One of your first questions will be whether to build a DADU using a Seattle Pre-Approved Design Plan or go fully Custom. Building a Detached Accessory Dwelling Unit (DADU) — also known as a backyard cottage — is an amazing way for Seattle homeowners to create more living space and long-term value. Whether it’s for rental income, multi-generational living, or a home office with a door you can actually close — a DADU is a smart investment in Seattle’s housing market. Before you get started, you will want to investigate your options thoroughly.
Here’s a clear look at the cost differences, benefits, and trade-offs so you can choose the right path for your property.
COST Category | Pre-Approved Design Plan | Custom Build |
Architectural design | Low (included/licensing fee only) | High ($20K–$60K+) |
Permit process | Faster and less expensive | More complex, longer review |
Construction | Moderate–High | Moderate–High (varies widely) |
Modifications | Limited | Fully customizable |
Unique finishes, layout, footprint | Some limits | Unlimited |
Typical Total Costs in Seattle:
Pre-Approved DADU: $350K–$500K
Custom DADU: $400K–$650K+
(Real costs depend on size, site conditions, utilities, finishes, and contractor selection.)
This is often the best route for homeowners focused on rental units or simple layouts.
A custom build gives you exactly the space you want — but with more design time and cost.
