Can I build a backyard home in Newcastle?
Since 2023, Washington law lets most city lots hold up to two more homes. Newcastle writes its own rules on top. Here’s what the code says, in plain English — and a free report on what may fit on your lot.
What Newcastle allows.
The headline standards. Tap any one for the full wording and the code section.
ADUs per lot2: up to two ADUs subordinate to a single-family dwellingFull wording
2: up to two ADUs subordinate to a single-family dwelling (NMC 18.31.030). A site may not have both an ADU and a Type II home occupation or home industry (NMC 18.31.030.G).
Max size1,000 sq ft max, 300 sq ft minFull wording
1,000 sq ft maximum and 300 sq ft minimum. The director may allow more if the unit is entirely on one floor and the extra area is needed for efficient use (NMC 18.31.030.B).
Max height24 ft for detached ADUsFull wording
24 ft for detached ADUs (NMC 18.31.030.D; NMC 18.12.030 note 26). Attached ADUs may be up to 30 ft.
Front setback10 ft in R-4/R-6 (same as the house)Full wording
Same as principal: 10 ft in R-4/R-6 (NMC 18.12.030 table; NMC 18.31.030.H).
Side setback5 ft in R-4 / R-6 (the ADU’s side setbacks total 10 ft)Full wording
5 ft minimum in R-4/R-6. When ADUs are on the lot, the ADU's side setbacks must add to 10 ft (NMC 18.12.030 note 12). A DADU may be sited at a lot line that abuts a public alley (note 25).
Rear setback5 ft for ADUs in R-4/R-6, reduced from 20 ftFull wording
5 ft for ADUs in R-4/R-6, reduced from 20 ft (NMC 18.12.030 note 24). Second-floor rear windows within 15 ft of the rear line must be semi-translucent. A DADU may be at a lot line that abuts a public alley (note 25).
Lot coverageR-4 / R-6: 35% building, 40% imperviousFull wording
R-4/R-6: 35% maximum building coverage and 40% maximum impervious surface (NMC 18.12.030). The ADU counts toward impervious and floor-area limits (NMC 18.31.030.H).
Parking1 per ADU, on top of the house’s; none near major transitFull wording
1 space per ADU in addition to the primary unit's; none within one-half mile of a major transit stop (NMC 18.31.030.E).
Where a city figure reads stricter than the state floor (RCW 36.70A.681), the floor may control — the report flags it for your parcel.
Living there, selling it, plans, time and fees.
Each answer is the code’s own wording, with the section it comes from. “Not published yet” means the city hasn’t posted a figure — the report asks the city, not the internet.
Are backyard homes allowed?
Yes, in single-family residential zones. An ADU may be in the principal structure or in a detached structure (NMC 18.31.010, 18.31.030.C).
Minimum lot size
None for an ADU in NMC 18.31; the zone minimum applies to new lots (R-4 7,500 sq ft, R-6 6,000 sq ft, NMC 18.12.030).
Do you have to live there?
No. NMC 18.31 as rewritten by Ord. 2025-671 contains no owner-occupancy requirement.
Can it be sold on its own?
Yes. ADUs may be rented or sold as condominiums, or fee simple within an approved unit lot subdivision (NMC 18.31.030.F).
Pre-approved plans
None found
How long the city takes
Not published
Impact fees
Not found; RCW 36.70A.681 caps ADU impact fees at 50% of the principal unit's
Plans Newcastle has already approved.
A pre-approved plan skips the design review: Newcastle has already checked the drawings, so the permit review is about your lot.
Loading the plans…
Pictures are the official pages’ own. A pre-approved plan is not a permit: the city still reviews your site.
The code is half the answer. Your parcel is the other half.
Lines and easements
Where the lot lines really are, which setbacks apply on each side, and whether a utility or access easement takes the corner you had in mind.
Critical areas and slope
Steep slopes, wetlands and streams come with buffers that no citywide number shows. The report reads the county’s maps for your parcel.
Sewer, septic, water, trees
Whether sewer reaches the lot or it’s on septic, where the water service is, and which trees a city protects — the things that decide where a unit can actually go.
What Newcastle allows, what could fit on your lot, and a planning cost range. If something won’t fit, the report says so — and shows what might.
Where this page comes from.
Read from the Newcastle municipal code and the city’s own pages on November 5, 2026. The same table feeds the HousUp report, so the report and this page never disagree. Cities amend their codes; before you design anything, confirm the current wording with the city.
2 sources, read November 5, 2026
Find out what your property could become.
Type your address. Within a few hours: what Newcastle allows, what could fit, and a planning cost range. Free, no obligation.