which matters when the person using it isn’t steady.
For posts, I use 6×6 instead of 4×4 anywhere a handrail is anchored. The lateral load on a handrail when someone catches themselves is significant, and a 4×4 with a single bolt has failed for me before on an older repair job in Tacoma. I won’t build that way anymore.
A few things I’ve learned about the permit office in Federal Way that aren’t in the handout.
Submit your ADU and deck plans together, as one packet, even though the city will let you do them separately. The reviewer flips between drawings. If they have to wait two months for the deck package, they re-review the ADU site plan, and that’s wasted time on both ends.
Include a topo sketch even on flat lots. Federal Way’s building department has been asking for finished grade elevations on more permits this year, especially after some of the lot-line drainage complaints that came in during 2024. A simple sketch with elevations at the four corners of the deck saves you a request-for-information loop.
The accessibility build is not a separate review in Federal Way for private residences. It goes through the regular building permit. If you’re doing this as a fully ADA-compliant build because you plan to rent the ADU through an agency that requires it, that’s a different conversation, and you’d want to talk to the city’s planning division before we draw anything.
If I were building an ADU on my own lot with the long-term plan of having a parent live in it, here’s roughly how I’d sequence it.
Build the ramp into the design before the ADU foundation gets poured. The cheapest accessible ramp is the one where the slab elevation was set with the ramp in mind. Clients have saved $2,000 to $3,000 just by lowering the ADU finished floor by 8 inches at the planning stage, which shortened the ramp by 8 feet.
Use a 48-inch wide ramp, not 36. Code says 36 minimum between handrails. 48 gives you room for a caregiver to walk beside the chair. The cost difference is around $400 to $600 in our work. The quality of life difference is bigger than that.
Put the ramp on the side of the deck that gets afternoon sun. Federal Way mornings stay damp until 10 or 11 most of the year. A ramp that catches the afternoon sun dries faster, stays cleaner, and lasts longer. I had a client in 2023 who insisted on the north-facing side because of the view from the kitchen, and that ramp grew moss inside of 14 months. We had to pressure-wash and re-treat it.
Build a covered landing at the top. The ADU door area should be at least partially sheltered. A 4-foot eave overhang or a small acrylic patio cover changes how the space actually gets used. The new deck builds we do often include covered entries for this reason, especially over ADU thresholds.
If someone calls me and says they’re thinking about an ADU and a deck and someone in the family uses a wheelchair, I usually drive out and look at the lot before I quote anything. The slope, the existing drainage, the location of the ADU on the site, and the route from the driveway to the ADU door all change the math by thousands of dollars. A 20-minute site visit saves both of us from a quote that doesn’t match reality.
The accessibility piece on an ADU deck is not the expensive part. The expensive part is doing it twice.
If you’re in the planning stage and want to talk through what your specific lot would need, you can reach us through our contact page, or take a look at how we approach custom builds for the broader Tacoma area we serve.