Zoned Development Capacity

One of the fundamental questions we face as land use planners is whether there is sufficient capacity to accommodate future residential and commercial needs. Zoned development capacity helps us to better understand where current zoning allows additional development.

Background

The Washington State Growth Management Act (GMA) includes a requirement that cities and counties periodically prepare a “buildable lands” report. The purpose of that report is for cities to document the amount of housing and employment growth that has occurred, and the amount of further development that could be allowed given existing development rules. King County published the most recent report in 2014 in support of the development of cities’ comprehensive plan updates in 2015.

We use and maintain a model to help the us understand the pattern of potential future development across the city.

About the Model

The zoned development capacity model compares existing development to an estimate of what could be built under current zoning. The difference between existing and potential development is the development capacity for new residential and commercial development. The model is based on land parcel information from the King County Department of Assessments.

Zoned capacity reflects the density permitted under the current zoning. In single-family and low rise multifamily zones, permitted unit density is measured by residential unit per parcel area. For instance, in the Single Family 5,000 zone, the model assumes one housing unit for each 5,000 square feet of parcel area. 

Density is measured as the Floor Area Ratio (FAR) for commercial / mixed-use zoning. FAR is the ratio of total building square footage to the lot area and is the standard metric used to regulate density in zoning ordinances. The model uses both the maximum density allowed by zone and observed density assumptions based on more than 15-years of development for each zoning category to calculate the range of potential development.

Zoned Development Capacity Figure 1

Development Capacity =
(Developable Land Area x Future Density Assumptions)
- Existing Development

Figure 1: Relationship of existing, observed, and maximum development.

Process

The model follows these basic steps:

  1. Determine developable land area (excluding water and shoreline protected areas of the parcel), the primary existing land use, and current zoning on the parcel.
  2. Determine total existing residential units, above-ground building square feet, and assessed values of the land and improvements using the most recent King County Assessor data.
  3. Calculate total potentially allowable development (total residential units and above-ground building square feet allowed) for each zone on a parcel using maximum and observed density assumptions.
  4. Calculate the ratio of improvement value to land value (ILR) and the ratio of existing residential units or above-ground building square feet to potential development (DR).
  5. Determine whether each parcel is likely to redevelop based on existing uses, ownership (public / private), and comparison of the DR or ILR to assumed thresholds.
  6. Calculate the capacity for residential units and non-residential floor area by applying assumptions of the mix of non-residential and residential uses and subtracting all existing development.
  7. For parcels where non-residential development is possible, calculate the approximate number of employees that could be accommodated in the new building floor area.

Assumptions

  • Existing parcel arrangement – land is not subdivided or aggregated and no new rights-of-way required
  • Development is unconstrained by time – the model doesn’t predict trends in the market or suggest when potential redevelopment might occur
  • Certain parcels are considered very unlikely to redevelop and are excluded:
    • Public facilities, cemeteries, churches, landmarked structures or transferred development rights, parks, schools, and private institutional sites
    • Development built within the past 15 years
    • Condominium ownership
  • Redevelopment will occur where existing development is less than 40 percent of the total currently allowed development on the parcel
  • An average of 1,000 gross building square feet per unit is used in mixed-use zones to convert floor area to units
  • ADU / DADUs are not included in the capacity model – this housing type currently makes up a small share of new housing units

Estimating Our Capacity for Growth

Information about capacity enables the us to determine the effects of proposed zoning changes, policy revisions, and development trends. It also aids in understanding how and where the 20-year growth targets identified in the City’s Comprehensive Plan could be accommodated.

Zoned Development Capacity Figure 2

Figure 2: Relationship of existing development and zoned capacity to potential development and planning targets.

As part of the Comprehensive Plan update, we used the zoned development capacity model to estimate the amount of development that could occur under current zoning for each urban village and how that relates to the City’s policies to accommodate the next 20-years of expected growth.

We published a Development Capacity Report that summarizes the capacity for housing and job growth by zoning category and for each urban village as of January 2014. The report shows that the City currently has sufficient zoned capacity for significantly more than the 70,000 housing units and 120,000 jobs expected to come to Seattle between 2015 and 2035.

Resources

More details on the model process, assumptions and output are included in the documents listed below.

MethodologyReports/DataPresentations
Zone Development Capacity Primer Development Capacity Report for Urban Villages Development Capacity Workshop
Model Assumptions for Zoning Designations Development Capacity Report for Land Use Zones Seattle 2035 Development Capacity Report

Data Dictionary
(for model output table)

Development Capacity Report for Council Districts  
  Development Capacity Parcel Output Table