Garage Conversion vs. DADU: Which Should You Build?

Both turn unused space into housing — but they differ a lot in cost, timeline, and what you end up with. Here's how King County homeowners should think about the choice in 2026.

Cost

- Garage conversion: typically $100,000–$250,000, because the structure, slab, and roof already exist. The final number depends on plumbing runs, ceiling height fixes, and insulation upgrades.

- New DADU: typically $250,000–$450,000+ for a ground-up backyard cottage with its own foundation and utility connections.

Timeline

- Garage conversion: usually 3–5 months of construction after permits.

- New DADU: 5–9 months of construction after permits, and permitting takes longer for new structures.

What you gain (and give up)

A conversion is faster and cheaper, but you lose covered parking and storage, and the footprint is fixed. A DADU costs more but is purpose-built: better layout, better sound separation, full-height ceilings, and typically stronger rent — $2,000–$3,200+ per month long-term near Seattle and Bellevue versus somewhat less for most conversions.

Permits

Both require permits. Conversions still need to meet habitable-space code — egress windows, insulation, heating, and often a ceiling-height fix on older garages. Under HB 1337, most urban lots in King County can add at least two ADUs, so a conversion now doesn't block a DADU later.

The bottom line

Tight budget or a solid existing garage → start with a garage conversion. Maximizing long-term rental income and property value → build a DADU. Not sure? Get a free consultation — we'll walk your lot and price both options as fixed bids.