How Much Does a Basement ADU Cost in Seattle?
If you’ve been eyeing your unfinished basement and wondering whether it could become a rental unit, guest suite, or multigenerational space — you’re not alone. Basement ADUs (Accessory Dwelling Units) are one of Seattle’s hottest housing strategies. But let’s be real: the first question on most homeowners’ minds is “What will this cost me?”
The Short Answer
It depends. Your budget hinges on the condition of your basement, how much plumbing/electrical work is needed, and the level of finishes you’re aiming for. But here’s what we typically see:
Design & Permits: $5K–$15K
Construction: $80–$250 per square foot
Fixtures & Appliances: $5K–$25K+
👉 For a 600–800 sf Seattle basement, most projects land somewhere between $100K and $180K, though luxury builds can go higher.
Where the Costs Hide:
Seattle basements aren’t always “plug-and-play.” Here are the common budget killers we see:
💧 Waterproofing & Drainage — Even a small leak can add thousands.
🚽 Plumbing Rework — If the sewer line is uphill or far from your planned bath, expect costs to climb.
⚡ Electrical Upgrades — A new kitchen or laundry often requires a panel upgrade.
🎨 Overdesign — Marble floors and budget ADUs don’t mix. Keep scope aligned with intent
➕ Scope Creep — “While we’re at it…” is the most expensive phrase in remodeling.
Why It’s Still Worth It
Yes, the upfront cost is real — but so is the payoff. In Seattle, basement ADUs commonly rent for $1,600–$2,500/month. That means many projects can break even in 4–7 years, depending on financing. After that, it’s cash flow, increased property value, and long-term flexibility.
Next Step: Run Your Own Numbers
If you’re serious about exploring, the best place to start is a clear, editable budget. That’s why we built our Basement to ADU Toolkit — the same system we use with clients, simplified for homeowners.
📦 Download the Toolkit here
📖 Or, read more in our guide: Basement ADUs in Seattle — What Homeowners Really Need to Know