ADU Cost on a 4,000–4,999 sqft Lot

Frequently asked

• Can I build an ADU on a 4,000–4,999 sqft lot in WA?

  In most cases yes — RCW 36.70A.681 preempts minimum-lot-size restrictions on ADUs across most WA single-family zones, so a 4,000–4,999 sqft parcel is generally buildable. The real constraint becomes lot coverage and setbacks geometry, not raw lot area. For 4,000–4,999 sqft parcels we run a coverage stress-test at feasibility: existing footprint + planned DADU + impervious driveway/walks against the zone's coverage cap and impervious threshold for stormwater.

• What size DADU fits on a 4,000–4,999 sqft lot?

  On a typical 4,000–4,999 sqft Puget Sound parcel, achievable DADU footprint usually lands 380–800 sqft as single-story, or up to 1,000 sqft as a two-story (which roughly halves the footprint for the same livable area). The pinch points are the rear and side setbacks (commonly 5 ft post-preemption), separation from the primary house (5 ft typical), and clearance from existing trees or septic systems. Two-story footprints often unlock 25–40% more livable area on 4,000–4,999 sqft lots while reducing impervious.

• Does a 4,000–4,999 sqft lot affect my ADU permit cost?

  Indirectly. Building permit fees scale with project valuation, not lot size, so a 600 sqft DADU costs the same to permit on a 4,000–4,999 sqft lot as on a larger lot. However, 4,000–4,999 sqft lots are more likely to need drainage review (because the new impervious is a larger share of the total parcel), which adds reviewer time and may add a drainage engineer scope. Our feasibility report tells you exactly which review tracks your 4,000–4,999 sqft lot will trigger.