Is Building a DADU Worth the Investment? ROI Analysis for King County

The Investment Case for King County DADUs

At $400,000–$650,000 all-in, a DADU is a major investment. The question every homeowner asks is: Is it worth it? The answer depends on your goals — rental income, property value, family housing, or some combination. But from a pure numbers perspective, King County DADUs consistently produce strong returns when planned well.

This analysis uses real market data from the King County rental and real estate markets to give you a clear picture of the financial case.

Revenue Stream #1: Rental Income

Rental income is the most immediate and measurable return on a DADU investment.

King County DADU Rental Rates (2026)

Studio/1-bed DADU (400–600 sq ft): $1,500–$2,200/month
1-bed DADU (600–800 sq ft): $2,000–$2,800/month
2-bed DADU (800–1,000 sq ft): $2,500–$3,500/month

Location Matters

Seattle neighborhoods: $2,200–$3,500/month — highest rents due to proximity to jobs and transit
Eastside (Bellevue, Kirkland, Redmond): $2,300–$3,500/month — strong tech employer demand
South King County (Renton, Auburn, Federal Way): $1,500–$2,500/month — lower rents but also lower construction costs
Unincorporated King County: $1,500–$2,800/month — varies widely by specific location

For detailed rental projections by area, see our rental income guide.

Annual Rental Income Projections

Using a mid-range 800 sq ft DADU at $2,400/month:

Gross annual rent: $28,800
Vacancy allowance (5%): -$1,440
Maintenance and repairs (5%): -$1,440
Property management (if used, 8%): -$2,304
Additional insurance: -$1,200
Net annual income (self-managed): $24,720
Net annual income (property managed): $22,416

Revenue Stream #2: Property Value Increase

A legally permitted DADU adds substantial value to your property.

How DADUs Affect Appraisals

Direct value addition: Appraisers assign value based on the DADU’s size, quality, and rental income potential
Income approach: For investment properties, appraisers capitalize the rental income stream — a DADU generating $24,000/year at a 5% cap rate adds $480,000 in value
Comparable sales: As more DADUs are built in King County, comparable sales data increasingly supports higher valuations

Estimated Value Increase

Conservative estimate: 20% increase in total property value
Moderate estimate: 25–30% increase
In high-demand areas: 30–40% increase, particularly when the DADU is income-producing

On a King County home valued at $800,000, a 25% increase means $200,000 in added equity — often approaching or exceeding the DADU’s construction cost. See our property value guide for details.

The ROI Math

Let’s run the numbers on a typical scenario:

Assumptions

DADU cost: $500,000 (all-in for 800 sq ft)
Monthly rent: $2,400
Financing: HELOC at 7% interest
Property value increase: 25% of $800,000 home = $200,000
Annual rent growth: 3%
Annual property appreciation: 4%

Year 1

Net rental income: $24,720
Interest expense: $35,000
Net cash flow: -$10,280 (negative — interest exceeds rent in year 1)
Property value gain: $200,000 (immediate equity creation)
Total return including equity: +$189,720

Year 5

Annual net rental income: ~$28,600 (with 3% annual growth)
Cumulative net rental income: ~$132,000
Remaining loan balance: ~$415,000 (assuming interest-only payments)
Property value gain: $200,000 + additional appreciation
Cumulative return: well into positive territory

Year 10

Annual net rental income: ~$33,100
Cumulative net rental income: ~$293,000
At this point, rental income alone has covered more than half the construction cost
Property value continues to appreciate

Break-Even Analysis

How quickly does a DADU pay for itself?

Cash-flow break-even (rent covers financing): Year 2–4, depending on interest rate and rental income
Full cost recovery from rental income: 12–18 years at current rates
Including property appreciation: Often break-even on day one — the DADU adds value approaching its cost immediately

Non-Financial Returns

The ROI calculation doesn’t capture everything a DADU provides:

Family housing: Keeping aging parents close, housing adult children, or providing space for caregivers has value beyond dollars
Flexibility: A DADU can serve as a rental today, family housing tomorrow, and a home office next year
Housing security: Rental income provides a financial buffer — if you lose your primary income, the DADU rent can cover your mortgage
Retirement asset: A paid-off DADU generating $30,000+/year in rental income is a significant retirement resource

When a DADU May NOT Be Worth It

Honest analysis requires acknowledging when a DADU isn’t the right investment:

Very low rental market: If your area commands less than $1,500/month, the payback period stretches significantly
Extreme site costs: Properties with severe slopes, poor access, or major septic issues can push costs to $700,000+, making the math harder
Short-term ownership: If you plan to sell within 2–3 years, the immediate property value increase may not offset the full investment
Over-leveraged: If financing the DADU strains your overall financial position, the risk may outweigh the return

Maximizing Your DADU ROI

Right-size the unit. Build the largest DADU your lot allows — rental income scales with size, but permit and utility costs are similar.
Design for rentability. In-unit laundry, good storage, private outdoor space, and a functional kitchen command premium rents.
Location awareness. DADUs near transit, employers, and amenities rent faster and for more.
Control costs. Work with an experienced builder who knows how to manage construction costs without cutting corners.
Consider long-term hold. The longer you own the DADU, the better the cumulative return as rents grow and the loan pays down.

APEX DADU Builds for Returns

We design and build DADUs with both livability and ROI in mind. APEX DADU helps you make decisions that maximize your return — from unit size and layout to finish level and rental strategy. Every project gets a custom financial analysis so you understand the investment case for your specific property.

Contact APEX DADU for a free ROI analysis for your property