> In Portland, I didn’t need a permit for the building because it was under the size threshold

You still need a structural building permit, beyond any electrical or plumbing permits, if it's both heated and used either as a living space or "for a business".

The city added the "for a business" wrinkle in 2024 and is very squirrely about whether using it as a work-from-home office counts as "for a business". It _does_ include working from it without bringing employees or clients into the buildling, but the way it's written implies that it's for people running a business out of their home address and not working for someone else's business.

It's a gray-enough area that I hope you got some solid legal advice. We ended up getting the permit to cover our bases in case I had to move to freelance/consulting self-employment.