Have you ever browsed r/raleigh or the DTRaleigh Community when the topic of density came up and not felt fully confident in your understanding of the terminology?

The “Development Jargon” series is about demystifying the language of development.

If you have suggestions for future demystifications, let me know on twitter!

Today I am going to explain density. I will do two things:

• define density
• discuss the two most common density frameworks.

Additionally, you will get some exercises at the end of the article that will allow you to test out your new found understanding and approach your next development conversation with more confidence!

Density Defined

Density is a ratio of one measure of area compared to another. Density should not be confused with intensity — the total amount of building area on a parcel — or height — how tall a building is — as they are not the same.

Density [Area] = Square Feet of Space / Square Feet of Land

Density [Units] = Total Dwelling Unit Count / Acres of Land

In commercial real estate density is usually described as the area of building divided into the area of land. In residential the units are typically switched into dwelling units per area of land. Though for residential, discussing density in terms of building area also works.

FAR

FAR or Floor to Area Ratio is a measure of density common in commercial real estate. To determine the FAR you measure the entire square footage of building area on a parcel and divide by the total land area using similar units.

If using square footage for example you would divide the total square footage of building by the total square footage of land. FAR can range from below 1:1 to over 50:1. The FAR can equal zero if there is no building on a property, but it can never be negative.

A commonly required conversion when calculating FAR is converting acres to square feet (sf). There are 43,560 square feet in an acre.

1 Acre = 43,560 Square Feet

As an example, if we have a single story commercial building that is 10,000sf on a one acre property, the FAR is 10,000sf/43,560sf or .23:1.

With FAR, the height of the building is irrelevant. If that building in the example above was two stories with each floor measuring 5,000sf, the total building area is still the same, so the FAR remains unchanged. If the building grows by an additional floor with the same footprint, however, the FAR doubles. 2×10,000sf/43,560 is an FAR of .46:1.

An FAR of 1:1 would be a 43,560sf building of any floor plate and height configuration that was built on a one acre parcel. Floor plate is a measure of the area of a single floor of a building.

An FAR of 100:1 would be a 2,000,000sf skyscraper on a 20,000sf (approximately .46 acres) parcel.

Units Per Acre

For residential development a more common measure of density is dwelling units per acre or units per acre. This measure can be potentially misleading in practice because rarely are the units being measured all the same size.

To measure density in dwelling units (du) per acre you count up all of the homes — be they apartments, single family, townhomes, or other — and divide by the total area of the parcels those homes occupy.

For example if a townhome community has 100 homes and sits on a total of 8 acres, you have a density of 12.5du/acre. Single family density is usually around 2–4du/acre, townhome densities are typically around 10–20du/acre, and other multifamily densities can range from 15–100du/acre.

Remember, density is a ratio of one thing to another, so a high dwelling unit count isn’t required for a high density. A quadruplex (four dwelling units) on a small lot could easily measure out at a density of 16du/acre.

It’s best to translate the dwelling units to square footage of building area and then to calculate the FAR. 10du/acre of McMansions calculate out to be a lot different than 10du/acre of studio apartments or Accessory Dwelling Units.

Conclusion

I hope you enjoy this explanation of density. Please take some time to try the exercises below. Remember when calculating FAR make sure the two areas you are comparing are in the same units of measurement, and when in doubt translate dwelling units to square footage of building area. Some of the exercises are intentionally challenging, so if you get them you know you truly understand density!

If you have any questions, please let me know! My email is [email protected]

Exercises

1. What is the density of a 5-story building with floor plates of 25,000sf on a 5 acre parcel?
2. Which is more dense, a 100,000sf office building on a 5 acre parcel or 20,000sf building on a 2 acre parcel?
3. How much more dense is a 300 unit apartment complex than a 50 home single family neighborhood if they occupy the same amount of land?
4. How tall is a building with an FAR of .5 if the site is 5 acres and the floor plate for the building is 54,450sf?

If you have suggestions for future demystifications, let me know on Twitter