Unfortunately, you can't go above 605' north of Union Street, which is along the left side of Rainier Square and Two Union.
Here's a
zoning map. The areas south of Union show "U" in their designations, which means commercial space is subject to floor area ratio (FAR) limits (which are pretty restrictive) but you can go as tall as the FAA guidance allows. To get very tall, practically, you need residential, which isn't subject to FAR. The
rest of the maps.
North of Union you see a lot of areas where commercial has one limit and residential has another, for example "DOC-2 500/300-550." This means residential buildings can have habitable floors up to 550', not counting the extra 10% for architecture, mechanical systems, and amenity space. Commercial towers can be 500' plus a little room for architecture and mechanical. The smaller figure is if you don't go for bonuses, or something like that.
Likewise, a lot of areas allow residential to 440'+44'. Six of those are underway. Much of this decade's wave was in those areas when it was zoned 400'+40', with the first under the new height starting in May 2019.