> Part of living in modern society is some basic minimum standards of quality and safety. Obviously those can be abused, but Seattle simply isn't doing that. 220sqft is a totally reasonable minimum. Requiring every unit have at least a bathroom and a non-bathroom sink is a totally reasonable minimum.
Requiring working fire sprinklers, smoke alarms, and functional electrics and plumbing is about quality and safety. A building with wiring that threatens to cause a fire or one with leaky plumbing that feeds toxic moulds is indeed a safety hazard, just as is tainted meat/dairy. However, minimum square-footage requirements / occupancy limits do not fall into "quality and safety" and only serve to increase the minimum cost of housing.
By mandating a certain level of luxury (and thus, of cost), the only effect is to make sure that people who can't afford it will end up homeless (or illegally subletting, squatting, or some other black-market, unsafe option). It's crucial to note that there is no backpressure mechanism at work here -- reducing housing options for people with the least money will not cause the construction of housing that meets those standards. Mandating luxury does not work without mandating supply as well. Such zoning laws ended SROs and rooming houses (by prohibiting any new construction) and such drastic reductions in the cheapest housing options, of course, caused homelessness crises.
> The goal of banning tenements is to prevent companies from exploiting people and profiting from them -- building tenements actually takes society away from the solution, by wasting land and money that could have been spent on housing.
Yes. Land-owners indeed inherently "profit" from people, as owning land lets them charge rents. But beyond that -- reducing zoning restrictions and occupancy limitations would increase the supply of low-end housing and thus decrease the market power of slumlords (by increasing competition). Giving renters available options they can afford lets them have the option of exit from their current renting situation.
> We could house everyone, we have the technology, we have the capital.
We indeed could, and it's a goal I support. However, creating a minimum standard for luxury of housing units without mandating supply will not get us there. I use the word "luxury" intentionally here -- from the perspective of someone who has had to sleep in homeless shelters, a dry room big enough for a bed and with a door that locks (for which one has the only key) is serviceable housing. To quote Paul Groth,
> a good hotel room of 150 square feet—dry space, perhaps with a bath or a room sink, cold and sometimes hot water, enough electric service to run a 60-watt bulb and a television, central heat, and access to telephones and other services—constitutes a living unit mechanically more luxuriant than those lived in by a third to a half of the population of the earth. As Dolores Hayden reminds us, many of the world's people would consider an American two-car garage an excellent dwelling in its own right.