Costs

How Much Does It Cost to Add a Room? Room by Room (2026)

What decides the price

Two rooms of the same size can be priced very differently. Three things decide it.

• Plumbing. Bathrooms and kitchens need water, drains and venting. The guide adds roughly $18 to $55 per square foot for plumbing alone.
• Out or up. Building out on the ground costs far less than adding a second story.
• The house you start with. Older homes often need a bigger electrical panel or turn up surprises once walls are opened.

Cost to add a room, by room

Figures are from homeadditioncostcalculator.com's Washington guide, last reviewed June 2026. They cover a finished, attached addition with basic design and permits, and leave out major site work and custom finishes.

1. Adding a bedroom

A bedroom is the cheapest room to add, because it needs only standard wiring, heating and flooring. The guide puts a typical 150 to 200 square foot bedroom at about $30,000 to $70,000 in Washington.

On septic, check one thing first. A new bedroom can mean the system has to be enlarged.

2. Adding a bathroom

Bathrooms are among the costliest rooms per square foot, loaded with plumbing, venting and waterproofing. The guide puts a new bathroom at $30,000 to $80,000, depending on size and fixtures.

The biggest saving is location. Put the new bathroom next to existing pipes, back to back with another bathroom or the kitchen, and you cut one of the largest line items.

3. Adding a family room

A family room costs about the same per foot as a bedroom, $150 to $300, because it is also a dry room. It is usually bigger, though, so the total climbs.

Most family room additions go off the back of the house, next to the kitchen. That needs space on the lot, so check your setbacks and lot coverage early.

4. Adding a sunroom

A sunroom runs $150 to $350 per square foot. The low end is a three-season room. The price rises fast once you insulate and heat it for year-round use, which is what most people want in our cool, wet winters.

5. Extending the kitchen

A kitchen extension is the most expensive room to add, at $200 to $500 per square foot. Plumbing, gas lines, ventilation and cabinets all add up.

A small bump-out is often enough to fit an island or a bigger pantry, and the guide puts bump-outs at just $85 to $200 per square foot.

Out, up or over the garage

Going up costs the most because the roof comes off, the structure below often needs strengthening, and you need stairs.

Costs people forget

• Design, engineering and permits, about 8 to 12 percent of the project.
• An electrical panel upgrade, often $1,500 to $4,000 in an older house.
• A geotechnical report, $1,000 to $3,000 on slopes or fill.
• Asbestos or lead testing in homes built before 1980.
• A contingency of 15 to 20 percent, because opening walls always turns up something.

Around Tacoma and Olympia

The guide puts Tacoma at $220 to $420 per square foot, below Seattle and the Eastside. It does not list Olympia separately, so Tacoma is the closest published benchmark for the South Sound.

For a deeper look at local pricing, see what a home addition costs in Olympia.

Cheaper ways to add a room

• A bump-out for a cramped kitchen or bathroom.
• Convert space you already have, such as a garage, attic or basement. The foundation and roof are already there.
• Keep plumbing rooms close to existing pipes.
• Keep the shape simple. A plain rectangle with one roof line costs less than corners and angles.

More options are in our home addition ideas.

Does adding a room pay you back?

Partly. National 2025 cost-versus-value figures quoted by the same guide put a midrange bathroom addition at about 53 percent of its cost recovered at sale, and a midrange primary suite at about 33 percent. A garage conversion recovers the most, at around 80 percent.

Most people add a room because they need the space and want to stay put, not to sell. That is usually the right reason.

Where to go from here

We design and build home additions across Thurston and Pierce counties, from single bedrooms and bathrooms to family rooms and sunrooms, with the permit handled as part of the job.

Tell us which room you need and roughly how big. We will tell you the best way to add it to your house.

Code requirements on this page were last checked on 21 October 2026. Codes change, so confirm current standards with your city or county before you apply.