First and foremost, my understanding of Copyright law is that you are the sole creator of the rendering, therefore you own the copyright. Period. If you had been an employee of the architect and were paid on a work-for-hire basis then the copyright would belong to them. The fact that you put their logos or names on the rendering doesn't give them any copyright privileges unless you specifically added "Copyright 2016 So-and-so Architect."
As for watermarking your images. Most people are ignorant about copyright law and will unknowingly use (steal) your work. My personal opinion is that a small watermark or copyright notice is appropriate and professional. Large watermarks across an entire image is distracting and cheapens your work (remember, this is just my opinion). A case could be made that if you're that sensitive about copyright, how difficult are you going to be to work with? Do you also have 20-page contracts and lawyers on speed-dial? I see my copyright mark cropped out and even cloned out of my images on occasion and it ticks me off, but I handle it with a polite call to the client and express to them how this hurts my business which in turn hurts my ability to do great work for them too (I've gotten a lot of new clients through the copyright watermarks on my renderings in the past). Also, if you're really, really awesome at what you do and your work is blatantly stolen and used in someone elses portfolio, your fellow CG artists will catch this and oust the offender in the equivalent of an online lynching by your peers. I've seen it happen over at CG Society quite a few times.
The most important part of this post though is that you should never work for free. That in itself hurts the community and the people who work very hard to make a living doing this work.
Joel