Additions & ADUs in Seattle, WA

Licensed & Insured | Family-Owned | Owner-Supervised Projects

✓ Licensed & Insured

✓ Family Owned

✓ Free Estimates

✓ Owner on Every Job

Expert Additions & ADUs Services in Seattle, Washington

Northwest Custom Homes & Remodel is proud to serve homeowners in the Seattle area with premium addition or ADU services. Whether you’re envisioning a complete transformation or targeted upgrades, our experienced team delivers exceptional craftsmanship and personalized attention on every project in Seattle and surrounding neighborhoods including local neighborhoods.

As a family-owned company with the owner present on every job site, we bring a level of dedication and quality control that larger contractors simply cannot match. Our deep knowledge of Seattle’s building codes, neighborhood aesthetics, and local permitting requirements ensures a smooth, stress-free experience from first consultation to final walkthrough.

Our Additions & ADUs Process

Every project in Seattle follows our proven six-step process, ensuring clear communication, on-time delivery, and results that exceed expectations.

Feasibility & Zoning Review
We evaluate your property for ADU potential including setbacks, utilities, access, and zoning compliance.

ADU Design & Floor Plans
Our architects create efficient floor plans maximizing livable space within allowable building dimensions.

Permits & Engineering
We manage permit applications, structural engineering, and all regulatory approvals for your ADU.

Site Prep & Foundation
The building site is prepared with utilities connections, grading, and foundation installation.

Framing & Systems
Walls, roof, electrical, plumbing, HVAC, and exterior finishes create a complete structure.

Finishing & Certificate of Occupancy
Interior finishes, appliances, final inspections, and Certificate of Occupancy complete your new ADU.

Additions & ADUs Styles We Offer in Seattle

Detached ADU

Standalone living spaces with full kitchens, private entries, and independent utility connections.

Garage Conversion

Transform underutilized garages into comfortable living spaces with minimal site disruption.

Second-Story Addition

Add living space above your existing footprint — bedrooms, offices, or complete suites.

Basement ADU

Convert unused basement space into a legal dwelling unit with proper egress and light.

Frequently Asked Questions — Additions & ADUs in Seattle

How much does an ADU cost to build in Seattle?

ADU (Accessory Dwelling Unit) construction in Seattle typically costs $200,000 to $450,000+ depending on size, type (attached vs. detached), and finishes. A 600-800 sq ft detached ADU averages $250,000-$350,000. Washington state and local incentives may help offset costs.

What are Seattle’s ADU regulations?

Seattle allows both attached and detached ADUs on most single-family lots following Washington’s HB 1337. Requirements include maximum size limits (typically 1,000-1,200 sq ft), setback requirements, and parking provisions. We navigate all local regulations and handle the permitting process for you.

How long does it take to build an ADU in Seattle?

An ADU in Seattle typically takes 6-10 months from permits to completion. The permitting process takes 4-8 weeks, followed by 5-8 months of construction. Attached additions and garage conversions may be completed faster (4-6 months construction) than ground-up detached ADUs.

Can I add a second story addition to my Seattle home?

In most cases, yes. Second-story additions are popular in Seattle for maximizing living space without expanding the footprint. Key considerations include existing foundation capacity, height restrictions, setback requirements, and views. We provide structural assessments to determine feasibility.