Guide · Newcastle · King County · Updated July 2026
How many units can you build in Newcastle?
If you own or are evaluating a residential lot in Newcastle, the first question is almost always the same: how many homes can legally go on it? Since Washington's HB 1110 middle-housing law took effect, the answer for most Newcastle lots is now more than one. Newcastle allows at least 2 units per lot on all residential land, with more where higher-density zoning applies.
Newcastle at a glance: at least 2 units per lot, with more allowed by lot size and zoned density, plus up to 2 ADUs under HB 1337. Source: NMC 18.19 (Ord. 2025-671).
What Newcastle's adopted code allows
Newcastle allows at least 2 units per lot on all residential land, with more where higher-density zoning applies.
The details: no transit tier — reported as the greater of 2 per lot or the zone's per-acre density, so larger or higher-zoned lots exceed 2. These are Newcastle's city-wide standards, per NMC 18.19 (Ord. 2025-671); the exact figure for a specific parcel — its zone, lot size, transit proximity, and critical areas — is what a Civexa report resolves.
What actually fits: floor area, height, and setbacks
HB 1110 and the city code set the right to a number of units; the zone's building envelope sets what physically fits. Floor-area ratio, lot coverage, height, and setbacks together decide whether those homes pencil as detached cottages, a townhouse row, or a stacked-flat building. Civexa computes this envelope for Newcastle zone by zone, so the unit count you see is one the zoning can actually hold.
What can shrink it: critical areas
Steep slopes, wetlands, streams, and flood zones can override the unit math on a specific parcel. A lot that qualifies for the maximum on paper may be constrained once environmentally critical areas are mapped. Civexa screens FEMA flood, slope, and county critical-area layers for every Newcastle address so you find this out before you make an offer, not after.
Get the exact number for your Newcastle parcel
The figures above are Newcastle's city-wide rules. The number that matters is the one for your address — its zone, lot size, transit proximity, and critical areas. Civexa turns that into a full feasibility report in about a minute: unit count, buildable envelope, a preliminary pro forma, utilities, and permit path.
Run a Newcastle feasibility report →
Frequently asked
How many units can I build on a residential lot in Newcastle?
Newcastle allows at least 2 units per lot on all residential land, with more where higher-density zoning applies. That is per NMC 18.19 (Ord. 2025-671). The exact number for a specific parcel still depends on its zone, lot size, transit proximity, and critical areas — run a Civexa report for the parcel-level figure.
Can I add ADUs on top of that in Newcastle?
Washington's HB 1337 allows up to two accessory dwelling units on a residential lot. In some cities they count toward the middle-housing total and in others they are separate — your Civexa report applies Newcastle's specific rule.
What unlocks the maximum in Newcastle?
No transit tier — reported as the greater of 2 per lot or the zone's per-acre density, so larger or higher-zoned lots exceed 2.