How Much Does an ADU Cost in Seattle?
ADU costs vary widely because an ADU is not a single product. It is a small home, a site project, a permitting process, and a construction project all at once.
Quick Cost Overview
The cost of a Seattle-area ADU depends on whether the project is a conversion, attached unit, addition, or detached structure. A detached ADU usually carries more site work, utility coordination, foundation, exterior envelope, and access considerations than an interior conversion.
Titan does not publish fixed ADU pricing without reviewing the property and scope. The table below is for planning conversations only, not a Titan quote.
Planning Ranges by Scope
| ADU direction | Relative budget level | Typical cost drivers |
|---|---|---|
| Interior conversion | Often lower than a new detached structure | Existing conditions, code upgrades, kitchen/bath additions, egress, ventilation, utilities. |
| Attached ADU or addition | Moderate to high depending on scope | Structural work, foundation tie-in, utility routing, exterior envelope, interior finishes. |
| Detached ADU | Often the highest project category | Site work, foundation, framing, exterior shell, utility connections, access, full interior buildout. |
| High-end custom ADU | Project-specific | Custom design, premium finishes, challenging site conditions, specialty windows, cabinetry, fixtures, and systems. |
Major Cost Drivers
The largest drivers are usually site conditions, structure type, utility work, foundation, framing, mechanical systems, windows and doors, cabinetry, bathroom and kitchen scope, and finish quality. Access also matters. A tight urban lot can make staging and material movement more difficult.
Soft costs should be part of the early conversation too. Design, engineering when needed, surveys, permit preparation, and professional coordination are part of many ADU projects.
Permits, Utilities, and Professional Services
Seattle ADUs typically require official review and permits through SDCI, but the exact permit path depends on the project. Utility work, electrical, plumbing, structural changes, and site work may introduce additional reviews or trade permits.
Homeowners should avoid comparing only construction square-foot numbers. An ADU budget has to account for the whole process from feasibility through closeout.
How to Build a Realistic Budget
Start with the intended use, then define the type of ADU, desired size, finish level, and site constraints. Add a contingency for unknowns, especially when tying into existing structures or utilities.
Homeowner Checklist
- Define whether you are considering attached, detached, or conversion ADU options.
- Separate construction costs from design, permit, engineering, and professional-service costs.
- Ask about site access, utilities, drainage, and staging early.
- Set a contingency for unforeseen conditions and scope changes.
- Avoid relying on generic online calculators as a final budget.
Frequently Asked Questions
What is usually the most expensive part of an ADU?
For detached ADUs, the structure, foundation, utilities, and full interior buildout are major drivers. For conversions, existing conditions and code upgrades can be significant.
Is a detached ADU more expensive than an attached ADU?
Often it can be, because it may require more site work, foundation, exterior shell, and utility coordination. The final answer depends on the property and scope.
Do permit fees determine the total cost?
No. Official fees are only one part of the total project. Design, engineering, materials, labor, site work, utilities, and coordination are usually much larger factors.
When can Titan provide a project-specific estimate?
A project-specific estimate requires review of the property, scope, drawings or concept, existing conditions, selections, and construction logistics.
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Planning an ADU?
Titan Build Group can help you think through scope, priorities, site conditions, and the information needed before a project-specific conversation.