Homeowners ask me about Seattle DADU setbacks more than any other subject, and most of the numbers they repeat back are wrong. Half of them are still quoting the old single-family code. The other half never noticed that Ordinance 127376 rewrote the Neighborhood Residential chapter, effective January 21, 2026. The actual numbers are easy to write down and maddening to apply to a real lot.
A detached ADU in a Neighborhood Residential zone needs a 15 ft front yard, 5 ft side setbacks (5 ft average, 3 ft minimum), a 5 ft rear setback — or 0 ft at a public alley, about 5 ft of separation from the house, no more than 50% lot coverage, and 32 ft height (37 ft pitched). Floor area caps at 1,000 sq ft (1,200 sq ft with three or more bedrooms). two ADUs per lot, and no off-street parking is required. Standards current as of Ordinance 127376, effective January 21, 2026.
Seattle DADU setbacks: the exact numbers under the 2026 NR code
Setbacks are measured from the surveyed lot line to the nearest point of the building. The table in SMC 23.44.090 sets them for Neighborhood Residential zones, and a separate footnote sets the DADU-specific rear line. Here is what applies to a typical single-house lot.
| Setback | Requirement | Code |
|---|---|---|
| Front yard | 15 ft on a lot with one or two dwelling units; 10 ft once the lot holds three or more | SMC 23.44.090 |
| Side | 5 ft average, 3 ft minimum — a flat 3 ft inside a frequent-transit service area on lots under 5,000 sq ft | SMC 23.44.090 |
| Rear (DADU) | 5 ft, except 0 ft when the rear lot line abuts a public alley | SMC 23.44.090, note 3 |
| Structure separation | About 5 ft between the DADU and the principal house | Modeled at 5 ft |
A detached ADU may not sit in the required front yard, so the practical effect is that the cottage lives behind the house, beside it, or on top of a converted structure. The side setback is the one with the footnote most people skip. It is a 5-foot average with no single point closer than 3 feet — except inside a frequent-transit service area on a lot under 5,000 square feet, where it is a flat 3 feet. Those transit areas are mapped on SDCI’s Property Information Map, and on a narrow lot the difference is two feet of building width. The front number moves too: 15 feet on a lot with one or two dwelling units, 10 feet once the lot holds three or more, which is worth knowing if middle housing is part of your plan.
The customized report measures your Seattle parcel's setbacks, lot coverage and buildable rectangle from City and County records, with the trees and critical areas that move the footprint.
The rear line: 5 feet, or 0 feet at a public alley — and why a private road is not an alley
The rear setback is the number that gets quoted wrong the most, and the alley question is why. State law says a city must let a detached ADU sit at the lot line when that line "abuts a public alley". Seattle's code carries the same rule: 5 feet of rear setback, or 0 feet if the rear abuts an alley.
The catch is the word alley. SMC 23.84A.002 defines an alley as "a public right-of-way not designed for general travel and primarily used or intended as a means of vehicular and pedestrian access to the rear of abutting properties." A platted alley on the City's right-of-way map counts. A private road through a shared lot, a driveway easement behind three houses, or a gravel lane somebody has used for forty years does not — it is not public right-of-way.
On the job site that distinction is worth real money. A lot with a true alley behind it gets the 0-foot rear line and a second way in for the excavator and the utility trench. The same-looking lot with a private lane behind it pays the 5-foot rear setback and runs every piece of equipment past the house down the side yard. I have watched homeowners argue this with a tape measure in hand. The alley either shows up as a named, platted right-of-way, or it does not. Which Seattle blocks actually have that platted alley — and which neighborhoods never got one — is its own question, covered in Seattle's alley lots.
Height, lot coverage, and floor area: the other three numbers
Setbacks only answer where. Three more numbers answer how much.
Height. SMC 23.44.070 caps structures at 32 feet, and allows 5 more feet when the roof is pitched, for 37 feet total. Two storeys fit inside that envelope, which is how a compact footprint still delivers the full floor area.
Lot coverage. SMC 23.44.080 caps structures at 50 percent of the lot, and it counts everything: house, DADU, garage, shop. A 5,000-square-foot lot allows 2,500 square feet of total footprint, house included. If the house covers 2,000 of it, the DADU has 500 left on the ground — which is why the two-storey cottage is so common.
Floor area. SMC 23.42.022 caps a DADU at 1,000 gross square feet with up to two bedrooms, and 1,200 square feet with three or more. Three things do not count toward that limit: up to 250 square feet of attached garage, underground stories, and up to 35 square feet of long-term bicycle parking. No off-street parking is required, owner-occupancy is not required, and no lot may hold more than two ADUs.
Setbacks tell you the box. Lot coverage and floor area tell you how much of that box you can actually use — and coverage is usually the one that binds.