How Much Does a Home Addition Cost in King County WA?

Adding space to your home is one of the biggest investments you can make — and one of the smartest. Whether you need a new bedroom, expanded kitchen, or a detached ADU, understanding costs upfront helps you plan with confidence.

What Drives the Cost of a Home Addition?

Several factors affect pricing in King County:

• **Size and complexity:** A simple bump-out costs far less than a second-story addition
• **Foundation work:** Slab vs. crawlspace vs. full basement each carry different costs
• **Plumbing and electrical:** Adding a bathroom or kitchen to your addition increases scope significantly
• **Permits and engineering:** King County requires structural engineering plans for most additions
• **Finishes:** Builder-grade vs. custom materials can swing the budget by 30% or more

Typical Cost Ranges in South King County

Here are approximate ranges for 2026:

• **Room addition (200-400 sq ft):** $40,000–$80,000
• **Primary suite addition:** $60,000–$120,000
• **Second-story addition:** $150,000–$300,000+
• **Sunroom or four-season room:** $25,000–$60,000
• **Detached ADU (400-800 sq ft):** $100,000–$250,000

These ranges include permits, engineering, construction, and standard finishes. Premium materials and complex site conditions push costs higher.

Do I Need Permits?

Yes. Every home addition in King County requires a building permit. Most also need:

• Structural engineering plans
• Energy code compliance documentation
• Setback and lot coverage review
• Possibly a SEPA (State Environmental Policy Act) review

A. Sanders Construction handles the entire permit process, saving you time and ensuring compliance.

ADUs: A Growing Opportunity

Washington state has expanded ADU (Accessory Dwelling Unit) allowances significantly. Many homeowners in Maple Valley, Renton, and Covington are adding detached ADUs for rental income, multigenerational living, or home offices. We evaluate your property's zoning and utility connections during a free consultation.

How Long Does a Home Addition Take?

Most additions take 3 to 6 months from permit approval to completion. The permit process itself can add 4 to 8 weeks. We coordinate the full timeline so you know what to expect before breaking ground.

Tips to Maximize Your Investment

• **Match the existing architecture:** Additions that blend seamlessly with your home's style protect resale value
• **Invest in insulation and windows:** Energy efficiency pays dividends in the Pacific Northwest climate
• **Plan plumbing runs efficiently:** Locating new bathrooms near existing plumbing saves thousands
• **Consider future needs:** Building slightly larger now is far cheaper than expanding again later

Get a Free Estimate

A. Sanders Construction has built additions throughout Maple Valley, Renton, Kent, Covington, and Issaquah. We provide detailed estimates with transparent pricing — no surprises. Call (206) 799-6580 or visit our contact page to schedule your free consultation.