FAR, Density Math,
and Real Unit Yield

How Seattle’s NR Rezone Translates Into Actual Buildings

One of the most common mistakes people make when evaluating development potential under Seattle’s new Neighborhood Residential (NR) zoning is assuming that zoning numbers directly translate into usable, sellable, or rentable space. They do not.

Terms like FAR, density, unit count, and height are abstractions. They are regulatory limits, not guarantees of feasibility. Turning those abstractions into real units depends on how floor area is measured, how circulation is configured, and how building code requirements interact with the zoning envelope.

This article walks through how FAR and density actually work together in practice—and where the gaps between “what’s allowed” and “what can be built” usually appear.

(For an overview of what changed in the NR rezone and why stacked flats matter, see our briefing on Seattle’s NR Rezone & the Stacked Flats Unlock.)

FAR Is a Zoning Metric, Not a Design Outcome

Under the NR rezone, stacked flats are allowed up to 2.0 FAR. On paper, that sounds straightforward:

  • 5,000 sq ft lot × 2.0 FAR = 10,000 sq ft of allowable floor area

But that number is only meaningful if you understand how FAR is measured and what it excludes.

In Seattle, FAR for zoning purposes is measured to the inside face of finished walls. This is not how buildings are marketed, sold, rented, or even typically cost-estimated by builders. That difference alone can account for an 8–12% delta between zoning floor area and real estate square footage.

That delta matters later when we talk about construction cost and sales value.
(We address this in detail in Inside Face vs Outside Face: How Square Footage Math Can Make or Break Feasibility.)*

Density Sets Unit Count, Not Unit Size

FAR controls how much you can build. Density controls how many dwellings you’re allowed to divide that area into.

Under the NR stacked flats provisions, density is generally calculated at approximately:

  • 1 unit per 450–500 square feet of lot area

On a 5,000 sq ft lot, this typically yields:

  • 10 units maximum

At first glance, that aligns cleanly with the FAR math:

  • 10,000 sq ft FAR ÷ 10 units = 1,000 sq ft per unit

But that assumes:

  • perfect circulation efficiency

  • no stairs, landings, or hallways

  • no structural thickness

  • no code-required common space

In reality, none of those assumptions hold.

Circulation Efficiency Is the Hidden Variable

The biggest driver of “real unit yield” on small multifamily sites is circulation efficiency—how much of the building is taken up by stairs, corridors, and shared space instead of dwellings.

On small stacked-flat projects, the difference between a single-stair and a two-stair building can be decisive.

Single-Stair Buildings (4 Units or Fewer per Floor)

Seattle is unusually permissive in allowing single-exit (single-stair) residential buildings under certain conditions. When you limit a floor to three or four units, you can often:

  • eliminate long double-loaded corridors

  • reduce stair area

  • improve unit access to light and air

In practice, well-designed single-stair buildings can achieve 85–90% efficiency, meaning that 85–90% of the zoning floor area ends up inside dwelling units.

TWO Stairs (Corridor) Buildings

Once you exceed four units per floor, a second exit stair is required. On small footprints, that stair and the corridor connecting exits consume a disproportionate amount of area.

On lots closer to 5,000–6,000 sq ft, this can drop efficiency into the 70–75% range, sometimes lower—meaning you “lose” hundreds to thousands of square feet to circulation.

This is one reason why chasing maximum unit count can backfire on small sites.

Example: 5,000 Square Foot Lot

Let’s walk through a realistic scenario using numbers consistent with projects you described.

Step 1: Zoning Envelope

  • Lot size: 5,000 sq ft

  • FAR: 2.0 → 10,000 sq ft zoning area

  • Density: 10 units

Step 2: Circulation & Measurement Adjustments

Assume:

  • 85% circulation efficiency (single stair, ~3 units per floor)

  • 8% difference between inside-face FAR and exterior-measured real estate area

Calculation:

  • 10,000 × 85% × 108% ≈ 9,180 sq ft of unit area

Step 3: Average Unit Size

  • 9,180 ÷ 10 units ≈ 918 sq ft average

That’s before considering:

  • Type A accessible unit bonuses

  • market preferences for family-sized units

  • structural or utility constraints

This is why a “1,000 sq ft per unit” assumption often collapses into the low-900s once real-world factors are applied.

Type A Units Can Change the Math—But Only Strategically

Seattle allows certain Type A accessible dwelling units to be exempt from FAR, lot coverage, and sometimes density calculations. This can effectively add “free” area to a project.

However:

  • the exemptions are not unlimited

  • they apply differently depending on total unit count

  • they introduce layout and cost implications

On projects with 10 units or fewer, Type A units can materially improve feasibility by allowing larger ground-floor units without consuming FAR. On larger projects, the benefit is more limited.

Height Limits Shape Yield More Than FAR

The NR height limits—42 feet for flat roofs and up to ~47 feet for pitched roofs—often allow four residential floors, sometimes five depending on configuration.

But height is not just about adding floors. It determines:

  • whether units can stack efficiently

  • whether elevators are triggered

  • whether stair runs become inefficient

In many cases, a four-story walk-up with well-proportioned floor plates outperforms a taller building that introduces new code or cost thresholds.

This is especially true when paired with single-stair layouts.

Why “Maximum Density” Is Rarely Optimal

A recurring theme in successful middle-housing projects is restraint.

On paper, zoning may allow:

  • 10 units at ~900 sq ft

  • or 8 units at ~1,150–1,250 sq ft

Depending on market conditions, construction costs, and financing, the 8-unit scenario often:

  • absorbs faster

  • commands higher per-unit pricing

  • avoids infrastructure or financing thresholds

This is where zoning math intersects directly with market reality and risk management.

How FAR and Density Fit Into Feasibility

FAR and density define the sandbox. They do not tell you how to play the game.

Real unit yield depends on:

  • circulation strategy

  • measurement conventions

  • building code thresholds

  • unit mix decisions

Understanding these relationships early prevents the most common development mistake: designing a project that technically complies with zoning but fails financially.

References & Additional Resources

We are always available to help guide you through feasibility, density, and yield questions for a specific site.
If you’d like to discuss how these concepts apply to your property, you can reach us at info@tsuga.studio