Cost Accounting in Local Government

Just How Scary is One Attached Dwelling Unit?

I have been completed over one hundred Development Impact Fee Calculation (DIF) studies since 1987 (a quarter of a century, yikes!).   These agencies ended up with an adoptable fee schedule capable of raising the revenues necessary to accommodate their General Plan new development.  But I also found that I had inadvertently generated some collateral information that I should probably share with you.  This month I will share one such “fact”.   It is this: Attached Residential Dwelling Units, consisting of apartments, condominiums and townhouses, are not the big service demand-generator you might think.  This is probably good news for you since my clients tell me that attached dwelling unit permits are pretty much the only housing permits being pulled.

It’s pretty well known that attached housing generates less traffic than other residential units, but what about public safety?  I can assure you that most police and fire chiefs can give you chapter and verse about how many times they have to respond to attached dwelling units.  Most of them were convinced that attached dwelling units are a huge problem.  But let’s look at the data.

Over the years I have compiled the data regarding the number of calls-for-service generated by the major land uses as the nexus for the public safety DIF’s.  I am limiting this article to only detached and attached residential dwelling units.  I compiled archived data from the inventory of DIF studies I have conducted over the years, and I found data from thirty-seven cities/fire districts regarding their public safety calls-for-service.  The process goes like this: Planning staff generates the number of existing detached dwelling units (DDUs) and attached dwelling units (ADUs) and then police and fire staff determines how many times they responded to those same known quantities.  (Side note: I was “advised” many years ago by a couple of city attorneys who advised me to stop using the terms Single Family Dwellings and Multiple Family Dwellings and due to their earnestness, I complied).  The result is a ratio of calls-for-service per unit of each type.  For example, City X has 3,000 DDUs and the police in the previous 12 months had 2,250 responses to those 3,000 DDUs, the result is a 0.75 calls-for-service per DDU per year, or stated another way, a 75% chance of requiring a police response within a year.  The same process was employed for fire responses.  As an additional side note Police calls, on average, run about 6 to 7 times greater in number than fire responses.   This process was uniformly employed in each city with the main variable being that each agency may have had differing methods of generating the needed data, as some had computerized data and others undertook hand-counts.

Other Variables.  The agencies had a variety of demographic reasons for the largest swings such as a farming community that had a large seasonal harvest population that tended to over-fill ADUs and a beach community with a past, but currently tamed, reputation for wild renters.  One was a well-known ski town with major ADU rental unit vacancy swings.  There was no discernible pattern of calls-for-service data with population, which ranged from a high of about 340,000 to a low of 3,500 with an average population of 68,707.  Since most cities fell within a “normal” ratio, I would think those at the extremes would want to know or find out why there was such a disparity between the two.

The Police Experience.  In terms of police response demands, 22 of 33 cities responded more to DDUs about 1.32 times more often than to the City’s ADUs.   The remaining 11 cities had the reverse, with similar 1.4 times more responses to an ADU than a DDU.  The overall ratio for all 33 cites was a combined 1.03 times higher (or 3% greater) response rate to ADUs than DDUs.  Certainly this difference does not seem to be anything to get worked up about.  The range was from high of 296% DDU over ADU annual calls-for-service demand to a low of 49% DDU over ADU calls-for-service demand.  The average police calls-for-service was about 0.93 annually for DDUs and abount 1.04 for ADU’s annually.

The Fire Experience.  In terms of fire response demands, 23 of 33 agencies responded more to DDUs about 1.54 (54%) times more responses than to any of the City’s average ADU.   The remaining 10 cities had the reverse, with a 1.34 times (34%) more responses to any one average ADU than any one DDU.  The overall for all 36 cites was a combined 1.12 times higher or 12% greater response rate to an ADU than to a DDU.  Again there seem to be nothing about these results to be concerned about (unless you were the high or low). The range was from high of 307% DDU over ADU annual calls-for-service demand to a low of 50% DDU over ADU calls-for-service demand. The average fire calls-for-service was about 0.157 annually for DDUs and about 0.140 for ADU’s annually.

So why do police and fire officials think that ADU’s are such a huge demand on their public safety resources?  Well frankly in a few cities, they simply are, but not all of them.  But how about the cities where I could show the public safety officials that an ADU was less likely to generate a response than a DDU?  I would simply ask them to identify the five major headache ADUs in town.  That is never a problem to do as there are always a few ADUs that generate many, many responses annually.  Then I ask them to name the five major headache DDUs, and again, they have no difficulty.   They generally realized that they had simply drawn conclusions from a few (or more than a few) ADUs in town, probably from early in their careers when they had been fire fighters or patrol officers.  Hopefully the data had some positive impact on how they felt about the construction of ADUs.

A Suggestion. Typically the public safety officials did identify that the problem DDUs and ADUs were often in poorly designed and poorly constructed complexes or tracts and had become problems over a long time.   I have no data to confirm or refute this, but in some cities I did visuals of the problem units and I do not doubt it.  My suggestion is that you tighten up all of your design and construction standards and do what you can to keep at-risk complexes and homes from slipping into becoming problem dwelling units

In a future newsletter, I will let you know about the average square feet of police station per sworn police officer.  Given there are no objective national standards, I’ll pass on the subjective information I have found.  It’s interesting.  Until then, have a good summer.

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2 thoughts on “Just How Scary is One Attached Dwelling Unit?

  • Jorge says:

    So happy to find this site. I have been researching ADUs for the past year (slowly) and bevleie your site will help me reach out to a broader community and find further information.I am currently in my last semester of graduate school at The University of Texas at Austin, getting my M.S. in Community and Regional Planning. I am doing my thesis on Accessory Dwelling Units. Specifically, a program feasibility study for my hometown of Denton, Texas. The thesis will also include 2 case studies that review current programs for ADUs in the country. One of my case study sites is Santa Cruz, CA, I have not decided on the other yet. (maybe Austin’s SMART Housing program). Any suggestions on other cities with successful ADU programs??? I know Portland and Seattle have great programs but I am trying to find a city less urban or west cost. I wish Austin had a stronger program. Darn! Again, so excited to see this site!!Thanks!

    • Scott says:


      I am sorry but I have limited data on Accessory dwelling units. very few of the cities I have worked with have significant ADU programs. Also, there is little information regarding demand rates for the “granny flats”. Polcie and Fire Department don’t keep data on them. Plus they are difficult to predict who will build them. I might suggest contacting cites that have 3/4 acre or one acre minimums allowing the addition of such units, or one with lots of alleys.

      Good luck and I’d enjoy seeing your final product. I can be reached via our website address and phone number.