Pre-Feasibility Report

11340 Alton Ave NE — DADU + 2-Unit Conversion

Site analysis, zoning compliance check, utility study, environmental review, and property record summary for the proposed conversion of a 1949 single-family residence into two rentable units, plus two ~1,000 sf DADUs built in the rear yard.

NR2 zone10,480 sf lotOlympic HillsSDCI jurisdiction

Property Overview

Single-family residence on a generally level 10,480 sf lot in the Olympic Hills neighborhood of north Seattle.

Address: 11340 Alton Ave NE; Seattle, WA 98125
Lot Size: 10,480 sf; ≈ 0.24 ac
Living Area: 1,700 sf; 850 main + 850 basement
Built: 1949; Condition: Very Good
Beds / Baths: 3 / 1.75; 1 full + 1 ¾
Garage: 240 sf; Attached
Zoning: NR2; Neighborhood Residential
Parcel: 407780-0218; King County

Location

Olympic Hills, north Seattle. Drag the map to explore the block.

47.7176° N, 122.3025° W (approx)

Photo gallery

Video tour

Project Goal

Increase the site from one rental unit to four — two units inside the existing footprint, two new detached units in the rear yard.

Phase 1 · SFH Rehab

Split the main house into 2 separate rentable units

Upstairs (main floor) becomes Unit A — 850 sf, 2 bed / 1 bath, retains original front entry.
Basement becomes Unit B — 850 sf, 1 bed / 0.75 bath. Needs a code-egress sleeping room and a private exterior entry.
Two scope items: (1) convert two basement windows to true egress, (2) swap the existing upstairs back door and side window to free up the basement entryway.

Phase 2 · DADU Build

Build 2 detached accessory dwelling units in the back yard

Two ~1,000 sf two-story DADUs, each with an integral garage.
Driveway access along the south side of the lot, extending from Alton Ave NE back to the DADU garages.
All-electric mechanical (heat pump + induction) to align w/ 2024 Seattle energy code and skip new gas runs.
Per Seattle SMC 23.44, NR zones allow up to 2 ADUs per lot (≤ 4 dwelling units total on this parcel).

Item 1 — Basement window-to-egress conversions

Per IRC R310, every basement sleeping room must have an emergency egress window: min 5.7 sf net openable, 24″ height, 20″ width, and sill no higher than 44″ above floor. Two current basement rooms have glass-block / hopper windows that need to be enlarged to egress wells.

Basement bedroom egress upgrade — Bedroom #1 — replace existing glass-block window with code-compliant egress window + window well.

Basement office / future bedroom egress — Bedroom #2 (currently office) — enlarge existing hopper window into a full egress + window well.

Item 2 — Entry swap (door ⇄ window)

Today the basement has only a bulkhead-style basement entrance, and the upper unit has a sliding back door onto the deck. To give Unit B a real ground-level entry, we'll convert the side window into a new door and convert the upstairs sliding door into a window so the units are properly separated.

Upper-unit back door (to convert to window) — Upper-unit back door → window. Eliminates conflict with the basement entry path below.

Existing basement bulkhead entry. Removing the upstairs door will free up this entryway.

Side window to convert to basement-unit door — Side window → entry door. Provides an entrance to the backyard & patio.

Item 3 — Backyard DADUs (×2) + driveway

Two roughly identical ~1,000 sf DADUs sited at the rear of the lot. Each has an integral single-car garage facing a new shared driveway extending from the south side of the existing driveway.

Backyard area where DADUs will be sited — Existing backyard. Deck can be made smaller or removed altogether; lawn area becomes the DADU footprint.

Side yard along DADU driveway path — Side yard — proposed driveway extension runs along here to the rear DADUs.

Conceptual site plan · not to survey scale

Zoning

The lot is NR2 (Neighborhood Residential 2). As of Seattle's October 2025 ADU update, the NR1/NR2/NR3 designations have been consolidated into a single NR zone with up to 4 units allowed per lot.

Zone

NR2 / NR

Units allowed

Up to 4

Max DADU height

18′ (23′ w/ green)

ADU side / rear setback

5′

Front setback

20′ avg

Max lot coverage

35% of lot

≈ 3,668 sf of building footprint on this 10,480 sf lot

Max FAR

0.8 – 1.2

Floor Area Ratio = total building floor area ÷ lot area

Off-street parking

None required

Project compliance check

Pre-feasibility sanity check, not a formal pre-app review. All numbers should be confirmed with SDCI Pre-Submittal Conference.

≤ 4 dwelling units on the lot

Pass

Main house split (2) + 2 DADUs = 4 units, exactly at the allowed cap.

DADU side & rear setbacks ≥ 5′

Pass (design driven)

Conceptual site plan keeps both DADUs ≥ 5′ from east and south property lines.

Combined lot coverage ≤ 35%

Likely pass

Existing house ≈ 850 sf footprint + 2,000 sf DADU footprints ≈ 2,850 sf ÷ 10,480 sf = 27%.

FAR (Floor Area Ratio) within allowance

Likely pass

Existing house = 1,700 sf + 2,000 sf DADUs = 3,700 sf total floor area. At 4 units on 10,480 sf, allowed FAR is ~1.2 → 12,576 sf max.

DADU max height 18′ (23′ w/ green building)

Design driven

Two-story DADU at 18′ flat-roof or 23′ pitched-roof w/ green features is achievable.

Parking

Pass

No off-street parking required for ADUs since 2019; integral garages are an amenity, not a requirement.

Tree protection (SMC 25.11)

Verify

Perimeter trees identified — most are outside the DADU footprint. Tier 2/3 tree confirmation required via arborist report.

On paper, the four-unit configuration is allowed by current Seattle code. A formal pre-application meeting with SDCI is the recommended next step.

Utility Study

Initial assessment of feasibility for connecting two new DADUs to existing utility infrastructure. SPU water/sewer map below is pre-centered on 11340 Alton Ave NE.

11340 Alton Ave NE

Source: Seattle Public Utilities — Water & Sewer interactive map. Use the SPU search bar inside the map to switch addresses.

Utility-by-utility feasibility

Sewer

Likely feasible

Public sanitary sewer in Alton Ave NE per Seattle Public Utilities. Two new DADU laterals can tap to the existing main; SPU side-sewer permit + capacity check required.

Sewer capacity

Confirm w/ SPU

Olympic Hills basin has known wet-weather flow; SPU will require a sewer availability certificate. Side-sewer cards on file should be pulled before design.

Water

Likely feasible

Served by Seattle Public Utilities. Each DADU needs a separate 3/4" meter (or shared meter w/ submeters). Upsize from existing 5/8" may be required for fire-flow at 3 units total.

Stormwater / drainage

Design-driven

Adding ~2,000 sf of impervious roof + driveway extension triggers Stormwater Code (SMC 22.800). On-site infiltration (drywell / dispersion trenches) likely; soils in this part of Olympic Hills tend to be glacial till — infiltration testing required.

Power

Feasible

Seattle City Light overhead service on Alton. Adding two units will likely need a service upgrade to 200A and separate meters per unit. Underground service to DADUs typical.

Natural gas

Optional

Puget Sound Energy gas main in street; existing service to main house. DADUs can be all-electric (heat pump + induction) to avoid new gas runs and align with Seattle's 2024 energy code.

Telecom / fiber

Feasible

Wave/Astound and CenturyLink/Ziply fiber available on the block. Pulling fiber to each DADU during trenching for power is recommended.

Property Survey

Known boundary, topography, and improvement details on record. A new boundary + topographic survey is recommended before architectural design.

Lot dimensions (assessor)

~80′ × 131′ rectangle

10,480 sf ≈ 0.24 ac

Year built

1949

76 years; Very Good condition

Bedrooms

Bathrooms

1 full + 1 ¾

Living area

1,700 sf

850 sf upper + 850 sf finished basement

Garage / deck

240 sf / 400 sf

Recommended additional studies

Boundary & topographic survey (1′ contours) by a licensed Washington PLS.
Geotechnical infiltration test — required for stormwater BMP design (drywell vs. dispersion trenches).
Tier 2/3 arborist report identifying exceptional trees within 25′ of any disturbance.
Side-sewer card pull from SPU + sewer availability certificate.
Locate utilities (locate ticket via 811) before final DADU siting.

Environmental Impact

Tree inventory and Environmentally Critical Area (ECA) screen.

Tree inventory

8 existing trees on the lot — four mature trees and four mid-size trees, all along the perimeter. One mid-sized tree sits slightly further in and is the only planned removal.

Rear yard trees on the back / east property line — East property line — mature evergreens form a privacy screen. These stay; DADUs maintain a 5′+ buffer.

View of backyard trees from the deck — Backyard from the deck — mix of mature corner trees and mid-sized interior trees. The center-rear specimen is the planned removal.

Mature · keep4

Large perimeter trees, one near each corner of the lot. Outside DADU footprints; protect during construction per SMC 25.11.

Mid-sized · keep3

Mid-sized trees scattered along the perimeter, outside the DADU footprints. Arborist assessment recommended before excavation begins.

Mid-sized · remove1

One mid-sized tree inside the planned DADU / driveway envelope. Confirm size class with an arborist.

Environmentally Critical Areas (ECA) screening

Olympic Hills sits on Lawton glacial till; the lot itself is generally level. The Seattle GeoData search below is pre-filtered to environment-tagged datasets within the parcel's bounding box, with 11340 Alton Ave NE pre-filled as the address — browse the results to see any mapped ECAs.

Sources: Seattle ECA hub · SDCI ECA code · Steep slope layer

Steep slope

None mapped

Landslide-prone

None mapped

Liquefaction

None mapped

Peat settlement

None mapped

Wetlands

None mapped

Riparian corridor

None mapped

Fish & wildlife habitat

None mapped

Flood-prone area

None mapped

ECA mapping is advisory; final designation confirmed by topographic / geotechnical survey at permit stage.

Property Records

Full record summary from the King County Assessor (parcel 407780-0218). Source data is authoritative — link to original record below.

Parcel Number

407780-0218

Site Address

11340 Alton Ave NE, Seattle, WA 98125

Owner

Kim, Won Suk + Yoohyun

Jurisdiction

Seattle

Property Type

Residential

Present Use

Single Family (Res Use / Zone)

Highest & Best Use (Land)

Single Family

Lot Size

10,480 sf (0.24 ac)

Topography

Generally level (per assessor)

Zoning

NR2 (Neighborhood Residential 2)

Restrictions

None on record

Year Built

1949

Stories

Living Sq Ft (Total)

1,700

First Floor

850 sf

Finished Basement

850 sf

Total Basement

850 sf

Bedrooms

Full Baths

3/4 Baths

Building Grade

7 — Average

Building Condition

Very Good

Heat Source / Type

Gas — Forced Air

Fireplaces

2 (1 single-story + 1 multi-story)

Attached Garage

240 sf

Deck

400 sf

Water System

Water District

Sewer System

Public

Road Access

Public, paved street

Last Sale

May 22, 2018 — $747,000 (Statutory Warranty Deed)

2026 Tax Year Appraisal — Land

$532,000

2026 Tax Year Appraisal — Improvements

$463,000

2026 Tax Year Appraisal — Total

$995,000

View original King County record

References

Public sources used to ground the analysis above. Add more here as the project moves forward.