Customer Service: (253) 677-0290
Customer Service: (253) 677-0290
By Daniel Rotaru, Owner and CEO at Decks Restore. WA Contractor License # CC DECKSRL797P2. Tacoma, WA. Updated September 2026.
The lot is flat where the house sits. You walk around the side, and the back yard falls away. Somewhere past the fence line there is a view of the water, or the Olympics on a clear day, and that view is the entire reason anybody wants a deck back there.
I get called out to these lots constantly, up above Ruston Way, off North 30th, up around Proctor, and the conversation almost never starts where the homeowner expects. They want to talk about board colors. I want to know what is holding the hill up, whether the city has that slope mapped, and where a concrete truck can physically park.
None of that is meant to scare anyone off. We build on these lots regularly and they produce the best decks in the city. But a slope deck is a different animal from a flat lot deck, and the differences show up in the budget long before they show up in the design.
Before I draw anything, I check whether the property sits in a mapped geologically hazardous area.
Tacoma regulates these under Chapter 13.11 of the municipal code, Critical Areas Preservation, which covers geologically hazardous areas along with wetlands, flood hazard areas, and a few other categories. The city’s mapping for erosion and landslide hazard areas was built from bare earth lidar and sorts the terrain into slope bands, so a lot of the North End that looks like an ordinary back yard is already classified.
A mapped hazard area does not mean you cannot build. I want to be clear about that, because homeowners hear “critical area” and assume the answer is no. What it usually means is that your permit needs documentation it would not otherwise need, most often a geotechnical report from a licensed engineer, and that review takes longer.
What I tell people: find this out in week one, not in week six after you have paid for a design. A geotech report costs real money and takes real time, and if it is coming, it should be the first thing in motion, not the last.
On a flat lot, “setback” means one thing. On a North End slope it can mean three, and they stack.
There is the zoning setback from your property lines, the ordinary one. There is a critical area buffer, which is measured from the top or the toe of the slope rather than from your lot line and has nothing to do with where your fence is. And then there is the structural setback, which is how far a footing has to sit back from the face of the slope so the soil under it is actually supporting it rather than sliding out from under it.
That last one catches people. The building code treats a footing near the top of a descending slope differently from one in the middle of a flat yard, because the load has to spread down into soil that stays put. In practice the footing either moves inland or it goes deeper. Usually deeper.
So a homeowner asks why the deck cannot come out another four feet toward the view, and the honest answer is that it can, but the posts holding it up have to start somewhere, and that somewhere just moved down the hill. Which brings us to the real cost driver.
Frost is not the reason we dig deep here. Our frost depth is shallow compared to most of the country. On a slope, depth is driven by two things: reaching soil with the bearing capacity the engineer specified, and clearing the slope setback.
The North End sits largely on glacial deposits, and what you find at three feet down can be entirely different from what you find at three feet down twenty feet away. I have dug two holes on the same lot and hit dense till in one and loose fill in the other, because somebody regraded that yard in 1974 and nobody wrote it down.
Every extra foot of depth costs money three times over. More excavation, more concrete, and taller posts above grade, which then need bigger lumber and bracing. A new deck of identical square footage can run substantially more on a North End slope than on a flat lot in South Tacoma, and almost none of that difference is in the part you can see.
This is the one nobody plans for.
Concrete has to get from the truck to the hole. If the truck can back down a driveway and the chute reaches, fine. If the hole is eighty feet behind the house and down a bank, you are looking at a pump truck, a long line of wheelbarrows, or a different foundation system entirely.
Same problem with materials. Twenty foot joists have to physically get to the back of the lot. On some of these properties the only route is through the house or over a neighbor’s yard with permission.
I have walked lots where the deck design was straightforward and the access was the entire project. When I give a number on a slope lot, a real share of it is labor moving things around, and I would rather say that out loud than bury it.
If the truck cannot reach, or the ground is saturated, or the slope setback is pushing footings uncomfortably deep, helical piles start to look reasonable.
They go in with a machine that fits through a gate. They screw down until they hit a torque value that corresponds to a load capacity, which means you get engineered confirmation of what each pile will hold rather than an assumption. There is no cure time. And on a hillside you are not excavating a big hole in the very soil you are trying not to disturb.
They cost more per point than a poured footing. On a steep North End lot they can still come out cheaper overall once you count the pump truck, the excavation, and the days.
Half the slope decks I look at involve a retaining wall somewhere, either one that exists and is failing, or one the design needs.
The rule of thumb most people have heard is that a wall over four feet needs a permit and engineering, measured from the bottom of the footing rather than from the ground you see. That part is roughly right. The part people miss is the word surcharge.
A wall shorter than four feet still needs engineering if something is loading the soil behind it. Deck posts sitting behind the wall are a surcharge. A driveway is a surcharge. Another wall above it is a surcharge. I have seen a homeowner stack a three foot block wall themselves, call it landscaping, and then set deck posts two feet behind it. That wall is now structural and nobody engineered it.
Terraced walls have the same trap. Two four foot walls close together do not behave like two independent short walls. If they are within a certain distance of each other, they act as one taller wall, and the lower one is carrying the upper one’s load.
And what actually kills retaining walls here is not the block or the height. It is water. Every failed wall I have taken out on the North End failed because the drainage behind it was never installed, or was installed and then clogged, and hydrostatic pressure did the rest over about a decade.
If you take one point from this, take this one.
Slopes fail because of water. Not usually because of the deck. The deck is often the thing that gets blamed because it is the newest object on the lot.
So on a slope build I care about where the roof downspouts discharge, because a downspout dumping at the top of a bank is doing more damage per year than anything else on the property. I care about where deck runoff goes. If we are installing a ceiling system underneath, I care a lot about where it discharges, which I got into in under-deck drainage in Tacoma.
Tightlining downspouts to a proper discharge point is not glamorous and it does not photograph well. It is also the single best money you can spend on a hillside property, and I bring it up on almost every North End estimate whether or not it is part of my scope.
An elevated slope deck ten or twelve feet off the ground is structurally a different building than a two foot deck.
Post size goes up. Lateral bracing becomes mandatory rather than optional. You need diagonal or knee bracing in both directions, and the connections at the top and bottom of each post matter as much as the posts themselves. On the taller ones we are often into engineered connections rather than anything prescriptive.
The homeowner sees the same deck surface either way. The frame underneath is roughly twice the work.
One design note while you are up there: on a view lot, spend the money on the railing. You are going to be looking through it every day, and a standard picket rail will cut the view you paid for. I compared the long term maintenance of the options in cable versus glass versus picket railing in Tacoma rain.
Some of the North End falls inside a designated historic district, and that adds a design review layer on top of the building permit. It generally applies to what is visible from the street, so a deck at the back of the house is often less affected than people fear. But it is another approval with its own calendar, and finding out about it early is much better than finding out about it late.
Do I need a geotechnical report for a deck on a hillside? Often, if the property falls within a mapped geologically hazardous area. It depends on the slope classification and the scope of the work. Check the city’s critical areas mapping for your parcel before you commit to a design.
Can I build a deck over an existing retaining wall? Sometimes, but not by assumption. The wall has to be evaluated for the added load, and most residential walls were never designed to carry deck posts behind them.
How much more does a slope deck cost than a flat lot deck? The visible part costs the same. The foundation, bracing, and site access are where the difference lands. On a steep lot with poor access, the substructure can be a large share of the total rather than a small one.
Will a deck make my slope less stable? A properly engineered deck should not. What destabilizes slopes here is uncontrolled water and unplanned excavation. That is why drainage comes up on every one of these jobs.
How long does permitting take on a critical area lot? Longer than a standard deck permit, because of the added review and any required geotechnical documentation. I walked through the general process in permits and regulations for deck construction in Tacoma.
Decks Restore is a custom deck builder serving Tacoma and Federal Way, and hillside work is a large share of what we do in the North End, Old Town, Proctor, Stadium District, and out toward Browns Point and Point Defiance. If your existing deck is on posts and you are not sure what condition it is in, start with our seven point inspection checklist, then have someone look underneath it.
For a new build, request a free quote and I will come out, walk the slope, and tell you what the foundation is likely to require before you spend anything on design. You can also see finished hillside projects in our gallery.
Decks Restore Tacoma office: 748 Market St #173, Tacoma, WA 98402 Federal Way office: 33530 1st Way S Suite 102, Federal Way, WA 98003 Phone: (253) 677-0290 Hours: Monday to Friday 9:00 AM to 5:00 PM, Saturday 9:00 AM to 2:00 PM, closed Sunday WA Contractor License # CC DECKSRL797P2 Serving Tacoma, Federal Way, Gig Harbor, Puyallup, Kent, Auburn, Covington, Bonney Lake, and the surrounding South Sound.
Comparing companies for a hillside project? Our page for deck builders in Tacoma covers process, pricing, and recent local work.