Building an accessory dwelling unit is a significant investment, and getting your budget right from the start can mean the difference between a smooth project and a stressful one. Whether you're planning a basement conversion or a new backyard cottage in the Seattle-Tacoma area, here's a detailed breakdown of the costs you should plan for.

Construction Costs

Construction is by far the largest line item in any ADU budget. In the Puget Sound region, expect to pay between $250 and $450 per square foot for new construction, depending on the complexity of the design, the builder you choose, and current material prices.

For a typical 600-square-foot detached ADU, that translates to roughly $200,000 to $350,000 in total construction costs. Attached ADUs and conversions tend to come in lower, often between $150,000 and $250,000, because they leverage existing structure, roofing, and sometimes plumbing.

Key factors that push construction costs higher include steep or difficult lot access, poor soil conditions requiring engineered foundations, high-end finishes, and custom architectural elements. Conversely, choosing a pre-designed or pre-approved plan can reduce both design and construction costs.

Permits and Fees

Permitting costs in the Puget Sound area typically range from $5,000 to $15,000, depending on your jurisdiction and the scope of the project. This includes:

Seattle has made efforts to reduce permit costs for ADUs, including waiving certain impact fees. Tacoma and other South Sound cities have followed suit with their own incentive programs. Always check with your local permitting office for the most current fee schedule.

Design and Architecture Fees

Hiring an architect or designer typically costs between $10,000 and $30,000 for a custom ADU design. This includes schematic design, construction documents, and sometimes construction administration (site visits during the build).

If budget is tight, consider using pre-approved ADU plans, which several Puget Sound jurisdictions now offer. Seattle's pre-approved plan program lets you skip the full design review process, saving both time and money. Pre-approved plans can cost as little as $2,000 to $8,000 to license.

Utility Connections

Don't overlook utility costs, which can add $10,000 to $30,000 or more to your budget. This covers:

Utility costs are generally lower for attached ADUs because you can often tap into the existing home's systems. Detached units nearly always require independent connections, which means trenching, new lines, and separate meters.

Contingency Budget

Every construction project encounters surprises. A 10 to 15 percent contingency on top of your total estimated budget is the standard recommendation. For a $300,000 project, that means setting aside an additional $30,000 to $45,000.

Common surprises include discovering substandard wiring or plumbing during a conversion, encountering rock or roots during excavation, and material price increases during the build. Having a contingency fund prevents these issues from derailing your project or forcing you to cut corners on finishes.

Financing Options

Several financing paths are available for ADU projects in Washington state:

When evaluating financing, factor in the potential rental income from the finished ADU. A well-located ADU in Seattle can generate $1,500 to $2,500 per month in rental income, which significantly offsets your carrying costs and can make the investment cash-flow positive within a few years.

Sample Budget Summary

For a 600-square-foot detached ADU in the Seattle area, here's a realistic budget range:

The wide range reflects the reality that every project is different. Getting multiple bids from qualified builders, being clear about your priorities, and building in a healthy contingency are the best ways to keep your ADU project on budget.