Choosing the Right ADU Size for Your Needs and Property

Accessory Dwelling Units (ADUs) have become increasingly popular as homeowners seek solutions for evolving housing needs, increased property value, and the potential for rental income. However, determining the ideal ADU size can be a complex process, influenced by many factors. Did you know that the right ADU size can significantly impact your return on investment and compliance with local regulations? In Portland, ADUs are capped at 800 square feet, while in Clark County, including the City of Vancouver, ADUs are can be up to 1,000 square feet.

This blog post will serve as a comprehensive guide, walking you through the critical elements that influence ADU size selection. We’ll delve into local regulations, property size considerations, intended use, budget constraints, and available incentives. To provide authoritative insights, we’ll draw from city planning documents, housing guides, and industry expert resources.

Navigating Local ADU Size Restrictions

Understanding and complying with local ADU regulations is paramount. These regulations vary significantly by city and county, impacting the maximum allowable size and placement of your ADU. Failing to adhere to these rules can result in fines, legal issues, and costly project revisions.

• General Overview: Regulations are not one-size-fits-all. Each municipality has its own specific set of rules governing ADU size, height, setbacks, and design.
• Portland, OR Specifics: In Portland, ADUs are limited to 800 square feet or 75% of the primary dwelling’s living area, whichever is less. Furthermore, detached ADUs must be positioned at least 5 feet from side and rear property lines.
• Vancouver, WA Specifics: Vancouver allows up to two ADUs per lot, with a maximum size of 1,000 square feet for each. It’s crucial to ensure that any ADU addition or new construction does not exceed the allowable lot coverage or encroach into required setbacks.
• Clark County, WA: Clark County allows up to two ADUs per lot, with a maximum size of 1,000 square feet for each. It’s crucial to ensure that any ADU addition or new construction does not exceed the allowable lot coverage or encroach into required setbacks.
• Consequences of Non-Compliance: Ignoring local regulations can lead to significant penalties, including fines and legal complications.

To illustrate the differences, consider the following table:

Table 1: ADU Size Regulations Comparison

How Your Property Influences ADU Size

Beyond local regulations, your property’s size, layout, and zoning play a significant role in determining feasible ADU options.

• Property Size and Footprint: Larger properties generally allow for larger ADUs. However, the ADU’s footprint must still comply with zoning regulations, regardless of your lot size.
• Zoning Regulations: Zoning dictates the type of ADU permitted on your property: attached (connected to the main house), detached (a separate structure), or a conversion (repurposing existing space like a garage). In Vancouver, WA, ADUs are permitted on all lots within urban growth areas that allow single-family homes.
• Setback Requirements: Setback requirements, as mentioned earlier, define the minimum distance an ADU must be from property lines. These vary by jurisdiction and zoning and can significantly impact the placement and size of your ADU.
• Lot Coverage Limits: Many jurisdictions impose lot coverage restrictions, limiting the total percentage of your property that can be covered by structures (including the main house and any ADUs).

Matching ADU Size to Your Needs

The intended use of your ADU is a crucial factor in determining the optimal size.

• Rental Income: If your primary goal is to generate rental income, maximizing the allowable size can be advantageous. A larger ADU may attract more tenants and command higher rent.
• Family Housing: When housing family members, prioritize accessibility, specific needs (e.g., accommodations for aging parents), and comfortable living space. The overall size may be less critical than thoughtful design and layout.
• Home Office/Studio: For a dedicated workspace, a smaller ADU (400-600 sq ft) may be perfectly sufficient, offering a quiet and productive environment.
• Guest House: The ideal size for a guest house depends on the expected frequency and duration of guest stays. Occasional visitors may only need a compact space, while frequent or long-term guests might require more amenities.
• Future Flexibility: Consider potential future uses for the ADU. Choosing a size that can accommodate various needs over time can add long-term value to your property.

The Financial Impact of ADU Size

It’s no surprise that ADU size directly correlates with construction costs. Larger ADUs require more materials, labor, and potentially more complex designs, all of which increase the overall expense.

• Cost Range: Building an ADU can range from $100,000 to over $300,000, and size is a major contributing factor to this wide range.
• Design Complexity: Larger ADUs often necessitate more intricate designs, potentially involving architects and engineers, which adds to the cost.
• Materials: The choice of materials significantly impacts the budget. Larger ADUs require more materials, and higher-end finishes will further increase expenses.
• Labor: More square footage translates to more labor hours, directly impacting the overall construction cost.
• Cost per Square Foot: While the total cost of a larger ADU is higher, the cost per square foot typically decreases as the size increases. This is due to certain fixed costs (like permits and design fees) being spread over a larger area.
• Financing Options: Explore various financing options, such as home equity loans or construction loans, to determine the best way to fund your ADU project.

Leveraging Incentives for ADU Construction

Several financial incentives and programs can help offset ADU construction costs, potentially influencing your size decision.

• Portland’s SDC Waiver Program: Portland offers a System Development Charges (SDC) waiver for ADUs, provided there’s a 10-year commitment to keeping the unit available for housing. This waiver can save significant costs, potentially encouraging homeowners to build a slightly larger ADU.
• Vancouver’s Tax Relief: Vancouver offers potential tax relief for up to three years for new ADU construction.
• Impact Fee Caps (Washington): In Washington State, impact fees for ADUs are capped at 50% of those for the primary residence.
• Other Potential Incentives: Explore other state or local incentives that may be available in your area. These could include grants, rebates, or expedited permitting processes.

Final Reflections

Choosing the right ADU size is a balancing act, requiring careful consideration of local regulations, property size, intended use, budget, and available incentives. There’s no one-size-fits-all answer, and the optimal solution will be unique to your circumstances.

It’s highly recommended to consult with professionals, like the team at MyADU, to navigate this process. We can help you assess your property, understand local regulations, and determine the perfect ADU size to meet your needs and maximize your investment. Check out our ADU Models page to see what might be a good fit for your property.

Reference List:

1. House Idea. (n.d.). Typically the cost to build an ADU can range from 100,000 to over 300,000 depending on factors like design complexity materials and labor. Retrieved from https://houseidea.com/blog/cost-to-build-an-adu
2. Clark County, WA. (2025). The total gross floor area living space not including the garage square feet of an accessory dwelling shall not exceed 1000 square feet. Retrieved from https://www.codepublishing.com/WA/ClarkCounty/html/ClarkCounty40/ClarkCounty40260/ClarkCounty40260020.html#:~:text=C.%20Development%20Standards.-,1.,d. 
3. Portland.gov. (n.d.). In Portland ADUs are capped at between 15 and 20 feet high and three quarters the size of the main house up to 800 square feet. Retrieved from https://portland.gov/ppd/residential-permitting/home-projects/accessory-dwelling-units
4. Portland.gov. (2020). Setback requirements for ADUs in Portland stipulate that detached structures must generally be at least 5 feet from side and rear property lines. Retrieved from https://www.portland.gov/sites/default/files/2020-02/title33_zoning_regs_ch33.205.pdf
5. City of Vancouver, WA. (n.d.). An ADU addition to an existing house or the construction of a new ADU shall not exceed the allowable lot coverage or encroach into required setbacks as prescribed in the underlying zone for the primary structure. Retrieved from https://cityofvancouver.us/business/building-construction/residential-building-permits/accessory-dwelling-units
6. City of Vancouver. (2024). *Under HB 1337 the City of Vancouver is required to adopt standards allowing middle housing developments of up to 4 units on lots in single family zoning districts citywide, and up to 6 units when located near Bus Rapid Transit (BRT) stations or if affordable housing units are proposed*. Retrieved From: https://cityofvancouver.us/wp-content/uploads/2024/07/07-23-24_PC-WS_Memo_Short-Term-Housing-Code-Change.pdf