Costs
What a Home Addition Costs in Olympia in 2026
The short answer
Budget roughly $220 to $420 per square foot for a finished, mid-range addition. That is a June 2026 Washington cost guide's range for Tacoma, the nearest market it publishes. Olympia is not published separately, and Seattle runs $300 to $550.
So a 400 square foot family room with no plumbing lands somewhere around $90,000 to $170,000 here. Add a bathroom or a kitchen and the number climbs fast.
That range already includes basic design and permit fees. It leaves out major site work and custom finishes, and the same guide says to hold back a 15 to 20 percent contingency on top.
Why an addition costs more per square foot than a new house
This is the part that catches everyone, so it is worth getting out of the way first.
Adding 400 square feet to the house you already own usually costs more per square foot than building the same space as part of a whole new house.
It sounds wrong. It is not.
A new house spreads its fixed costs across 2,000 square feet or more. One excavation, one crew mobilization, one roof, one set of connections. An addition pays many of those same fixed costs and spreads them across 400 square feet.
An addition also has to join an existing building, and that work has no equivalent on a new build.
- Cutting into a wall that is currently holding the house up.
- Tying a new roof into an old one so it does not leak in twenty years.
- Matching siding and windows that may no longer be manufactured.
- Bringing the old structure up to current code where the new work touches it.
- Working around a family living in the house while you do it.
None of that appears on a new build, and all of it is skilled work that takes time.
What an addition costs in 2026
These are the June 2026 Washington figures for a finished, mid-range addition, by how you add the space.
| How you add the space | Per sq ft | What drives it |
|---|---|---|
| Build out, single story | $150 to $350 | A new foundation and tying into the roof |
| Build up, second story | $350 to $700 | Reinforcing the structure, stairs, a new roof |
| Over a garage | $200 to $450 | The existing slab helps, but it needs a load check |
| Bump-out, 2 to 4 feet | $85 to $200 | No full foundation, the cheapest space you can add |
What that means in Thurston County
By city, the same guide puts Tacoma at $220 to $420 per square foot and Seattle at $300 to $550. Olympia, Lacey and Tumwater are not published separately, so Tacoma is the closest guide there is.
So read the table for the shape of the cost, which approach is expensive and which is not, rather than as a quote.
Anyone quoting you Seattle numbers for an Olympia job is quoting the wrong market. Anyone quoting the state average for a kitchen with a new roof over it is quoting the wrong project.
Then there is plumbing. The same guide adds roughly $18 to $55 per square foot for it, so a kitchen or bathroom costs far more than a bedroom of the same size. You are paying for water in, waste out, ventilation and the finishes that go with them.
A bedroom or family room with no plumbing is the cheapest square footage you can add to a house.
Out or up: what a second story addition costs
Building out means extending on the ground. Building up means adding a story.
Up looks efficient because you are not giving up any yard. It is usually the more expensive of the two.
| Building out | Building up | |
|---|---|---|
| New foundation | Yes, and it drives the cost | No |
| Existing roof | Tie the new one in | Remove it entirely |
| Structural work below | Little | Walls and footings may need strengthening |
| Living in the house | Usually possible | Often not, once the roof is off |
| Limited by | Lot size, setbacks, septic, critical areas | What the existing structure can carry |
Taking a roof off is the line people underestimate. The house is open to the weather, and this is western Washington. That means temporary cover, a tighter schedule and, frequently, moving out for part of the job.
Going up also tends to trigger a structural engineer, because the walls and footings underneath were designed to carry a roof, not another floor.
The costs people leave out of the budget
The construction number is the one everybody compares. These are the costs that decide whether the project lands on budget.
| Cost | Typical | When it applies |
|---|---|---|
| Design, engineering and permits | About 8 to 12% of the project | Every addition, often inside the headline range |
| Geotechnical report | $1,000 to $3,000 | Slopes or fill, often with a structural engineer on top |
| Electrical panel upgrade | $1,500 to $4,000 | When an older house cannot carry the new load |
| Contingency | 15 to 20% | Every addition, and you will use some of it |
On a $150,000 addition, the contingency alone is $22,500 to $30,000. That is not padding. It is the difference between the number in the brochure and the number you actually pay.
On contingency
A 15 to 20 percent contingency is not pessimism. Opening up an existing wall is the only way to find out what is inside it.
Old wiring, undersized footings, rot where a gutter has been leaking for a decade, a beam someone cut through during an earlier remodel. None of it is visible when the quote is written.
A contractor who tells you there will be no surprises in a 1970s house has either not opened one before or is not telling you the truth.
What the permit costs in Olympia
Nobody can give you an exact permit fee in advance, because it is not a flat price.
Olympia works out building permit and plan review fees from the International Code Council valuation data, under OMC 4.36.010. It then adds a technology fee of 4.1 percent to permit and plan review fees.
The fee scales with the value of the work. So the same 400 square feet costs more to permit as a kitchen than as a bedroom, because the valuation is higher.
The Olympia Permit Center is on 360.753.8314 if you are unsure what to submit.
If your property is in the Olympia urban growth area but outside the city limits, the county permits your project, not the city. That is a different fee schedule, a different technology fee and a different counter. We covered that distinction in the Thurston County ADU guide, and it applies to additions just the same.
What makes a Thurston County site expensive
Two houses of the same age, the same size, the same addition. One comes in at the bottom of the range and one at the top. Usually it is the site, not the building.
- Septic. More bedrooms means more load. If the system was sized for the house as built, an addition with a bedroom in it may need the system enlarged or replaced. This is the single most common reason a Thurston project stalls.
- Critical areas. Wetlands, steep slopes, habitat and geological hazards all carry buffers, and a great many lots here have at least one.
- Shoreline. If you are near water, the Shoreline Master Program applies on top of everything else. In this county that is a lot of lots.
- Access. A crew that can back a truck to the work costs less than one carrying materials around a house on a narrow lot.
- What is already there. A simple rectangular footprint on a flat lot with good access sits at the bottom of every range in this guide.
Find the septic answer before you pay anyone to draw plans. It is the cheapest question you will ever ask and the most expensive one to answer late.
Addition or ADU?
These get compared constantly, and they solve different problems.
| Addition | ADU | |
|---|---|---|
| What it is | More of the same house | A separate, self-contained home |
| Kitchen | Usually not a second one | Required |
| Can it be let out | Not as a separate dwelling | Yes |
| How many allowed | Governed by setbacks and lot coverage | One rural, up to two in a UGA |
| Best for | A growing family, more living space | Family nearby, or rental income |
If you want the space connected to your own, you want an addition. If you want somebody to be able to live independently, or you want rent, you want an ADU, and Thurston County changed those rules in December 2025.
How to compare addition quotes
Three quotes for the same addition can differ by tens of thousands of dollars, and the cheapest is not automatically the one hiding something. Often it is the one that has left work out.
- Check whether design, engineering and permits are inside the number or outside it. This alone explains most of the gap.
- Check the allowances. A quote with a $6,000 window allowance and one with $18,000 are not the same project.
- Ask what happens when the wall is opened and something is wrong. Get the answer in writing.
- Ask who pulls the permit. If the answer is you, that is unpaid work coming your way.
- Ask for the schedule in writing, including who covers the house if the roof comes off.
- Check the license at Washington L&I before you sign anything. It takes a minute and it is public.
The one red flag that matters most
A quote that is a single number on one page is not a quote. It is a guess you cannot check.
You want the work broken into lines you can compare against somebody else. Any contractor who will not do that is asking you to trust a number you have no way to interrogate.
Where to go from here
We build home additions across Olympia, Lacey, Tumwater and the rest of Thurston County, and we handle the permit work as part of the job.
Tell us the address and roughly what you want to add. We will tell you which rules apply to your lot, what the site is likely to demand, and a real number rather than a range.
Common questions
How much does a home addition cost in Olympia?
Budget roughly $220 to $420 per square foot for a finished, mid-range addition. That is a June 2026 Washington cost guide's range for Tacoma, the nearest market it publishes, and it includes basic design and permit fees. Olympia is not published separately, and Seattle runs $300 to $550. A 400 square foot family room with no plumbing lands roughly between $90,000 and $170,000, before a 15 to 20 percent contingency.
Why does an addition cost more per square foot than building a new house?
A new house spreads its fixed costs across the whole building. An addition pays many of the same fixed costs across a few hundred square feet, and it has to join an existing house on top of that. Cutting into a load-bearing wall, tying new roof into old, matching siding and bringing the old structure up to code are all skilled labor with no equivalent on a new build.
Is it cheaper to build out or up?
Building out is usually cheaper. Going up avoids a new foundation but means removing the existing roof, which exposes the house to weather, often forces you to move out, and frequently means strengthening the walls and footings below. It also tends to trigger a structural engineer.
What is the cheapest kind of addition?
A bedroom or family room with no plumbing. Kitchens and bathrooms cost far more per square foot than a bedroom of the same size, because you are paying for water in, waste out, ventilation and the finishes, not for floor area.
Do I need a permit for a home addition in Olympia?
Yes. Olympia calculates building permit and plan review fees from International Code Council valuation data under OMC 4.36.010, then adds a technology fee of 4.1 percent. The fee scales with the value of the work.
How much should I hold back for surprises?
Fifteen to twenty percent. Opening up an existing wall is the only way to find out what is inside it, and old wiring, undersized footings and rot behind a long-leaking gutter are not visible when a quote is written.
Should I build an addition or an ADU?
An addition if you want the space connected to your own house. An ADU if you want somebody to live independently or you want rental income, because an ADU is a self-contained home with its own kitchen. Thurston County changed its ADU rules in December 2025 and now allows up to two per lot inside the urban growth areas.
Why are my three quotes so different?
Usually because they do not cover the same work. Check whether design, engineering and permits sit inside or outside each number, and compare the allowances for windows, flooring and fittings. A quote that is a single figure on one page cannot be compared against anything.
Code requirements on this page were last checked on 7 October 2026. Codes change, so confirm current standards with your city or county before you apply.
Planning a deck in Thurston County?
We handle the permit work as part of the build. Tell us the site and what you want.
Where this applies
We work in every one of these towns, and each has its own page covering the rules that apply there.