How Much Does a Sunroom Cost in Seattle & the Eastside? 2026 Price Guide

Three-season vs. four-season: the price fork

The first decision sets most of the budget. A three-season room — lighter framing, simpler glass, no heat — built over an existing patio or deck typically runs $80–$200 per square foot, or $130–$300 per square foot as a new structure, which is why most land in the $25,000–$50,000 range. A four-season room is a true addition: in 2026, custom four-season construction runs roughly $200–$400 per square foot installed nationally, and Seattle-area projects usually price in the upper half of that range. A 200 sq ft four-season room commonly lands between $50,000 and $90,000 here, and larger rooms with premium glass and finishes can pass $120,000. If you mainly want covered outdoor living rather than conditioned space, a patio cover or outdoor living space delivers that for far less.

What actually drives the cost

Four line items explain most of the spread. Foundation: a four-season room needs a code-compliant, insulated foundation ($500–$6,000+ depending on site and size). Glass: glazing can make up the majority of the wall area, and stepping from double-pane to triple-pane low-E windows moves the price meaningfully — but it is what keeps the room warm in January. Heating and cooling: a ductless mini-split heat pump, the usual choice here, runs about $3,000–$7,000 installed and comfortably serves rooms up to roughly 400 sq ft; tying into existing ductwork can cost anywhere from $2,300 to $20,000 depending on your system. Electrical: new circuits, lighting and outlets are part of every conditioned room — see our plumbing and electrical page for how we handle that work in-house.

Permits and the energy code

Every city in King County treats a sunroom as an addition, so plan on a building permit plus electrical and mechanical permits when the room is wired and heated. Because a four-season room is conditioned living space, its insulation, glazing and heating must meet the Washington State Energy Code — this is the single most common reason DIY and bargain sunroom quotes fall apart at plan review. Our guides to construction permits in Washington and Seattle vs. Eastside permit timelines cover what to expect city by city; permit costs themselves typically run a few hundred to $1,500.

Sunroom, patio cover or full addition?

A sunroom is usually the middle path. A patio cover gives you dry outdoor space for a fraction of the cost; a conventional home addition gives you fully integrated square footage but costs more — see our 2026 home addition cost guide for that comparison. A four-season sunroom splits the difference: real year-round living space, more glass and light than a standard addition, and a faster build. Timing also matters less than people think — winter is often a smart time to start, since design and permitting happen indoors.

Budgeting your project

Get the glass, heating and foundation decisions priced together, not as allowances. Blue Lake Construction designs and builds sunrooms and four-season rooms across Seattle, Bellevue and the Eastside on fixed-price, fixed-duration contracts that cover design, permits and construction. Tell us about your project and we will give you a fixed, itemized price before any work starts — or call (206) 786-5988.