April 23, 2018
Multifamily’s Problem Child
Posted by Clifton Harness
Inside Corner Units
Need I say more? These units are the hardest to design. These units are the hardest to lease. These units are the hardest to live in. For the most part, if inside corners can be avoided, avoid them.
Fair warning this is a very niche topic within the multifamily industry and I will try my best to make it entertaining and simple to understand.Inside corner units occur anytime a multifamily building turns any kind of corner. These units perform the critical task of joining two colliding building masses at the human level. They are the reason higher density can simply be achieved.
Inside Corner Math is Whacky
These units are mathematically difficult because of two basic factors:
 not enough external skin for glazing
 too much net rentable area
If we compare this kind of unit to an inline unit of 28 foot depth, a corresponding inside corner unit would have a unit depth of 39 feet. A simple formula would be √2 * Unit Depth (thank you pythagoras).
In order to have the same worstcase depth as an inline unit? Divide unit depth by √2. In our 28′ example, an inside corner unit would need to be roughly 20′ deep.
The ratio of NRSF to Skin is the main ruling principle of solving inside corner units. The better the ratio? The more viable it is, in general, as a unit.
Mathematical Unit Requirements
The inside corner unit viability ratio (NRSF Requirement / Glazing Requirement) tells us how viable a unit is to become an inside corner unit:
Studios  1 Bedrooms  2 Bedrooms  3 Bedrooms  
Living Space Skin Requirement  12′  12′  12′  12′ 
Bedroom Skin Requirement  0′  12′  24′  36′ 
NRSF Requirement  500  800  1000  1400 
Viability Ratio (NRSF / Glazing LF)  42  33  28  29 
With this scenario, Studios are the most viable kind of inside corner unit, and 2 bedrooms are the least viable. Since the bedroom skin requirement and then NRSF requirement can fluctuate, this is not an established fact, but rather a ruleofthumb for this specific set of parameters.
Geometric Viability
There are four general strategies to improve geometric viability. The better the geometric viability, the more likely the mathematical viability. Both these things need to be close in order for a valid unit to be created.

Provide Corner Spacing
 Without this base strategy, an inside corner unit would have no glazing. This is required for basic viability.

Carvie Out Lobbies
 This strategy creates space off of the common corridor for an elevator or other mechanical spaces

Inset the Facade
 This strategy reduces NRSF area and increases glazing

Bifurcate
 This strategy reduces the NRSF requirement, but glazing remains constant
The table below is for orthogonal inside corner units related to this diagram.
Scheme:  Base  Lobby  Inset  Lobby + Inset 
Corner Spacing (#1)  8′  8′  8′  8′ 
Unit Depth  30′  30′  30′  30′ 
Lobby Depth (#2)  0′  10′  0′  10′ 
Inset Depth (#3)  0′  0′  10′  10′ 
Total Skin  16′  16′  32′  32′ 
Viability Ratio (NRSF / Glazing LF)  86.25  80  31  28.3 
Remember–the lower the viability ratio, the better.
Matching the Math with the Geometry
If we think about this kind of problem and attempt to apply an algorithm to it, we get some interesting results. In this case, I have asked the solver to hit a unit specific unit mix that includes studios, 1 beds and 2 beds. The inside corner unit can bifurcate if necessary, to lower overall NRSF demand, and it can also inset if it must to improve matching unit and geometric viabilities. There is another level of detail that needs to take place, like how a bifurcated unit is actually solved at the roomtoroom level, or how a lobby carveout would assist in creating better inside corner units. There are also many folks that chamfer at 45 degree angles various walls to assist with how the unit connects to its surroundings. Codifying these parameters assists in the overall possible solution set.
Codified Inside Corner Unit Types
If one could customize geometric viability, the following parameter toggles could be used:
Inset The Facade  Carve Out Lobbies  Bifrucate 
Straight Inset  Straight Carve  Angled 
Chamfer  Chamfer  ZigZag 
Straight Inset + Chamfer  Chamfer + Straight Carve  Angled ZigZag 
Double Inset  Double Carve  Vertical Circulation 
Double Inset + Chamfer  Dedicated Carve  Avoid The Issue 
Codified CrossProduct
Basic math for the six methods of designing the skin, the six methods of addressing the common corridor, and the three methods of bifurcating the unit gives us 108 options (rendered below). Hopefully, within at least one of these 108 options, we can find a geometrically viable unit.
Or you can, you know, just avoid making them in the first place:
Solving Inside Corner Units for Test Fit
Why did I write this article and create a method of analyzing inside corner unit viability? We are going to do a new feature within TestFit to allow inside corner units to be customizable (more than they are right now). Why did I need to come up with 110 ways of solving inside corner units? We have to build a flexible product for all of our users. We have to build flexible algorithms for users to customize. Inside corner units are one of the hardest design issues within multifamily, and a tool to solve them quickly and easily should be robust, and simple.
Stay tuned for news about the new Inside Corner Unit solver by subscribing to updates.
Clifton Harness is the CEO of TestFit.io. Send him an email at clifton@testfit.io if you have a better way of solving inside corner units!