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Newcastle ADU and detached ADU rules

Newcastle ADU and DADU rules, including the 2026 code

Newcastle's current code allows up to two ADUs and now gives detached units specific size, height, alley, rear-yard, and parking rules. The unit still counts toward the site's floor-area and impervious-surface limits.

Reviewed August 14, 2026 against adopted city code and official city guidance

Quick definitions. An accessory dwelling unit (ADU) is a complete additional home on the same property. A detached ADU (DADU) is in its own building. FAR or GFA limits how much enclosed floor area the property may contain.

Bottom line

What the current rules mean

A DADU may be feasible if a 300- to 1,000-square-foot unit fits the lot's development standards. A detached ADU is capped at 24 feet high, the rear setback is generally reduced to five feet where an ADU is present, and one parking space is normally required per ADU unless the transit exception applies.

Up to 2ADUs with a principal homeAttached, detached, or converted configurations.
300–1,000Square feet per ADUA single-floor conversion can receive a narrow efficiency exception.
24 ftDetached ADU heightAttached ADUs may reach 30 feet.

What is allowed on one lot

Up to two ADUs are permitted as subordinate to a single-family dwelling when the project complies with Chapter 18.31 and the applicable zoning and building rules.

  • ADU inside or added to the principal house.
  • ADU in an existing garage or other legally established structure.
  • One or two new detached ADUs.
  • Conversion of a nonconforming structure, subject to building and life-safety review.

Official sources: NMC Chapter 18.31: accessory dwelling units · RCW 36.70A.681: limits on local ADU regulation

Size, height, setbacks, and site capacity

Maximum size
An ADU must be at least 300 and no more than 1,000 square feet. If the unit occupies one complete floor, the Community Development Director may allow only the additional area needed to use that floor efficiently.
Height
An attached ADU may be up to 30 feet. A detached ADU is limited to 24 feet, measured using Newcastle's building-height method.
Setbacks
Except for the ADU-specific rules, single-family setbacks apply. The rear setback is reduced to five feet when one or more ADUs are on the lot. Side setbacks for the ADU/middle-housing unit add to 10 feet under the current table notes, and a DADU may sit on a lot line abutting a public alley.
FAR, lot coverage, or impervious area
The ADU is included in applicable impervious-surface and floor-area limits. Building coverage, hard surface, critical-area tracts/buffers, and the base zone must be checked separately.

Official sources: NMC Chapter 18.31: accessory dwelling units · NMC 18.12.030: residential dimensions

Parking, occupancy, renting, and separate sale

Parking
One off-street space is required per ADU, independent of the principal unit's parking. No ADU parking is required within a half-mile walking distance of a major transit stop. Parking can be collocated at the parent-lot level for condominiums or a unit-lot subdivision.
Owner occupancy
No owner-occupancy requirement applies.
Short-term rental
A detached ADU may not be used as a short-term rental under Newcastle's short-term-rental code. Other units must still comply with Chapter 18.30 and licensing or operational requirements.
Separate sale
ADUs may be rented, sold as condominiums, or sold fee-simple through an approved unit-lot subdivision.

Official sources: NMC Chapter 18.31: accessory dwelling units · NMC Chapter 18.30: short-term rentals and home occupations

Utilities, fire access, and the permit path

The city will not require a separate utility connection solely because of the ADU, but the owner must follow each utility provider's capacity, meter, connection, and service rules.

  1. Confirm the zone, parent-lot unit count, dimensional standards, critical areas, trees, access, and surface calculations.
  2. Confirm the required Newcastle land-use decision and building-permit application for the proposed attached, detached, conversion, condominium, or unit-lot configuration.
  3. Submit coordinated site, architectural, structural, energy, civil, utility, and critical-area/tree documents.
  4. Resolve serving-utility conditions even where a separate connection is not required by the city.
  5. Complete all related permits, inspections, and any condominium or unit-lot recording steps.

Official sources: NMC Chapter 18.31: accessory dwelling units · NMC 19.07: permit application types

Rules that can change the answer for one property

A citywide ADU allowance does not prove that a detached unit fits on a particular lot. Resolve these items before buying plans or fixing the DADU location.

  • A site may not contain both an ADU and a Type II home occupation or home industry.
  • The five-foot ADU rear yard does not erase critical-area tracts, buffers, their additional building setback, or easements.
  • A second-story rear-facing window closer than 15 feet to the rear line must satisfy the code's translucent-window condition.
  • The ADU and its driveway, parking, walkways, and utilities consume impervious or hard-surface capacity.
  • Trees, steep slopes, wetlands, streams, drainage, fire access, and retaining walls can materially reduce the code-compliant area.

Official sources: NMC 18.12.030: residential dimensions · NMC Chapter 18.24: critical areas

Prepared for a future address check

What a reliable DADU feasibility screen must verify

Newcastle has enough codified numeric rules for a detailed address screen, but the engine must use parent-lot unit count, zone dimensions, the special ADU rear/side yards, 24-foot height, and three separate site-surface controls correctly.

Jurisdiction and zoning

Confirm the address is actually governed by Newcastle and select the correct base-zone standards.

Existing and proposed units

Count the principal dwelling, existing legal ADUs, middle-housing units, and the proposed attached or detached configuration.

Surveyed parcel geometry

Use legal lot lines, front/rear designation, alleys, access easements, and existing structures rather than a tax-map rectangle.

Capacity calculations

Test the ADU against the rules that apply locally: FAR or GFA, building or lot coverage, impervious or hard surface, and landscaping.

Mapped and field constraints

Screen critical areas and buffers, shorelines, slopes, flood hazards, significant trees, drainage, and recorded easements.

Utilities and emergency access

Verify water, sewer or septic, fire flow, hydrant and apparatus access, power, meters, and trench routes.

The screen must not treat the five-foot rear yard as permission to cross a critical-area buffer, easement, tree-protection zone, or fire-access requirement, and it must flag the Type II home-occupation conflict.

Primary sources

Official Newcastle and Washington references

These are the sources used for this guide. Check them again before filing because ordinances, fee schedules, and permit forms can change.

NMC Chapter 18.31: accessory dwelling unitsCurrent count, size, location, height, parking, tenure, utility, and compliance rules.NMC 18.12.030: residential dimensionsZone standards and current ADU rear-yard, side-yard, alley, and height notes.NMC Chapter 18.30: short-term rentals and home occupationsDetached-ADU short-term-rental prohibition and home-business rules.NMC Chapter 18.24: critical areasWetland, stream, geologic, flood, habitat, buffer, and reasonable-use rules.NMC 19.07: permit application typesCurrent administrative permit classifications, including ADUs.RCW 36.70A.681: limits on local ADU regulationWashington's current baseline for two ADUs, size, height, parking, owner occupancy, conversions, alleys, fees, and critical-area exceptions.

A practical first-pass checklist

  1. 1

    Confirm the parcel is inside Newcastle city limits and identify its current zone.

  2. 2

    Map the principal home, lot lines, easements, trees, utilities, driveway, critical areas, and buffers on one current survey.

  3. 3

    Test the ADU footprint against setbacks, height, floor-area or coverage limits, hardscape, parking, and construction access.

  4. 4

    Ask the utility providers to confirm water, sewer, power, meter, side-sewer, and capacity requirements before fixing the design.

  5. 5

    Have the city confirm the complete land-use, building, fire, right-of-way, tree, and utility permit list in writing.