Update:
For unknown reasons, I can't reply to comments. A lot of people are talking about this topic but will not publicly comment. I think there are problems. I think the SC administration has some reasons, though it's hard to determine exactly who is making decisions so that a direct conversation or negotiation can take place. It is my hope that by beginning a more public conversation, people can get involved and change can become possible.
The Ethnic Cultural Center is UW's facility, but I have seen plays and readings there, so I know it is possible to rent (though not for how much). I perceive that the Erickson is in a different situation, and in an area where people are begging for locations to mount their productions.
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Space to perform in Seattle is at a premium already and seems to be getting worse. While there is are two new spaces being built (one for Theater Schmeater that is an example of what I see as the future avenue we must find a way to advocate for with new development, and the 12th Avenue Arts building), each of those spaces will not actually enlarge the pool of available spaces for rentals in any meaningful way. (I am not going to lay out my rationale for that statement here.)
Rentals, theater spaces where people can rent space for performance purposes, are few and full, and vary in their ability to house the requirements of productions. Some of them are uncomfortable to sit in as audience members, and most of them are simply too small for any but productions with four or five people and a single set, maximum.