Thread by @volkerj2

NIMBYs frequently oppose ADUs on the grounds that #ADU residents will overwhelm on-street #parking. Are they right?

@cgthigpen and I take a look in our new study in @JTLU2008

A🧵on our findings: 1/11

https://twitter.com/JTLU2008/status/1497329959274258433

@cgthigpen and I surveyed Sacramento homeowners to investigate the supply and sufficiency of residential parking for single-family homes and what that means for ADUs:

jtlu.org/index.php/jtlu…

2/11

Our first step was to estimate actual garage use. We found that the most common individual use was storage (75%), followed by car parking (63%), use as an extra room (12%), and living space.

3/11

We then added the estimates of garage use to the self-reported availability of other off-street parking to estimate total off-street parking supply. In our most *conservative* scenario, households on average had 2.9 off-street spaces and only 2.3 cars.

4/11

Again using our most *conservative* assumptions, we found that >75% of households had enough off-street parking available to park all their cars.

5/11

And now to the fun part: with our estimate of average total residential parking (1.1 on-street + 2.9 off-street = 3.9 total spaces) in hand, we subtracted the average number of household vehicles (2.3) to estimate total parking sufficiency.

6/11

We found that households had an average of 1.6 more parking spaces available to them than they had vehicles.

7/11

What does this mean for ADUs?

Previous research (e.g. by @profchapple @JakeWegmann) indicates that ADU residents have ~1 car on avg. That means the avg single-family home in Sacramento could add an ADU and still have ≥0.6 surplus parking spots: 1.6 surplus - 1 spot = 0.6

8/11

We also scaled our findings up to NBHD block level. In related work, @volkerj2 found nearly 50% of Sac homeowners could be open to adding ADUs. So we estimated the effect of adding 5 ADUs to a hypothetical 10-house block. We found there'd still be 12 vacant parking spots.

9/11

But what if those ADUs replaced existing off-street parking (e.g. garage conversions)? We estimated there would still be 7 surplus parking spots on the block even if all 5 ADUs replaced off-street parking: 12 surplus spots - 5 displaced cars from garages = 7 surplus spots.

10/11

Overall, our findings counter NIMBYs’ claims that #ADUs will overwhelm existing #parking supplies in single-family neighborhoods.

jtlu.org/index.php/jtlu…

11/end