MODERN ADDITIONS
Additions are where most contractors get into trouble — roof tie-ins, foundation underpinning, and continuity of the weather envelope are unforgiving details. We treat additions like new construction grafted onto an existing home, with engineered connections and fully blended exteriors.
Deliverables
SPEC_04.A- 01Structural + foundation engineering
- 02Roof + envelope tie-in detailing
- 03Underpinning where required
- 04Selective demolition + protection
- 05New space framing, MEP, finish
- 06Whole-home punch list
Spec sheet
SPEC_04.B- +LVL & steel structural members
- +Self-adhered roof membrane
- +Continuous exterior insulation
- +Quartz or sintered-stone surfaces
- +Wide-plank engineered hardwood
DETACHED ADU
Stand-alone backyard cottages built from the ground up — engineered for rental income, multi-generational living, or your home office.
02 //ATTACHED ADU
Seamlessly integrated additions — basement, ground-floor, or wing conversions that maintain the architectural integrity of your existing home.
03 //GARAGE CONV.
Convert an under-used garage into a high-end studio, office, or rental quarter without surrendering your driveway.
ARCHIVE_02 / Field Reports
Recent Additions projects
Additions questions
SPEC_04.FAQReal questions from Seattle, Bellevue, Kirkland and Eastside owners evaluating a modern additions project. Code references are current as of 2025 — confirm with us during feasibility.
Q.01
Do I need a permit for a home addition in Seattle?
Yes — any addition that increases floor area, alters the building footprint, or modifies structural framing requires an SDCI building permit. Mechanical, plumbing, and electrical sub-permits run in parallel. We handle the entire SDCI submittal and inspection cycle in-house.
Q.02
How does a second-story addition work on a Seattle Craftsman or 1920s home?
We start with a structural assessment of the existing foundation and stud walls — Craftsman-era framing was usually 2x4 at 16 in. on center with no shear panels. Most second-story additions require foundation underpinning, a new continuous load path, and shear-wall retrofits to meet current Seismic Design Category D2 in Puget Sound.
Q.03
Will an addition affect my lot coverage or setbacks?
It will. Seattle single-family zones cap lot coverage at 35% and require 5–20 ft setbacks depending on zone. We run a buildable-area diagram during feasibility to confirm the addition fits before any design fees are spent.
Q.04
How do you tie a new roof into an existing one without leaks?
Pacific Northwest rain is brutal on bad tie-ins. We use a self-adhered ice-and-water membrane lapping a minimum of 24 in. onto the existing roof, install kicked-out flashing at every wall-to-roof intersection, and re-shingle a full course beyond the new framing line. Every detail is built per ASHRAE 160 moisture-control guidance.
Q.05
Can I live in the home during an addition project?
Usually yes for side or rear additions, often no for second-story or whole-home reframes. We build a temporary weather barrier (TWB) and dust partition, sequence demolition to one zone at a time, and provide a rotating-room schedule so families can stay through most of the build.
Q.06
What's the typical cost of a home addition in the Puget Sound area?
Our addition range is $220K–$750K+ depending on scope. A 400 sq ft kitchen extension lands around $250K–$320K; a full second-story typically runs $450K–$700K. Addition cost-per-square-foot is higher than new construction because of tie-in and protection work.
Official resources & sources
- Source ↗WSEC 2021 Residential Energy CodeEnvelope, HVAC, hot-water and air-sealing requirements
- Source ↗WA HB 1110 — Middle housing lawStatewide middle-housing zoning preemption
- Source ↗WA HB 1337 — ADU statewide rulesTwo-ADU baseline, no owner occupancy, parking limits
- Source ↗WA Dept. of Commerce — ADU resources
- Source ↗WA L&I contractor license lookupVerify Golden State ADU Builders Inc · GOLDESA747LZ
- Source ↗Washington State Building Code CouncilStatewide adopted codes (IRC, IBC, WSEC)
MODERN ADDITIONS builder in every Puget Sound city
In-depth guides on modern additions
- cost · 7 minFive hidden ADU costs that wreck Puget Sound budgets
- roofing · 8 minRoofing for Pacific Northwest rain: materials, details, lifespan
- foundation · 9 minFoundation inspection checklist for Puget Sound homes
- process · 7 minFixed-price vs time-and-materials: which contract to sign
- cost · 9 minSeattle ADU financing: 7 loan products compared (2026)
- foundation · 9 minPacific Northwest foundation types: slab, crawl, pier, basement (2026)
- process · 7 minSeattle electrical panel upgrade for ADUs: 100A to 400A (2026)
- decks · 7 minEastside deck cost & permits: cedar, IPE, composite (2026)
Frequently asked
What is included in MODERN ADDITIONS?
Our MODERN ADDITIONS scope includes design through completion: feasibility, schematic design, construction documents, permit submittal and review management, construction with one self-performed framing crew plus L&I-licensed MEP subcontractors, all required WA inspections, certificate of occupancy, and warranty. One contract, one license number, one warranty — not a coordination headache across five vendors.
How is MODERN ADDITIONS different from a generic remodel?
MODERN ADDITIONS carries code, permit, and construction details that a one-off remodeler hasn't repeated enough times to optimize. Examples: IECC R406 envelope path selection, fire separation when units are within 5 ft, side-sewer slope requirements, IRC Appendix AQ when permitted as a tiny house, separate utility metering rules. We do this scope every month — repetition is what makes our first-cycle approval rate and our schedule reliability stand out.
What is the typical price range for MODERN ADDITIONS?
MODERN ADDITIONS pricing in 2026 lands $330–$380/sqft all-in depending on size, lot conditions, finish tier, and AHJ. The wide range is real — small lots with difficult access, soils requiring pier-and-beam, or service upgrades push the upper end; flat lots with good access and a catalog plan land at the lower end. Our feasibility report gives you a ±15% number tailored to your specific lot in two weeks.
What permits does MODERN ADDITIONS require?
Standard MODERN ADDITIONS permit stack: building permit (covers structural, mechanical, plumbing under combined review), side-sewer permit, water meter capacity check, electrical permit, drainage review where applicable, addressing application. Critical-areas review triggers on lots mapped for steep slope, wetland, riparian, or landslide hazard. We pull all permits as a pass-through line in our contract — not marked up.
Go deeper: Read the King County side sewer permits for ADUs: cost & timeline (2026) guide
How long does MODERN ADDITIONS take from contract to completion?
9–14 months end-to-end is typical for MODERN ADDITIONS: 6–8 weeks design, 8–16 weeks permit, 18–26 weeks construction. Faster on catalog plans with pre-app meetings. Schedule variance is driven by AHJ workload, owner selection pace, and any field-discovered conditions. We publish a Gantt at contract and update it weekly.
Go deeper: Read the Eastside deck cost & permits: cedar, IPE, composite (2026) guide
What's the warranty on MODERN ADDITIONS?
One year workmanship, two years mechanical (HVAC, plumbing rough, electrical rough), 10 years structural per RCW 4.16.310. Manufacturer warranties pass through directly: LP SmartSide siding 50-year, GAF Timberline HDZ shingles, Marvin window 20-year, Bosch/GE appliances 1-year. Documented in a written warranty packet at closeout.
Do you self-perform MODERN ADDITIONS work or subcontract everything?
We self-perform framing, exterior weather barrier, interior trim, and project supervision with W-2 carpenters. MEP trades (plumbing, electrical, HVAC) are subcontracted to L&I-licensed specialty contractors we've worked with 5+ years — WA requires separate PL, EL, HVAC/R licenses that a GC cannot legally self-perform without them.
What's the change-order policy on MODERN ADDITIONS?
Every change order is written, priced, and signed before work proceeds. We never charge T&M without a not-to-exceed. Owner-initiated changes carry 15% OH&P; field-discovered conditions (rotted sheathing, undocumented buried oil tank, unmapped sewer) are priced at cost +10% with photo documentation. Average MODERN ADDITIONS project sees 4–7 change orders totaling 3–6% of contract value.
What financing works for MODERN ADDITIONS?
HELOC (fastest, 8.0–9.5% APR), renovation loan (Fannie HomeStyle / Freddie CHOICERenovation, 7.1–7.9%), construction-to-perm (best for ground-up), or cash-out refi (only if existing rate above market). WSHFC's ADU pilot at ~3.5–4.25% applies to ADU-scoped MODERN ADDITIONS projects for income-qualified owners.
Do you provide references for MODERN ADDITIONS work?
Yes — we provide 5+ MODERN ADDITIONS references on request: past clients in your target city, your target neighborhood when possible, and at your target scope. References include name, phone, project address, and a one-paragraph project summary. We refresh the reference list quarterly so the contacts are recent and reachable.