Accessory Dwelling Units (ADUs) have become one of the most talked-about trends in housing across the Pacific Northwest and for good reason. With increasing demand for flexible living arrangements, rental income, and multigenerational housing, homeowners in Bellevue are looking to ADUs as a smart, future-ready solution. While much of the ADU spotlight has focused on Seattle, 2025 is shaping up to be a defining year for Bellevue homeowners ready to take advantage of this movement.

At Evergreen Remodelers, we’ve seen a major increase in Bellevue clients exploring ADUs whether it’s a basement conversion, backyard cottage, or garage apartment. But with shifting regulations, zoning rules, and construction costs, it’s not as simple as putting up a small unit and listing it on Airbnb. This guide breaks down everything Bellevue homeowners need to know about ADUs in 2025 what they are, how to build one, and why now is the right time to start planning.

What Is an ADU and Why Is Everyone Talking About Them?

An Accessory Dwelling Unit, or ADU, is a secondary residential unit built on the same property as a primary home. It can be attached (like a converted basement or above-garage apartment) or detached (like a backyard cottage). These units are fully functional living spaces, typically including a kitchen, bathroom, sleeping area, and separate entrance.

What makes ADUs so attractive in cities like Bellevue and Seattle is their versatility. They can be used as long-term rentals, guest quarters, home offices, housing for aging parents, or even downsizing options for homeowners who want to stay on their property while earning passive income.

With housing affordability top of mind in 2025, local municipalities have started easing restrictions to make ADUs more accessible. And for Bellevue residents, where home values are high and property lots are relatively generous compared to Seattle’s urban core, the opportunity is even more appealing.

The Bellevue Advantage: Why ADUs Make Sense Here

While Seattle has led the region in terms of ADU policy reform, Bellevue is quickly catching up. As of 2025, Bellevue continues to encourage accessory dwelling units as part of its broader housing strategy. For homeowners, this means relaxed design standards, quicker permitting, and more options for placement and occupancy.

Compared to Seattle’s denser neighborhoods, many Bellevue properties have larger lots, more yard space, and easier access to off-street parking all key factors when designing a compliant ADU. Additionally, Bellevue’s steady rental demand and strong property values make building an ADU a strategic investment that can pay off in both flexibility and financial return.

Whether you live in neighborhoods like Crossroads, Bridle Trails, or Somerset, adding an ADU is one of the most practical ways to extend your living space without leaving your community.

Types of ADUs Bellevue Homeowners Are Building in 2025

The beauty of ADUs lies in their adaptability. Depending on your lot, budget, and long-term goals, there are several types of ADUs you can consider:

Detached ADUs (DADUs)

Often referred to as backyard cottages, these are standalone structures separate from the main house. They’re great for homeowners with larger yards and offer the most privacy for both parties. Many include 400–800 square feet of living space and can be built as single-level or loft-style units.

Attached ADUs

These units are physically connected to the main house, usually as a bump-out addition or over-the-garage apartment. They share one or more walls with the main home but have their own entrances, kitchens, and utilities. Attached ADUs are ideal for those who don’t want to reduce yard space but still want an independent living unit.

Interior Conversions

These include basement or attic conversions. They are usually the most cost-effective option, as they don’t require new foundations or major exterior work. For homeowners with unfinished or underused basements, this is often the quickest path to creating a legal ADU.

Each type comes with its own permitting and design considerations, but all can dramatically enhance your property’s functionality and value when done right.

What Are the 2025 Rules for Building an ADU in Bellevue?

ADUs but it’s still critical to understand what’s required before you begin. The city continues to refine its housing regulations, so staying current is key.

Here are the main zoning and building considerations:

Zoning Requirements: ADUs are generally allowed in all single-family zones, but setbacks, lot size, and unit size restrictions apply. Detached ADUs may be limited to 800–1,000 square feet depending on the lot.
Lot Coverage and Height: There are limits on how much of your property can be covered by structures. Detached units must also meet maximum height restrictions, which are usually around 20 feet.
Parking: In many Bellevue neighborhoods, off-street parking is required for an ADU unless specific waivers apply. This can impact your design and placement options.
Occupancy Rules: Bellevue generally requires the homeowner to occupy either the main house or the ADU. This limits investor-only usage but encourages long-term neighborhood stability.
Utilities and Access: Separate plumbing, electrical, and sewer connections may be required. In some cases, shared systems can be permitted with inspection and approval.

Permitting can take several weeks to a few months, depending on your project’s complexity. At Evergreen Remodelers, we guide clients through the permit application, site plans, engineering, and all interactions with the City of Bellevue to ensure a smooth process from start to finish.

How Much Does It Cost to Build an ADU in Bellevue?

The cost of building an ADU in Bellevue depends heavily on the type, materials, design, and site conditions but here’s a general 2025 breakdown:

Basement Conversion: $80,000–$150,000
Attached ADU: $120,000–$200,000
Detached ADU (New Build): $180,000–$350,000+

Factors that influence cost include:

Site prep and foundation work
Plumbing and electrical system
Kitchen and bathroom installations
Heating, insulation, and ventilation systems
Finishes, fixtures, and interior design

While upfront costs can be significant, many Bellevue homeowners recoup much of the investment through rental income, increased property value, or lifestyle flexibility like housing adult children, caregivers, or aging relatives without leaving home.

How Long Does It Take to Build an ADU?

From design to final inspection, building an ADU in Bellevue usually takes 6 to 12 months, depending on the scope. The general timeline includes:

Design & Planning: 4–8 weeks
Permitting: 6–12 weeks
Construction: 4–6 months (longer for detached units)

This process can vary based on weather, contractor availability, permit backlogs, or design changes. At Evergreen Remodelers, we streamline the experience by handling everything from architectural design to project management, so you don’t face unnecessary delays.

Why 2025 Is the Right Time to Build an ADU in Bellevue

There’s no question that ADUs have gone from a niche trend to a mainstream housing solution and Bellevue homeowners are in a prime position to benefit. With rising housing demand, favorable policy changes, and record-high home equity, 2025 represents a window of opportunity for smart investment.

Here’s why now is the time:

Flexible zoning laws make it easier to build
Rental demand remains strong across King County.
Construction costs are stabilizing after pandemic-era volatility.
ADUs boost long-term property value and resale appea.l

For homeowners thinking long-term whether it’s housing family, aging in place, or creating a passive income stream ADUs offer unmatched versatility.

Work with Evergreen Remodelers to Design and Build Your ADU

Building an accessory dwelling unit isn’t just about adding square footage it’s about adding possibilities. From the initial design phase to securing permits and completing construction, Evergreen Remodelers brings the local knowledge, craftsmanship, and service you need to bring your vision to life.

We help Bellevue homeowners every step of the way whether you’re converting a basement into a legal rental or constructing a detached cottage from the ground up. Our team understands the city’s evolving codes, lot conditions, and architectural styles that keep your property compliant and desirable.

📞 Schedule a consultation today to explore your ADU options and see how Evergreen Remodelers can help you transform your property into something more.

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