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Accessory Dwelling Units (ADUs) have become one of the most popular ways for Washington homeowners to create additional living space, accommodate family members, generate rental income, or increase property flexibility.

Recent changes to Washington State housing laws have made ADU construction more accessible in many communities, leading more homeowners to explore the possibility of building an ADU on their property.

However, while ADUs can provide significant benefits, many projects encounter delays, unexpected costs, or permitting challenges because important planning considerations were overlooked early in the process.

At Goodwin Construction, we’ve worked with homeowners throughout Washington State to help navigate the planning, design, permitting, and construction process. With over 25+ years of experience and an in-house designer, we understand where projects often run into challenges and how early planning can help avoid them.

If you’re considering building an ADU, here are some of the most common mistakes homeowners make and how to avoid them.

What Is an ADU?

An Accessory Dwelling Unit (ADU) is a secondary residential living space located on the same property as a primary home.

ADUs can be:

• Attached to the primary residence
• Converted from an existing garage
• Built as a detached backyard structure
• Integrated into a basement or existing living area

ADUs are commonly used for:

• Multi-generational living
• Guest accommodations
• Rental opportunities
• Home offices
• Flexible living space

Quick Answer: What Are the Most
Common ADU Planning Mistakes?

Some of the most common mistakes homeowners make include:

• Assuming every property qualifies for an ADU
• Not researching local regulations
• Underestimating permitting timelines
• Ignoring utility requirements
• Focusing only on today’s needs
• Overlooking site constraints
• Making design changes during construction
• Choosing a contractor without ADU experience

Many of these issues can be avoided through proper planning before construction begins.

1. Assuming Every Property Qualifies for an ADU

One of the biggest misconceptions is that every residential property automatically qualifies for an ADU.

While Washington State has expanded ADU opportunities in many areas, local jurisdictions still maintain development standards that may affect what can be built.

According to the Washington State Department of Commerce, local regulations can still address factors such as setbacks, building placement, height, utility requirements, and other site-specific development standards. 

Factors that can influence ADU eligibility include:

• Zoning requirements
• Property dimensions
• Setbacks
• Easements
• Environmental restrictions
• Utility availability

Before investing in plans or design work, it’s important to verify what is allowed on your specific property.

2. Not Understanding Local ADU Regulations

Many homeowners begin planning based on information they find online without realizing that regulations vary by city and county.

Requirements may differ regarding:

• Maximum ADU size
• Building height
• Setbacks
• Parking requirements
• Design standards
• Occupancy requirements

While recent Washington State legislation has expanded ADU opportunities and reduced some local restrictions, homeowners should not assume the same rules apply everywhere. Under RCW 36.70A.681, cities and counties must allow ADUs and cannot impose certain restrictions that conflict with state law, but local jurisdictions still administer permitting processes and development standards that can affect what is ultimately approved on a specific property.

Understanding local requirements early can prevent costly redesigns later.

3. Underestimating Permitting Timelines

Permitting is one of the most overlooked aspects of ADU construction.

Many homeowners assume construction can begin shortly after plans are completed.

In reality, ADU projects often require:

• Building permits
• Site review
• Utility review
• Structural review
• Stormwater review
• Other jurisdiction-specific approvals

Permitting timelines can vary significantly depending on project complexity and local workload.

Planning for these timelines early helps avoid frustration and unrealistic expectations.

4. Ignoring Utility Requirements

Utility infrastructure is often more complex than homeowners expect.

Depending on the project, an ADU may require coordination involving:

• Electrical service
• Water connections
• Sewer systems
• Stormwater management

Utility requirements vary based on the property and local regulations.

Early utility planning helps identify potential limitations before they affect the project schedule.

5. Designing Only for Current Needs

Many homeowners focus solely on their immediate goals without considering how the ADU may be used in the future.

For example:

• A home office today may become living space later.
• A family suite may eventually become a rental unit.
• Guest accommodations may later support aging family members.

Flexible design often provides greater long-term value and usability.

When planning an ADU, it’s worth considering how your needs may evolve over time.

6. Overlooking Site Conditions

Every property presents unique challenges and opportunities.

Factors that may affect construction include:

• Slope and grading
• Drainage
• Tree preservation requirements
• Utility locations
• Construction access
• Existing structures

Site conditions can influence both permitting and construction planning.

A thorough site evaluation early in the process can help identify potential challenges before they become costly issues.

7. Making Major Design Changes During Construction

Changes made after construction begins can affect:

• Budget
• Timeline
• Materials
• Permitting
• Labor requirements

Even small design changes can have a ripple effect throughout the project.

Finalizing the design as much as possible before construction begins helps reduce delays and unexpected costs.

At Goodwin Construction, our in-house designer works closely with homeowners during the planning phase to help align design decisions with project goals before construction starts.

You can learn more about this approach in our blog  Design-Build Services in Washington State: A Smarter Way to Build.

8. Choosing a Contractor Without ADU Experience

ADU projects often involve unique challenges related to:

• Site planning
• Utility coordination
• Permitting
• Residential construction
• Local regulations

Choosing a contractor with experience in ADU construction, can help streamline the process and reduce potential setbacks.

Homeowners should look for contractors who understand both construction requirements and local permitting processes.

Why Early Planning Makes a Difference

Many ADU challenges occur long before construction begins.

Investing time in early planning can help homeowners:

• Understand property limitations
• Improve budgeting accuracy
• Reduce permitting delays
• Coordinate utilities
• Avoid unnecessary redesigns
• Improve project efficiency

The earlier potential issues are identified, the easier they are to address.

How Goodwin Construction Helps Homeowners Plan ADU Projects

Every ADU project is different, but a structured planning process helps create a smoother experience.

Our approach typically includes:

Initial Consultation

Understanding your goals, property conditions, and long-term plans.

Site Evaluation

Reviewing property characteristics that may affect design and construction.

Design Coordination

Working with our in-house designer to develop a plan that aligns with local requirements and project goals.

Permitting Support

Helping coordinate approvals and communication with local jurisdictions.

Construction

Managing the project with ongoing communication and coordination throughout the build.

Key Takeaways

Building an ADU in Washington State can be an excellent way to create additional living space, increase property flexibility, and support long-term goals.

However, successful projects depend on careful planning, realistic expectations, and understanding the requirements that apply to your property.

Many of the most common ADU challenges can be avoided by addressing permitting, utilities, site conditions, and design considerations before construction begins.

At Goodwin Construction, we help homeowners navigate every stage of the ADU process with clear communication, detailed planning, and coordinated project management.

If you’re considering building an ADU, scheduling a consultation early can help you better understand your options, timeline, and overall project scope before construction begins.