Home Addition Cost in Seattle [2025 Data]

Home addition costs in Seattle average $60,530, and most projects total between $23,975 and $104,563. Prices depend mostly on the type of home addition you want and the size of the add-on you’re building, but there are other factors to consider, too. For example, building out costs a third to a half as much as building a second-story addition. Luckily, the lot sizes in Seattle provide enough space to build out.

Home Addition Cost Factors

The type and size of the home addition you’re building are the most important cost factors to consider, but things like material quality, permits, and site preparation needs can all affect your pricing in Seattle, Washington, too.

Size

Home additions cost between $80 and $250 per square foot if you’re building out and between $300 and $600 per square foot if you’re building up. The cost to add a room that’s 10x10 to a house in Seattle averages about $16,000 if you build out and around $47,000 if you build up.

The average lot size in Seattle is 0.153 acres, which is around 6,700 square feet. This is close to the national average, and it means most residents will have enough space to build out rather than up to keep costs down. Of course, lot sizes vary widely, especially in the more densely populated sections of the city, so you may only have the option of building up.

The price you pay within those ranges depends, in part, on the complexity of the room and its finishes. For example, adding a bathroom will often cost more than a bedroom since it requires additional plumbing and electrical work.

Materials

As you can imagine, it takes many kinds of building materials to complete a custom home addition. Material prices vary widely, so your price could push closer to $104,563 if you choose luxury or high-end materials at each step of the way.

The materials you choose for inside your home addition mostly depend on budget and personal preference, but local home addition costs in Seattle stray from the national average, more so because of the exterior materials. 

When choosing siding and roofing options, it’s a good idea to pick materials that can stand up to frequent rainfall and heavy moisture to avoid rot, mold growth, and shortened lifespans. It’s also wise to pick materials that insulate well and boost home efficiency, which can reduce heating costs through the long, cold winters.

The tables below should give you an idea of how different material choices can affect your pricing. Ask your home addition contractor what materials are best for your project within your budget.

Foundation Materials Cost:

Insulation Materials Cost:

Roofing Materials Cost:

Siding Materials Cost:

Window Frame Materials Cost:

Type

The cost to add on a room varies based on the type of home addition you’re building and the size of the room you’re adding. There are a few different types of home additions, and the ones that involve building out, like sunrooms, bumpouts, and detached garages, will always be more affordable per square foot than second-story additions. The average lot size in Seattle is big enough that most residents can opt to build out and save money if they’re willing to sacrifice some of their yard space.

Prices can also range within a given type of home addition due to the different demands you have for the space. Rooms like kitchens and bathrooms require plumbing work that a bedroom doesn’t, while an unfinished attic home addition may not require electrical or HVAC work that a bedroom needs. The cost per square foot for your home addition will vary based on utilities and the finishes required for how you plan on using the extra square footage.

Site Prep

On average, site prep for a home addition costs between $1,500 and $5,000. You may not need any site prep in some cases, while other home addition projects may require grading land, demolishing concrete patios, removing trees, and other work that can add to your total.

Most homeowners in Seattle have enough yard space to build out, which also saves money on home addition costs. In most cases, this does mean that site prep costs will tend to be higher, especially since the soil in Seattle has a high clay concentration and can take longer to prep than it would in other areas. Still, you’ll spend less than you would on a second-story addition even after accounting for site prep costs.

Permits

All major construction projects in Seattle, including home additions, will require building permits. Permit fees can get expensive, so you should always budget for these separately based on your project. 

The City of Seattle charges permit fees based on construction type, square footage, project cost, and other factors, but the average building permit cost for a home addition in the city, based on the average home addition cost in the area, is $2,223.05. You can use the permit fee calculator on the city’s website to estimate your permit costs.

Home Addition Pro Costs

Hiring a homebuilder in Seattle to construct your home addition is going to be costly, but hiring a pro is necessary to keep your project legal and safe. On average, you’ll pay between $24,200 and $36,300 on labor alone in Seattle, which makes up between 40% and 60% of your total. This is higher than you’d pay in most cities, in large part because the cost of living in Seattle is much higher than the national average.

For the most accurate estimate possible, you should also consider sales tax, which your professional will charge on top of the construction cost. The sales tax rate in Seattle is 10.35%, which is well above the national average. Given the typical cost of a home addition in Seattle, you’re looking at sales tax totaling $6,264.86.

Does a Home Addition Increase Property Value?

Yes, a home addition will increase property value in Seattle. On average, home additions provide a return on investment (ROI) of between 20% and 50%. Given the average cost of building one in Seattle, you’re looking at a value bump of between $12,100 and $30,300. The average price of real estate in Seattle is $562 per square foot, so you can also use this number to estimate value added based on the size of your home addition.

You can maximize your ROI by choosing materials that stand up to local weather. For example, pick insulative siding and roofing materials that can help keep your living space warm through the cold winters, and choose moisture-resistant siding and window frame materials that will last longer with exposure to frequent rain and heavy moisture.

How Angi Gets Its Cost Data

Home is the most important place on earth, which is why Angi has helped more than 150 million homeowners transform their houses into homes they adore. To help homeowners with their next project, Angi provides readers with the most accurate cost data and upholds strict editorial standards. We survey real Angi customers about their project costs to develop the pricing data you see, so you can make the best decisions for you and your home. We pair this data with research from reputable sources, including the U.S. Bureau of Labor Statistics, academic journals, market studies, and interviews with industry experts—all to ensure our prices reflect real-world projects. 

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