ADU Builder on Mercer Island, WA

Build a detached backyard cottage, attached ADU, or garage conversion on your Mercer Island property with a licensed design-build contractor. From the North End and Mercerdale to First Hill, East Seattle, and the South End, Blue Lake handles zoning checks, permits, and construction on one fixed-price contract — adding value, rental income, or multigenerational space to your home.

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Woodinville

Master bed and bathroom addition.

White house with dark brown roof undergoing roof renovation with scaffolding and tiles stacked on roof.

Fall City

Ada extension side of the house.

Modern white house with large windows, a brown roof, and a paved pathway surrounded by green grass.

Burien

Second story extension for a sunroom.

Wooden framing of house extension under construction with two workers on top and ladders around.

Check Out Some of Our Latest Work.

In the gallery, are some of the projects we have done for clients in the various service areas we offer.

Modern office meeting room with concrete table, chairs, shelves with books, and industrial style decor.

Long wooden fence with metal posts along a sidewalk and grass in a residential area.

Modern garage with a pool table, bike on wall, and a Tesla parked outside glass garage door.

Modern basement with wood floors, two patterned chairs, a white sectional sofa, and a gray patterned rug.

Black metal fence separating grassy area with trees from a yard with shrubs and a house in the background.

Open kitchen and living area with laminate wood flooring, white cabinets, and stainless steel appliances.

Modern outdoor patio with wooden furniture, cushions, potted plants, and a hot tub beside a white house.

Three white-framed sash windows looking out to a sunlit backyard with a wooden fence and house beyond.

Modern small house with white and wood-paneled exterior, tall windows, and black door in a garden setting.

Empty wooden deck with railing surrounded by green pine trees in sunny outdoor setting.

Glass sunroom attached to white house under construction with ladder and tools on ground.

Side view of a gray house with white trim, windows, and bushes along the foundation on a sunny day.

Wet stone patio with stairs leading to a wooden deck and green lawn with bushes in the background.

Covered patio with brown cushioned chairs, a fireplace, ceiling fan, and a swimming pool in the background.

Wooden frame structure being built in front of a large two-story house with gray siding and white trim.

Glass sunroom attached to brick house with wicker furniture and ceiling fans, surrounded by greenery.

Back patio with wooden deck and pergola attached to beige house under clear blue sky.

Modern kitchen with white upper cabinets, green lower cabinets, stainless steel appliances, and wood flooring.

Modern kitchen with white cabinets, marble island, stainless steel appliances, and hexagon tile backsplash.

Shower with blue and gold marble tiles, hexagonal floor tiles, built-in shelf with bottles, and bronze showerhead.

Small bathroom with marble floor, black vanity, lit mirror, toilet, and bathtub with floral shower curtain.

Worker on ladder painting upper part of house beige while lower part is brown under blue sky.

A person sweeping a two-level wooden deck outside a gray house with white trim windows.

Corner bathtub with gray tiles, silver faucet, soap dish, and large windows showing autumn trees outside.

Modern glass shower with marble tile walls and hexagonal floor tiles, built-in seat, and silver fixtures.

White right arrow on teal circular background, indicating forward or next action.

Why Choose Blue Lake?

Blue outline of a house with a location pin symbol on the left side.

Local Family-Owned Expertise in Washington State

We, a locally family-owned and operated renovation company in Washington state, bring a wealth of experience. We've successfully completed thousands of construction projects. When you choose us, you're not only selecting a team; you're partnering with experts who understand the local scene, offering top-notch service with a straightforward and reliable approach.

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Collaborative Design Precision

We collaborate closely with architects and designers in the initial stages, ensuring a workable concept within budget before committing to the developed design. This collaborative approach not only saves time but also minimizes re-design costs. Our builders are prepared to tackle any task, motivated to surpass your expectations.

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Innovative Communication and Transparency

To enhance communication and transparency, we've developed a construction project management app. This tool provides instant updates for all parties involved, giving you a hands-on experience with the build of your home. This commitment to efficient collaboration and technological innovation sets us apart, ensuring a seamless and satisfying construction experience for our clients.

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Stress-Free Build Experience

You will have one point of contact who manages all aspects of your project from planning to completion. Their goal is to create an exciting and stress-free experience by understanding your requirements, delivering a quality service through clear communication to you and the trades, and then project managing the build.

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Fixed Price and Project Duration Assurance

We provide you with a detailed scope of work with a fixed price and a fixed duration. We deliver on time and on budget.

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Total Peace of Mind

We take care of everything from the moment we meet you until we deliver a finished and professionally cleaned beautiful home, all with great attention to customer satisfaction.

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Unified and Skilled Teamwork

Our team includes builders, handymen, plumbers, electricians, painters, carpet installers, kitchen makers, landscape experts, and professional cleaners. All working together and communicating on one platform.

Two-story house with a beige garage door, stone accents, lawn, and wooden fence under a blue sky.

Not sure how to start? Get a free consultation.

One of our experts will get back to you as soon as possible.

(206) 786-5988

ADU Questions, Answered

How much does an ADU cost on Mercer Island?

Most attached ADUs and garage conversions on Mercer Island run $150,000–$300,000. Detached ADUs (DADUs) typically run $250,000–$450,000+ depending on size, site conditions, utility connections, and finishes. Blue Lake builds on a fixed-price, fixed-duration contract, so the number you sign is the number you pay.

How long does it take to build an ADU?

Plan on 4–8 months of construction after permits are issued. Mercer Island's permit review is generally quicker than Seattle's, so a realistic start-to-finish timeline is 8–12 months including design and permitting. We manage the whole process for you.

Can I build an ADU on my lot?

Most likely yes. Under Washington's HB 1337 and Mercer Island's updated residential code, most single-family lots can now have up to two accessory dwelling units. We verify your lot's zoning, setbacks, and utility connections before design begins and handle the full permit package.

Can I rent out my ADU?

Yes. Long-term ADU rentals are allowed on Mercer Island and throughout King County, and Mercer Island ADUs commonly rent for $2,000–$3,200+ per month thanks to top-rated schools and quick I-90 access to both Seattle and Bellevue. We'll flag any owner-occupancy or short-term rental rules that apply to your address.

Serving all of Mercer Island — the North End, Mercerdale, First Hill, East Seattle, and the South End — plus neighboring Bellevue, Newcastle, and Factoria. Licensed Washington general contractor #BLUELLC760C3.

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