A well planned home addition or ADU can change how your entire property works. Instead of squeezing one more function into an already overloaded floorplan, you add the space you actually need: a primary suite, a larger family room, a proper home office, a guest suite, or a rentable unit. For many of our clients in Seattle and on the Eastside, an addition or ADU is the move that lets them stay in the neighborhood they worked hard to get into.
Cornell's Quality Construction has been building home additions and ADUs across Greater Seattle and the Eastside for more than 30 years. We understand the zoning, permitting, structural, and logistical complexity of adding onto an existing home in this market. Our team pairs European-style craftsmanship with disciplined project management so that a complicated addition feels controlled instead of chaotic.
Whether you are planning a second story in Wallingford, a family room addition in Shoreline, or a detached ADU in Bellevue or Kirkland, our job is the same. Design and build a structure that looks like it has always been there, works for how you live, and meets or exceeds local code.
Who We Work With
Most of our additions and ADU clients fall into a few clear groups:
- Families in Seattle and the Eastside who love their location but are out of space and do not want to move farther out.
- Homeowners planning for multigenerational living who need a self contained suite for parents, adult children, or long term guests.
- Owners who want a detached backyard cottage or ADU for rental income while keeping the main house private.
- Clients who want a real home office, studio, or hobby space that is physically separated enough to focus.
- Long term owners who intend to age in place and need a main floor bedroom suite and accessible bath.
They are typically thoughtful, mid to high income homeowners who understand that a home addition or ADU is a serious investment. They want a contractor who can manage zoning, engineering, and permitting, not just pour a slab and hope it passes inspection.
What a Cornell's Addition or ADU Project Includes
Every site and jurisdiction is different, but a typical addition or ADU project in Greater Seattle or the Eastside includes:
Feasibility and Planning
- Reviewing your lot, existing house, and local zoning rules to understand what is actually possible.
- Discussing options: second story additions, rear or side additions, bump outs, attached ADUs, or detached DADUs.
- Identifying constraints early, such as setbacks, height limits, lot coverage, parking requirements, and tree protection.
Design and Engineering
- Working with your architect or our design partners to create a layout that meets your goals and fits the existing structure.
- Structural engineering for new foundations, beams, and framing that tie the new work into the old house correctly.
- Detailing exterior elevations so the addition or ADU feels like part of the original home, not an obvious bolt on.
Permitting and Coordination
- Preparing and submitting permit sets to Seattle, Bellevue, Kirkland, or your local jurisdiction.
- Responding to plan review comments and coordinating any required revisions.
- Managing inspections and sign offs for foundation, framing, mechanical, electrical, plumbing, and final occupancy.
Construction and Finish Work
- Foundation work appropriate to the site: slabs, crawl spaces, or full basements where feasible.
- Framing, roofing, and weatherproofing that integrate with existing structures without creating future leak points.
- Full interior build out: insulation, drywall, trim, cabinets, flooring, tile, and fixtures to match the level of the main home.
You are not just adding square footage. You are adding long term value and functionality to one of your largest assets, and we treat it that way.
Types of Additions & ADUs We Build
We handle a wide range of addition and ADU projects across Greater Seattle and the Eastside:
Primary Suite Additions
New main floor or second floor suites with a bedroom, walk in closet, and primary bath. These are popular in older Seattle homes that never had a true primary suite.
Family Room and Kitchen Expansions
Rear or side additions that allow for larger kitchens, casual dining, and family gathering spaces. Often combined with reworked decks or patios to improve indoor outdoor flow.
Second Story Additions
Adding a full second level on top of a one story home in neighborhoods like Shoreline, Ballard, and parts of the Eastside. This requires significant engineering and careful integration with the existing structure and foundation capacity.
Attached ADUs (AADUS)
Self contained units within the existing envelope or as an attached wing: separate bedroom, bath, small kitchen, and dedicated entry. Ideal for long term guests or family.
Detached ADUs / DADUS and Backyard Cottages
Smaller stand alone structures in the backyard that serve as rentals, guest houses, or offices. These demand precise site planning to meet setbacks, height limits, parking rules, and utility connections.
Garage Conversions and Over Garage Additions
Converting existing garages into conditioned living space or building new living space above them, with structural upgrades as required.
High-Quality Construction Applied to New Space
Additions and ADUs expose bad work quickly. What we build has to tie into the original house and still look and feel right from every angle. Our focus on attention to detail shows up in:
- Rooflines, soffits, and trim that align cleanly with the existing structure.
- Windows and doors that are sized and placed in proportion to the rest of the house.
- Siding, flashings, and weatherproofing details that keep water out and age well in our climate.
- Interior transitions that feel seamless: consistent floor heights, trim profiles, door styles, and finish quality.
We pay as much attention to how the addition or ADU connects to the original house as we do to the new space itself.
Where We Build Additions & ADUs
We design and build additions and ADUs throughout our service area:
- Seattle: Capitol Hill, Queen Anne, Ballard, Fremont, Wallingford, Green Lake, Ravenna, Madison Park, Madrona, West Seattle, Magnolia, and other neighborhoods where lots are tight and zoning is nuanced.
- Eastside: Bellevue, Kirkland, Redmond, Sammamish, Issaquah, Mercer Island, Medina, Clyde Hill, Hunts Point, Yarrow Point, Newcastle, Woodinville, and Bothell, where lots may support larger additions or detached ADUs.
- North & South King County: Shoreline, Lake Forest Park, Kenmore, Renton, Kent, Auburn, Federal Way, and nearby suburbs, where many homes have room to expand and zoning supports creative addition and ADU solutions.
Because we work across all of these jurisdictions, we understand not only the written rules but also how local reviewers and inspectors tend to interpret them.
Our Additions & ADUs Process
Additions and ADUs involve more steps than a standard interior remodel. Our process keeps them organized and predictable:
- Initial consultation and feasibility
- We walk your property, review the existing house, and listen to what you want the new space to solve.
- We discuss high level feasibility for an addition or ADU on your lot, including rough thoughts on direction, size, and potential constraints.
- Concept design and budgeting
- Working with an architect or designer, we develop concept plans that fit your goals and respect zoning limits.
- We prepare a preliminary budget range based on the concept so you can decide whether to move forward before detailed drawings.
- Detailed design, engineering, and permits
- The design team develops full plans. Structural engineering is completed where required.
- We prepare and submit permit applications, then manage responses and revisions with the city.
- Construction planning
- Once permits are in motion, we finalize schedule, phasing, site logistics, and how we will protect existing areas of your home and yard.
- If you will live in the home during construction, we plan access routes, temporary protections, and staging accordingly.
- Construction
- We build the foundation, frame the structure, and tie into existing roof and walls with careful weatherproofing.
- Mechanical, electrical, and plumbing systems are installed and inspected.
- Interior finishes are completed to match or intentionally complement the main house.
- Completion and handoff
- We walk the new space with you, test systems, review finishes, and develop a punch list if needed.
- You receive information on warranties, systems, and maintenance for your new addition or ADU.
Budget, Timeline, and Expectations
Additions and ADUs are more complex than interior remodels. There are real cost and time drivers we address directly:
- Design, engineering, and permits are a meaningful part of the budget. We structure the process so you have clear checkpoints before committing to the full spend.
- Site conditions, access, and existing structure all affect cost. Tight urban lots, steep slopes, and difficult soil can require additional work.
- Timelines are longer than a single room remodel. We will give you realistic expectations for design, permitting, and construction instead of optimistic guesses.
Our clients are not looking for the cheapest ADU builder or addition contractor. They want a partner who can get the project approved, built correctly, and finished to a standard that matches the rest of their home.
Why Homeowners Choose Cornell's for Additions & ADUs
Homeowners across Seattle and the Eastside choose Cornell's for additions and ADUs because we:
- Have decades of experience integrating new structures with existing homes in this region.
- Understand local zoning, permitting, and inspection practices and manage them on your behalf.
- Deliver high quality, durable construction that looks like it was always part of the house.
- Keep job sites organized and communicate clearly about progress, decisions, and tradeoffs.
- Treat additions and ADUs as long term investments in your property, not shortcuts to quick square footage.
Start Planning Your Addition or ADU
If you are considering a home addition, second story, or ADU in Seattle, on the Eastside, or in North or South King County, you need more than a contractor who can frame a box. You need a builder who can navigate rules, design constraints, and construction complexity while protecting the home you already live in.
Contact Cornell's Quality Construction to discuss your addition or ADU project. We will review your lot, talk through options, and give you a clear view of what it will take to add the space you need in a way that fits your home and your life.