Deck Cost Calculator

Why a price per square foot lies to you

Every deck article on the internet gives you a dollar figure per square foot. I understand why. It is the only number that fits in a headline. The problem is that a deck is not a slab, and the square footage is one of the smaller levers on the price.

Here is a real example from my own jobs. I built a low deck with no railing that came out to roughly forty dollars a foot. I built a small hillside deck the same year, on eight foot posts with cable railing on three sides, and it landed at more than twice that per foot. Same contractor, same county, same standards. The difference was not the decking. It was everything holding the decking up.

So if you take one thing from this page, take this: the framing, the height and the railing decide your price far more than the square footage does.

1. Size, and the part of size that actually matters

Square footage sets your material quantity and roughly how many days the job runs, so it does matter. But the shape matters too. A simple rectangle is the cheapest deck per foot you will ever buy, because every board runs full length and every cut is square. Once you add angles, a wraparound, or a second level, the framing underneath gets complicated and the labor climbs faster than the footprint does.

The other size effect people do not expect is that small decks cost more per foot than big ones. The permit, the footings, the mobilization and the railing do not shrink with the deck. Spread across a hundred square feet, those fixed costs land hard. Spread across four hundred, they disappear into the number.

2. Height off the ground

This is the lever most homeowners never see coming, and around here it is often the biggest one. A deck sitting a step off grade needs shallow footings, short posts and frequently no guardrail at all. The same deck eight feet in the air needs deeper footings, tall posts, beams sized for the load, lateral bracing, guardrail the whole way around and a stair run to get down.

Once you pass thirty inches above grade the code requires a guardrail, and that single line in the code book is a real jump in cost. It is worth knowing which side of it your deck falls on before you fall in love with a design.

Ground matters as much as height. On the hillside lots around Kelso and Kalama the footings have to reach something that will hold, and on the river bottom near Woodland the ground stays soft longer than it looks. That is site work you cannot see in the finished deck and cannot skip.

3. Decking material

Pressure treated is the least expensive way to get a deck, and it is a legitimate choice. It needs cleaning and sealing on a cycle, and a board here or there will cup or check over the years. If you are comfortable maintaining it, it works.

Cedar costs more and looks better doing it. It is stable, it smells like a deck should, and it takes a stain beautifully. It still wants to be cleaned and sealed, and if it is not, it will gray. Some people want the gray. Most do not realize it is coming.

Composite, meaning Trex and TimberTech, costs the most up front and gives you back every weekend you would have spent on a deck you already paid for. It does not rot, it does not splinter, and it carries a long manufacturer warranty. TimberTech is my default and costs a bit more than Trex. Both are far more money than wood on day one and frequently less money by year fifteen.

Worth saying plainly: the framing under all four of those is the same pressure treated lumber. Choosing composite does not buy you a better structure. It buys you a surface that does not need you.

4. Railing, which is its own project

Railing surprises people more than any other line. It is priced by the linear foot, not the square foot, and on a small deck the railing can approach the cost of the deck surface itself. A wood railing built on site is the least expensive. Composite railing costs more and matches the decking. Aluminum and cable systems cost the most by a wide margin, and they are what people point at in photos, because they do not block the view.

If you have a view worth protecting, the railing is where that money goes. If you do not, a well built wood rail does the same job for a fraction of it.

5. Stairs

Stairs are priced per step and they are more work than they look. Stringers get cut and hung, treads and risers go on, a landing gets built at the bottom, and a handrail runs the length of the flight. A tall deck needs a long run, and a run past a certain height needs a landing in the middle of it. That is why height and stairs move together on a bid.

What the permit costs, and when you need one

Replacing decking boards like for like generally does not require a permit. A new deck does. Guardrail work on a deck more than thirty inches above grade does, under IRC R312.

Cowlitz County does not charge a flat fee for a permit. It assesses a building permit fee based on the valuation of the work, then adds a plan review fee equal to sixty five percent of that, plus a few smaller add ons. I built a calculator that runs your number through the county's actual adopted fee schedule.

If your property sits inside the city limits of Kalama, Longview, Kelso, Castle Rock or Woodland, that city runs its own schedule and the number will differ. I pull permits in house either way.

Cowlitz County permit cost calculator →

What no calculator can tell you

A ballpark is a starting point, not a bid. The things that move a real number are the things somebody has to stand in your yard to see. Whether the old deck has to come out first. What the rim joist behind your siding is actually made of. Whether a truck can reach the back of the lot. Whether the ground under the footings is what the neighbor's ground was.

I have thirty years of this and I am on the job every day. Electrical goes to licensed trades. The carpentry is mine. Start with the ballpark so you know what range you are in, then I will come out and turn it into a real number in writing.