David Sucher, Seattle urbanist (and PT reader) has a helpful analysis. From the Seattle Times:
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I’M a literal kind of guy, so I was struck by something Seattle City Council District 5 candidate-elect Debora Juarez said: “While I support increasing the diversity of the housing stock in these zones by encouraging mother-in-law apartments and backyard cottages, I don’t think that building triplexes on every block is going to significantly alleviate our housing affordability crisis.”
I was interested in the image of “triplexes on every block” and wondered if, literally, one triplex on every block would be significant.
So I thought, “OK, let’s do the math.”
… by multiplying 15,740 lots by two, you’d have 31,480 additional housing units. That’s the power of a small change, and allowing one triplex per block could add a significant number of new homes to Seattle.
There are only two neighborhood impacts of any importance:
• The architectural scale (envelope) of the structure
• Parking
No matter how many other factors one may want to add, the real impacts that concern people are building scale and parking.
… if you had a 3,000-square-foot house on your lot, a proposal for a new triplex could be no bigger than 3,000 square feet. It could be configured as the owner preferred, such as three 1,000-square-foot units or one 1,800-square-foot unit with two rental units of 600 square feet each. But these would all be within the “envelope” already allowed by the single-family zone.
With good design, a triplex can appear not to have three separate entrances. Neighbors and casual passers-by would not be able to tell if the house had three units or one.
As to parking, let’s make it simple and require one off-street parking space per dwelling unit. So a triplex would require three off-street parking spaces with the potential (as already allowed by the city rules) to decrease the requirement in neighborhoods that have high transit availability or bountiful on-street parking.
And here’s a crucial point: Even greater impact is already allowed in Seattle single-family zones, and we live with it.
