Updated: Sep 15, 2020
In July I wrote about the State Legislature requiring easier paths to increasing residential Accessory Dwelling Unit (ADU) construction (SB 1069). This action is no doubt a massive intrusion into local planning and is rife with problems, but it’s too late to debate those points. Now it’s time to deal with them and try to figure out what the impact of an ADU is.
As a recap, one major type of ADU is one that can be cobbled from existing excess space inside of the initial detached dwelling (i.e. an unused bonus room or a master bedroom, a breezeway, garage or similar existing underutilized space). The second major type of ADU is one that is constructed separate and outside of the existing detached dwelling. These ADUs can range up to 1,200 square feet. The impact of the former will be difficult if not impossible to calculate as any additional person or persons living in space converted from within an existing detached dwelling will be indistinguishable from the original occupants of the detached dwelling. The demand resulting from the latter type of ADU probably can be calculated. I have explored the various infrastructure of the average city and have concluded the following:
Circulation System Demands – There is not adequate data on ADUs with which to calculate the additional demand they may generate on this critical infrastructure. The Institute of Transportation Engineers (ITE) does not have this category in their list of traffic generators simply because of the great difficulty in measuring it.
Law Enforcement Demands – Surely these units will generate police-related calls-for-service, but how many? This is difficult to determine because most police agencies do not generally keep track of land-use demands, as they instead focus on the type of crime (burglary, assault, noise, etc.).
Fire Suppression/Paramedic Demands – Again, some calls-for-service can be expected as people who live in the new ADU’s will need assistance from time to time, but there is no information as to how many additional calls-for-service there will be. Fire departments keep excellent calls-for-service records but the National Fire Information Reporting System (NFIRS) used by every fire department does not have a special category for Accessory Dwelling Units.
Storm Drainage Runoff Demands – Finally we have found one that can be measured. If an accessory unit is built new with a pad and roof top that creates another 750 square feet of impervious space, the additional storm water runoff that can be measured and thus calculated. If the ADU is constructed within the existing detached dwelling, there will be no additional runoff and thus no additional demand upon your storm drainage system
Public Use Facilities – Your community centers, senior centers, gymnasiums, libraries and similar facilities can all be expected to see some increases in demand, but by how much? A 600 square foot ADU is capable of housing two or more people. A 1,200 SF ADU is capable of being three bedrooms. I know because I grew up in a three bedroom 1,500 SF home in a suburb outside of Los Angeles. There were four of us in that home but neighbors with the very same floor plans sometimes often had up to six people living in theirs.
Utility Demands – Water and Sewer demand can probably be determined for a new ADU, most likely on a fixture basis as long as it is new ADU that is separate from the “main” house and not converted space within the existing “main” house. Those of you with your own utilities will probably see water demand by existing detached dwellings increase over time as homeowners take economic advantage of unused space within their home. Sewer demand is usually a function of water use, so that could increase also and probably can be determined for a new separate accessory living unit.
What is most likely to occur from all of this is that, over time, we will see an increase in the average number of persons living in a detached dwelling and in turn an increase in the average demands from detached dwellings (where we can measure demands), as vacant spaces within the existing detached dwelling are converted to indoor apartments and back-yard separate ADUs (again, up to 1,200 S.F.).
Clearly, some unknown amount of ADUs will be likely be constructed in your agency’s service boundaries, and it appears that the best (and only) way to deal with the unknown demand is to have a development impact fee schedule for ADU’s. But given the difficulties in identifying a scientific methodology it will be difficult to determine what that should look like given the difficulties on calculating demand. I am currently working on a method, maybe something involving the average size of a detached dwelling unit. I’ll pass it along when I have completed my calculations.