Go Far®What additions cost by type
In Snohomish and King County, most additions price out between roughly $300 and $550 per square foot, with complex or high-end projects going beyond that. What that means in practice depends on the kind of space you are adding. A bump-out — pushing a kitchen, dining room, or bath out a few feet on cantilevers or a small foundation — is the entry point, often priced in the tens of thousands. A single-room addition such as a bedroom, family room, or home office typically lands in the low-to-mid six figures once foundation, framing, roofing, and finish work are counted. A primary suite addition with a full bath and closet costs more per square foot than a plain room because of the plumbing and tile involved (see our primary suite addition cost guide). And a second-story addition — building up instead of out — is the largest undertaking of all, since it involves engineering, a new roof structure, and living arrangements during construction, but it can nearly double a home without giving up any yard.
Why additions price differently than remodels
A remodel reworks space that already exists inside your walls. An addition builds new house: excavation and foundation, framing, roofline tie-in, siding, windows, insulation, and all-new mechanical, electrical, and plumbing runs — before a single finish goes in. That building-envelope work is why a 300-square-foot addition can cost more than a full kitchen remodel, and why per-square-foot numbers for additions run higher than people expect. The upside: you are creating square footage that did not exist, which is the one thing a remodel can never do.
What moves the number most
- Foundation and site conditions. Flat lot with easy access is one price; slopes, drainage work, or tight setbacks are another. This is where Puget Sound lots vary most.
- Wet rooms. Every bathroom or laundry hookup inside the addition adds plumbing, venting, and waterproofing cost.
- Up versus out. Ground-floor additions spend money on foundation; second stories spend it on structure and engineering. Which wins depends on your lot and your plans.
- Matching the existing house. Blending rooflines, siding, and flooring so the addition looks original — not bolted on — takes craftsmanship, and it is worth paying for.
- Design and permits. Additions need drawings, structural engineering, and city permits. Around here that process is measured in months, so start it early.
Addition, ADU, or move?
If the new space is for family under your own roof, an addition usually wins. If it could earn rent or house relatives independently, compare it against an attached ADU or a backyard cottage (DADU) — Washington’s ADU-friendly rules make those strong alternatives on many lots. And if you are weighing adding on against buying a bigger house, we ran the numbers in our guide to addition vs. moving in the Puget Sound market — with today’s interest-rate math, improving usually beats moving.
How to budget without regret
Set the total you are comfortable with, then hold 10–15% back as contingency before you commit to finishes. Insist on an itemized, written price so you can compare bids line by line — our guide to choosing a general contractor in Washington covers what to look for. And remember that an addition is priced in decades: it changes what your house is worth and how your family lives in it every day.
Get a real number for your addition
Ranges are for articles — your project deserves an exact figure. Go Far Construction has built additions across Snohomish and King County for over 12 years, and every estimate is free, itemized, and no-obligation. Tell us what space you need, and you will know what it actually costs to build it.