Two ADUs or a Fourplex? What You Can Build on a Seattle Lot

Since January 2026, “how many homes can I build here?” has a real answer in Seattle's Neighborhood Residential zone, and it is usually more than one. The number comes out of your lot area, and then three exceptions and one floor-area limit decide whether you can actually use it.

Short version: most NR lots now pencil to somewhere between three and six dwelling units on paper. Two of those may be accessory dwelling units. For the large majority of homeowners we talk to, the choice is not really “fourplex or not” — it is whether to keep the house and add one or two ADUs, or to treat the lot as a redevelopment site. Those are very different projects. Here is the arithmetic that separates them.

This is a planning-stage explainer, not a zoning determination. Lot-specific answers come from SDCI, and the numbers below are the code standards as of October 2026.

Start with one unit per 1,250 square feet

The base density in the NR zone is one dwelling unit per 1,250 square feet of lot area for attached and detached units (SMC 23.44.060.A.4). Stacked flats get a denser allowance — one unit per 600 square feet — and there are narrower bonuses for stacked development that retains a large tree or hits a Green Factor score.

One detail is worth knowing because it changes answers at the margin: when the division leaves a fraction, any fraction over 0.85 counts as one more unit (SMC 23.44.060.D.1). So:

• A 4,000 sq ft lot: 3.2 units, which rounds down to 3.
• A 5,000 sq ft lot: exactly 4.
• A 6,000 sq ft lot: 4.8 — the fraction is under 0.85, so still 4.
• A 6,100 sq ft lot: 4.88 — over the line, so 5.
• A 7,500 sq ft lot: exactly 6.

Critical-area land comes out of the lot area before you do that division. Riparian corridors, wetlands and their buffers, submerged land and shoreline setbacks, and designated steep-slope non-disturbance areas are all subtracted (SMC 23.44.060.D.6). A nominally 7,000 sq ft lot with a creek buffer across the back can calculate out considerably smaller.

The three exceptions that matter more than the base rule

The base number is not the ceiling. Three exceptions in SMC 23.44.060.C do most of the work, and all three require the lot to be free of those same critical areas — riparian corridors, wetlands and buffers, submerged land and shoreline setbacks, and steep-slope non-disturbance areas.

• Any lot under 5,000 sq ft may have up to 4 dwelling units, regardless of what the 1,250 sq ft math says. A 3,200 sq ft lot that calculates to 2 units can still have 4.
• A lot under 7,500 sq ft within a quarter-mile walking distance of a stop on a major transit service may have up to 6.
• A lot under 7,500 sq ft farther than a quarter mile from major transit may also reach 6 — but only if at least two of the principal units are income-restricted for 50 years, at 60% of median income for rentals or 80% for ownership, with annual reporting to the Office of Housing and, for ownership product, a non-profit steward.

That third route is a genuine affordable-housing program with ongoing obligations, not a box to tick. It is the right tool for some owners and a poor fit for most single-family homeowners adding a backyard home.

“Major transit” is not the same as “frequent transit”

This is the single most common error in online guides to the new rules, and it matters because the two terms are defined separately in SMC 23.84A.038 and used for different purposes.

• Major transit service means commuter rail, light rail or streetcar, and bus rapid transit routes already operating or funded and projected for construction in an applicable six-year transit plan. The 6-unit exception keys off a quarter-mile walking distance to a stop on one of those.
• A frequent transit service area is broader — within 1,320 feet of a bus stop on a frequent route, or 2,640 feet of a rail station, as mapped by Director's Rule. That definition drives other standards: reduced setbacks on small lots, a lot-coverage bonus, and the 1,500 sq ft ADU size in Lowrise zones.

A frequent bus line down your arterial does not get you to six units. If a guide tells you “six near frequent transit,” it is describing a rule that does not exist.

One honest caveat: the code does not define how that quarter-mile walking distance is measured, and unlike the transit service areas, it is not tied to an adopted Director's Rule map. SDCI publishes a quarter-mile major-transit walkshed layer on its mapping tools, and that is the practical starting point — but if your lot sits near the edge of it, treat the answer as something to confirm with SDCI rather than something to design around.

ADUs count toward density — and you can only have two

Here is the sentence that reshapes the decision. When you calculate density, accessory dwelling units count as dwelling units alongside principal units (SMC 23.44.060.D.5 and 23.42.022.J). Separately, no lot may have more than two ADUs (SMC 23.42.022.C).

Put those together on a 5,000 sq ft lot with a four-unit allowance and the menu looks like this:

• House + attached ADU + detached ADU = 3 of your 4 units, with one principal unit still available.
• House + 2 ADUs + one more principal unit = 4 units, which is the cap.
• House + 3 ADUs = not permitted, no matter what the density number says.
• Four principal units and no ADUs = also 4 units, and a very different building program.

If you already have an ADU, it is occupying one of those slots. That is worth checking before anyone draws anything — our guide to having a house and a DADU on one lot covers the configuration side in more detail.

Two ADUs or more principal units: the differences that decide it

Once you know your unit count, the real question is what kind of units. ADUs and principal units are treated differently in ways that have more practical effect than the headline number.

• Size. An ADU is capped at 1,000 sq ft with two bedrooms or fewer, and 1,200 sq ft with three or more. A principal unit has no unit-level size cap — it is limited only by floor area ratio, lot coverage and setbacks.
• Rear setback. An ADU gets a 5-foot rear setback, and none at all against an alley. Principal units need 15 feet with one or two units on the lot, 10 feet once you are at three or more.
• Parking. No off-street parking is ever required for an ADU. Principal units default to one space per two units, with exemptions that commonly apply — units under 1,200 sq ft, anything inside a major transit service area, and a waiver for retaining a qualifying tree.
• Street improvements. The City cannot require street improvements as a condition of permitting an ADU. Principal units can trigger sidewalk, curb and ramp work, though three-to-nine-unit projects in NR get reduced requirements.
• Selling it later. An ADU can be sold as a condominium unit, but it can never sit on its own fee-simple unit lot. Principal units can be unit-lot subdivided.

Both paths skip two things people expect to face: there is no design review in the NR zone, and an ordinary code-compliant residential project is categorically exempt from SEPA review. That removes months that competitors and older guides still budget for.

The sale question is the one owners most often get wrong in both directions. Washington law protects selling an ADU as a condominium unit, and as of July 2026 there is appellate authority on the point — we cover what changed in can you sell a DADU separately in Washington.

Floor area usually runs out before unit count does

This is where most four-unit daydreams meet the code. Density tells you how many front doors. Floor area ratio tells you how much building, and in NR it scales with how dense you actually go (SMC 23.44.050, Table A):

• Less dense than one unit per 4,000 sq ft: FAR 0.6.
• Down to one per 2,201 sq ft: 0.8, or 1.0 if the project meets the family-size standards for stacked units near a school.
• Down to one per 1,601 sq ft: 1.0, or 1.2 on the same terms.
• One per 1,600 sq ft or denser: 1.6 for attached and detached units, 1.8 for stacked flats or for courtyard-type development in a frequent transit service area, and 2.0 for stacked units that retain a Tier 1 tree, two Tier 2 trees, or hit Green Factor 0.6.

Two changes here catch people out. First, lots under 5,000 square feet may always use up to 2,500 square feet of chargeable floor area, or the FAR figure, whichever is greater — a meaningful floor for small lots. Second, and more important: ADU floor area is now chargeable to FAR. The old exemption for ADU floor area in NR zones was struck by Ordinance 127376. Any guide, calculator or plan set written before 2026 that treats a backyard cottage as FAR-free is giving you a number that is too large.

Lot coverage runs alongside it at 50 percent, rising to 60 percent for stacked flats or for courtyard-type development in a frequent transit service area. You also owe amenity area — 20 percent of the lot, 25 percent for stacked units — and driveways and parking do not count toward it.

Work a 6,000 sq ft lot through that and the shape of the problem is clear. Four units at 1,500 sq ft each is 6,000 sq ft of building. At the 1.6 tier you are allowed 9,600 sq ft, so FAR is not binding — but 50 percent coverage caps your footprint at 3,000 sq ft, and 20 percent amenity area takes 1,200 sq ft of the yard out of play. The limit is rarely the unit count. It is footprint, setbacks and the yard you have left.

Trees and critical areas quietly reset everything

Seattle's tree code is the most common late-stage surprise on these projects. A regulated tree carries a tree protection area with a radius of one foot for every inch of trunk diameter, and in NR zones a tree protection area is not buildable area. Neither are slivers of land less than 10 feet wide. Where multiple trees are involved, the analysis runs on the sum of their protection areas.

A single mature Tier 2 tree in the middle of a back yard can take a six-unit allowance and leave room for a house and one cottage. It is worth identifying before you pay for design — our guide to tree protection rules for a Seattle DADU walks through the tiers, the protection-area math and what removal actually costs when it is allowed at all.

So which one is right for your lot?

We build both, and the honest answer is that they suit different owners rather than different lots.

Adding one or two ADUs keeps the house you live in, keeps the project on conventional residential financing, keeps the permit in the middle-housing queue, requires no parking, and cannot trigger street improvements. It is the lower-risk path, and it is what most homeowners are actually asking about when they ask how many units they can build.

Going to four or six units is a development project. It usually means demolishing or substantially reworking the existing house, a larger capital stack on commercial terms, more units to lease or sell, and a design driven by coverage and amenity area rather than by what you want in a back yard. The upside is real; so is the complexity.

What we would not do is let the density number drive the decision. A four-unit allowance on a lot with a protected tree, a steep slope and 50 percent coverage is often a worse project than a well-sited house and DADU on the same ground. For what the backyard-home version costs, start with DADU costs in Seattle or run numbers in our ADU cost calculator.

How to check your own lot this afternoon

• Get your exact lot area from the King County Parcel Viewer — not the figure on a listing.
• Open the SDCI Property Information Map and look for critical areas, your zone, and whether you fall inside the quarter-mile major-transit walkshed.
• Subtract any critical-area land, divide by 1,250, and apply the 0.85 rounding rule.
• Check whether the under-5,000 or under-7,500 exceptions apply to you.
• Remember that up to two of your units may be ADUs, and that both count toward the total.
• Find your FAR tier, then test it against 50 percent coverage and 20 percent amenity area.
• Identify every tree over about 12 inches in diameter and sketch its protection area.

If you want a second set of eyes on that, we do feasibility review as the first step on every project — the point is to find the binding constraint before anyone pays for drawings. Our guide to whether you can build an ADU on your lot covers the single-unit version of this check, and how big a DADU can be covers the size standards in detail.

Building in Seattle or elsewhere in the Puget Sound region and want the arithmetic run on your address? Tell us about your lot and we will tell you what it supports — including when the answer is “less than you hoped.”