Building Efficiency 101

I was green, fresh out of college, and had just planned my first building. My boss simply asked, "What is the efficiency of this building"? I had no idea. "Let me check...Ill get that to you as soon as possible"

12 grueling hours, a sloppy excel sheet, and a CAD file with far too many polylines later, the answer was 79.5% efficient. "Why is it so inefficient?" I never stopped to consider what the metric actually meant.

Building efficiency is the ratio of usable area to non-usable area. In multifamily its generally a ratio of net-rentable area to all other areas. Today I will go into the weeds of multifamily efficiency--one of the building optimization metrics.

Part One: Consider the Building Components

Time to roll out the building planning legos...




For this test I built an interactive spreadsheet for the local efficiency of theoretical building components. Using two variables, unit depth and corridor width, we can compute a huge amount of data for a dozen building components. Dead-end corridors are the most efficient building component, quickly followed by double-loaded corridors and 90 degree turns. None of this is huge news, until we look at how inefficient it is to wrap and line a parking structure. Splitting corridors from the corner of a garage is the least efficient component.

That plan I drew years ago? It had four Wrap C (76%) conditions (see diagram above) and one Liner D (73%) conditions per level. This is why the building was inefficient.

Part Two: Getting More Efficient

Let us assume I am planning a project with 27' deep units and 6' wide corridors. I currently am under pro-forma by about 1,000 NRSF. I should consider using a 5' corridor. This boost in 1.66% efficiency for a 100,000 NRSF project could gain up to 1600 NRSF.





Part Three: Diminishing Returns




I ran the math from a 2' deep unit (would never happen) to a 40' deep unit (does this happen?). Boosting the unit depth has diminishing returns, but meaningful gains if boosting the unit depth a couple of feet is possible.


Part Four: Using Residential Engine to Test Building Efficiency

I did 120 site plan iterations on the same property in Residential Engine. This generated a massive amount of data to use for backing up the "building component" theory above.

Here is the spread of efficiencies. As low as 79.78% and as high as 85.16%




Site Plan Scheme 36 is the least efficient design at 79.78%:




Site Plan Scheme 231 is the most efficient design at 85.16%:





Part Five: Using TestFit for "Advanced Metrics"

We built out an export function to tell us several atypical metrics from the 120 plans generated above:

  • Percent of Double Loaded Corridors
  • Number of 3 and 4 way intersections
  • Linear feet of corridor

No shock here, but designing with double-loaded corridors improves efficiency.




This also was not much of a shock, but more intersections generally means a more inefficient building.




Similar to the one above, but more a pronounced trend:




Any thoughts? Test Fit is an instant multifamily site planning application. Send me an email:

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