Home Addition vs. ADU: Which Is Right for Your Washington Home in 2026?

What's the difference between a home addition and an ADU?

A home addition expands your existing living space — a larger kitchen, a primary suite, a family room, or a second story — and everything stays part of one home. An accessory dwelling unit (ADU) is a self-contained second home on the same lot, with its own kitchen, bathroom, and entrance. It can be attached to the house, converted from existing space, or built as a separate backyard structure (a DADU).

When is a home addition the better choice?

Choose an addition when the goal is more room for the people already living in the house. A bump-out that fixes a cramped kitchen, a primary suite over the garage, or a second story that doubles a rambler's footprint keeps daily life under one roof — no second kitchen, no separate utilities, and the new square footage reads as part of the main home when it's appraised. If what you really want is a bigger house, an addition almost always serves that goal more directly than an ADU.

When is an ADU the better choice?

Choose an ADU when the space needs independence: housing for aging parents or an adult child, a long-term rental for income, or flexibility to downsize into later. Because an ADU is a legal dwelling, it can generate rent an addition never will — and Washington's recent ADU legislation (HB 1337) has pushed cities across King County to allow more units on more lots than just a few years ago. Zoning details still vary city by city, so our guide to Seattle's DADU rules is a good starting point, and we confirm current local rules during design.

What do they cost in 2026?

In our current King County project pricing, attached ADUs typically run $150,000–$300,000 and detached ADUs (DADUs) $250,000–$450,000+, because a DADU is effectively a small standalone house — foundation, roof, utilities, and all. Additions vary more widely with scope and structure; see our 2026 home addition cost guide and ADU cost guide for detailed ranges. Two budget-friendlier paths worth knowing: a garage conversion or basement finishing can often create an ADU-style suite inside the structure you already own.

Which adds more resale value?

It depends on the buyer and the neighborhood. An addition raises the home's finished square footage, which appraisers compare directly against neighborhood comps. An ADU adds a second income-capable unit, which some buyers prize and some appraisers value conservatively — treatment genuinely varies by market and lender. Our honest take: build for how you'll use the space in the next ten years, and treat resale as the tiebreaker rather than the driver.

How do permits and timelines compare?

Both need building permits and both involve design, engineering, and city review. Additions that touch structure require engineering but follow a familiar residential review path. DADUs involve siting, setbacks, utility connections, and sometimes design review — often a longer road in total. Either way, permit queues differ sharply between Seattle and Eastside cities; our permit timeline comparison shows what to expect, and we run permitting in parallel with design so it adds as little calendar time as possible.

Talk through both options with one builder

Blue Lake Construction designs and builds home additions, ADUs, and DADUs across Seattle, Bellevue, Kirkland, Redmond, and the Eastside on fixed-price contracts — so you can compare both paths for your actual lot and budget before committing to either. Request a free consultation and we'll walk the property with you.