The addition of one triplex per block would have negligible neighborhood impact and help meet Seattle Mayor Ed Murray’s goal of 50,000 new housing units.

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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.”

According to the City of Seattle, there are 15,740 blocks in single-family zones. (The city defines one block as having two “block faces.”) That would mean 15,740 lots on which to build a triplex.

But let’s further assume that most lots already have a house on them. So each triplex would add two additional units per block.

So 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.

Such a small change as “one triplex per block” would go a long way to Mayor Ed Murray’s goal of adding 50,000 new housing units in the next decade.

More important, one triplex per block would have invisible neighborhood impacts. That’s a bold statement, so why do I offer it?

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.

As to architectural scale, the mayor’s Housing Affordability and Livability Agenda (HALA) Committee report proposed that a triplex in single-family zones would have to be within the “building envelope” now allowed in single-family zones. That means that 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. City building codes state one house is allowed to have one “household” with one parking space. A household is defined as a housekeeping unit consisting of any number of related people or eight or fewer non-related, non-transient people. And there is no owner-occupant requirement.

So one could rent out rooms in a five-bedroom house — on the large side, but by no means unusual — and there could easily be eight unrelated people living there, and each one — stated to scare you — could own one car. In fact, many single-family households own two, three or even four cars and parking overflow is absorbed on-street.

Of course this is all a thought experiment to show the potential for “cumulative impact,” but it’s done in a positive way to demonstrate that even small changes can have huge impacts.

The uniqueness of allowing just one triplex per block would encourage immediate housing development, which is what the city says it wants.

We need more opportunities to build housing and my proposal would have no discernible neighborhood impact. I urge the Seattle City Council to take up this idea for discussion and action.