Home additions in Seattle, Bellevue and Kirkland: what we design
A home addition adds floor area to the house you already own. It can be a rear or side extension, a second story over all or part of the house, a dormer that gives a low attic full headroom, or a garage, porch or sunroom brought inside the heated envelope. Seattle's permit department, SDCI, also lists a house lift among additions and remodels: the house is raised so that a full-height floor fits underneath. DeZZign designs these projects for owners in Seattle, Bellevue, Kirkland and the rest of the Puget Sound area, from the first check of what the lot and the structure allow to permit drawings and visits during construction.
About our portfolio, plainly: it has no house addition to show, and the houses in this page's gallery are new buildings in Ukraine. The closest work differs in type but shares the method. In Residence 11, a top-floor apartment in Kharkiv, the pitched roof left enough height for a second level, and 65 m² on the developer's plan became 103 m², terraces included; the sections are in our article 8 interesting design solutions. For Copper Hall we designed the reconstruction of a 1990s building in central Kharkiv, with a second floor cantilevered over the first. For a trichology clinic in the same city, the entrance group was designed to grow by 10 m².
The survey comes first: what the existing house can carry
Every addition leans on the old house somewhere: on its foundation, an exterior wall, the roof or the systems. Before any sketch we find out what that house actually is. Original drawings help when they exist, but they show what was planned decades ago, not what was built and changed since. Our survey of the existing building covers:
- a measured drawing of the plans, sections and elevations, with floor and ceiling heights;
- the foundation: its type, depth and condition, often checked with a small test pit where the addition will meet it;
- the framing: joist and rafter sizes and spans, posts and beams, and whether the roof is site-framed or built with engineered trusses, which cannot be cut without a redesign by the engineer;
- moisture and movement: stains in the crawlspace or attic, rot at the sills, cracks that point to settlement;
- traces of earlier work done without a permit, better found now than by an inspector mid-build.
The structural engineer we work with reads the same findings and tells us what the foundation and walls can take. That answer often settles the choice between building up and building out before design time goes into the wrong option. On older houses, testing for lead paint and asbestos before demolition is a sensible first step; our floor plan redevelopment page covers what Seattle homes of past decades tend to hide inside their walls.
Building out or building up: rear, side and second-story additions
A ground-floor extension is usually the simpler structure: a new foundation, walls and roof beside the old ones, with the existing house left largely as it is. It costs lot area, though. In Seattle the combined footprint of the house, covered areas and decks 36 inches or more above the ground counts toward a lot coverage limit, and the percentage depends on the zone. Setbacks keep it away from property lines, and on a narrow city lot that often leaves the rear garden as the only direction to grow.
A second story keeps the garden but loads the old house. SDCI notes that a second-story addition may require structural upgrades to the existing house. In practice that can mean new footings under concentrated loads, new posts and beams where the upper walls do not line up with the lower ones, and new shear walls, because in the Puget Sound region a house must also resist earthquakes and wind. The roof comes off for weeks, so the work sequence has to protect the rooms below from rain. A full stair with its landings also takes a real piece of the ground floor, and we find its place first, because it decides the plan of both levels.
Between the two sit partial solutions: a second story over the garage or one wing only, a dormer along one side of the attic, or a house lift that creates a full-height lower floor. We sketch the options the survey allows and compare them on the same terms: area gained, garden lost, structural work, weeks with the roof open and the view from the street.
Where old meets new: settlement, roofline and floor levels
Most problems with an addition appear at the junction rather than in the new room, so the junction gets its own drawings.
Settlement. The soil under the old foundation finished compressing long ago, while the soil under the new footing will compress during its first years under load. If the two are joined rigidly without an engineered connection, the difference shows up as a crack along the line where they meet. The engineer chooses between doweling the new footing into the old one and separating them with a joint, and the design then hides or expresses that line in the siding and finishes.
The roof. Where a new roof meets an old wall or runs into the old roof, water looks for a way in. We avoid valleys that collect fir needles, draw the flashing at every roof-to-wall joint and check that gutters and downspouts can carry the added roof area. Matching the pitch is the easy part; the eave height, overhang depth and fascia profile are what make a roof look original.
Floor levels. An old floor on joists over a crawlspace and a new floor on a slab rarely end up at the same height by accident. A step of an inch or two between rooms trips people and looks like an error, so the new floor height is set from the survey and the structure is designed to meet it. If the new rooms are meant for parents or for later years, the route between old and new stays step-free.
The old exterior wall becomes an interior one. Openings cut into it need headers sized by the engineer, the old cladding is removed or stays hidden behind the new finish, and the insulation and air barrier of the addition must connect to those of the house without gaps. We draw that connection as a detail instead of leaving it to the framer.
As for appearance, one honest option continues the house exactly, with the same siding exposure, window proportions and trim. The other sets the addition apart with a recess, a change of material or a glazed link, like the transparent attached pergola we designed on Lesnoe, a new house near Kharkiv, to carry the living room toward the garden. What reads badly is the near match: siding almost the same width, a window almost the same height.
Garage, porch and sunroom conversions into living space
Turning an attached garage or a three-season sunroom into a heated room adds living space without new footprint, and it is easy to underestimate. Under the Washington State Energy Code, WAC 51-11R-50200, any nonconditioned or low-energy space that is altered to become conditioned space has to be brought into full compliance with the code.
A garage built as an unheated room usually has an uninsulated slab set below the house floor, uninsulated walls and a large door opening. Converting it means insulating the floor or building a new one at house level, insulating walls and ceiling, and filling the door opening with a wall that sits on a proper footing, because a slab edge is not designed to carry one. If part of the garage stays a garage, the fire separation between it and the living space is redrawn.
A sunroom has the opposite problem: lots of glass that loses heat through a Seattle winter and overheats on a July afternoon. Meeting the code then means better glazing, fewer glass panels, or the envelope calculation the code allows, which compares the heat loss of the house before and after. Where the room can stay outdoors, a covered deck or terrace may serve better: see terrace and outdoor living design.
Heating, electrical capacity and drainage for a bigger house
The same WAC section says that additions shall not create an unsafe or hazardous condition or overload existing building systems. A new 400 sq ft room adds heat loss, lighting, outlets and often a bathroom or laundry, all running through equipment sized for the original house. We check each system with the engineer or the trade that will sign for it:
- heating: a furnace and ducts sized for the old house often cannot push enough air to the far end of an addition, and a separate ductless heat pump for the new rooms is one answer; new ducts installed as part of an addition have to meet the duct requirements of the energy code;
- the attic: where an addition larger than 150 sq ft adjoins existing ceilings with attic spaces, those attics must meet the code's envelope requirements, which usually means more insulation over the old rooms as well;
- electrical service: a heat pump, an induction range or a car charger may exceed what the existing panel can supply, and a service change may need its own application to Seattle City Light;
- plumbing and drainage: a new bathroom needs a drain with enough fall to the existing building sewer, and the added roof area sends more rainwater to downspouts that must discharge somewhere the city accepts.
These checks belong to the concept stage: a panel upgrade or a new heating zone found after approval costs a redesign and a permit revision.
Permits for a home addition in Seattle, Bellevue and Kirkland
In Seattle a house addition needs a construction addition/alteration permit. A small one may qualify for a subject-to-field-inspection (STFI) permit; SDCI describes these as construction permits for small projects that meet certain criteria, typically reviewed and issued within a few business days (Building Connections, November 3, 2026). Its own example is a small single-story addition. Sites with environmentally critical areas often do not qualify, and a project that needs a geotechnical report does not.
Two rules help owners of older houses. According to SDCI, existing portions of the house that are not being remodeled do not need to be upgraded to current codes, although the structural effects of the addition must be accounted for in the design and the permit submittal. The energy code, WAC 51-11R-50200, applies to the addition as new construction without requiring the unaltered part of the house to comply, except where the existing-buildings chapter says otherwise. An addition of 150 sq ft or less is not required to earn the additional efficiency credits of section R406; a larger one is, and we choose those options at the concept stage, not at permit intake.
SDCI also notes that reconfiguring spaces or replacing wall surfaces in more than 60 percent of the existing house is likely to make the project a substantial alteration, which brings its own requirements. A large addition combined with a deep remodel of the old rooms should be checked against that threshold before the scope is fixed. Bellevue, Kirkland and unincorporated King County have their own permit departments and zoning codes, and energy code editions change, so we check the current rules for your parcel on the project date. Our architects develop the documentation to local codes; drawings that require a local license are signed by licensed professionals we work with.
Living in the house while the addition is built
Staying at home during a ground-floor extension is realistic when the sequence is planned so that the new part is built, roofed and closed against the weather first, while the old exterior wall still stands between the crew and the family. The opening between old and new is cut near the end, behind a dust barrier, and finished fast; until then the house keeps its kitchen, its bathroom and its heat.
A second story is a different case. With the roof open, the rooms below are exposed to rain and dust, and power and water are shut off in stages, so living elsewhere for that phase is often the simpler plan. Decide it before signing the contract, since it changes what each week of delay costs you. Staying means agreeing on the crew's entrance, storage for materials, closed rooms and how children and pets are kept away from the work. During supervision we visit the site regularly, check the work against the approved drawings and coordinate with the contractor on your behalf; the notes from an architect working on American job sites explain why we follow a build that closely.
What moves the timeline and cost of a home addition
For the Seattle area our design fee, the timeline and supervision are discussed individually, because two additions of the same size can be very different jobs. These are the factors that move them:
- what the survey finds: a foundation that needs underpinning or framing that needs reinforcing changes both the design and the build;
- building up rather than out, with structural upgrades and weeks of open roof;
- the permit path: an STFI permit or full plan review, a critical area, a geotechnical report;
- the systems: whether heating, the electrical service and the drains can be extended or have to be replaced;
- matching: siding, windows and trim that are no longer made need a choice between custom fabrication and a deliberate change of material;
- phasing around a family that stays at home.
We give a detailed proposal after the first consultation and a site review. The work follows four stages: site and zoning review, concept and design with 3D visualizations, permit documentation, and construction supervision. If the first stage shows that the lot or the structure will not take the addition you want, learning it there costs far less than after the concept.
Who does what on a home addition
The existing house brings questions a new building never has, so an addition involves more people than its size suggests:
- DeZZign's architects survey and draw the existing house, develop the options and the concept, prepare the permit and construction drawings and coordinate the consultants;
- a structural engineer designs the foundations, beams, connections to the old structure and lateral bracing;
- a land surveyor maps the boundaries and levels when setbacks or slopes are tight;
- the electrician and the heating designer confirm service capacity and heating loads;
- the contractor prices the drawings, plans the sequence with us and builds;
- you set the program and the budget, decide how the family lives during the works and approve each stage.
When an addition fits, and when another service fits better
An addition is the right project when the house works but is too small, when the lot has room or the structure can carry another floor, and when you want to stay where you are. If the space you need is already inside the walls, a reworked plan usually costs less than new floor area: see remodeling design. A second household with its own door is often better served by a backyard cottage or an attached unit, which we cover under ADU and DADU design.
When the survey shows that the old house needs more work than it is worth, compare the addition with a new house planned from scratch on our house plans page. If what bothers you is how the house looks rather than its size, start with facade design, and for the whole range of our building work see architecture.
