The answer will vary depending on which institution, and your experience. I know someone who did a PhD in architecture without completing a master's (at Berkeley, though he did do master's course work). But he was in his 30s with several years of professional experience. Much will depend on whether your application shows an already honed research topic.

I applied to do a PhD in arch. history after completing the MArch, and was told I'd need to apply to the separate MA program in arch. history before getting accepted into the doctoral track. (They wanted me to get scholarly research fundamentals that are not part of professional arch. studio training.) I politely declined.

I went across town and applied directly into a PhD program in urban planning, and was accepted. They saw credentials as adequate for entry into scholarly training. It all depends on the school, and on how focused you are able to present yourself.

All students coming out of the University of Hawaii's School of Architecture come out with a PhD in Arch, although I'm told that it is not a very highly regarded diploma.

An ArchD (at least as described on the UH website) is not the same as a PhD in architecture. The first seems to be a professional, studio-based curriculum. The second is an academic, research-based degree.

That is interesting, though, about UH's approach.

I'll be getting a Ph.D. w/out earning an M.Arch. beforehand. I have an M.A. in Art/Architectural History and an M.A. in History, plus a two-year design degree and several years of professional design and historic preservation experience (plus a couple of B.A. degrees thrown in). So far, everyone else I've met in the program has an M.Arch., but it really seems to be a matter of how one's credentials and experience fit w/one's research goals. If you want a Ph.D., you'll have to be able to prove that you have *some* aptitude for research (instead of a pure design background), but there are a lot of ways to do that.

To add to citizen's comments on Berkeley, I was also accepted to the Ph.D. program there. That program requires that anyone w/out an M.Arch. start out w/an introductory design studio the first semester. So, they've got a system in place to make sure historians have at least some (little) experience designing. Personally, I think a better requirement would be to make historians take a Construction Materials and Methods course instead of a contemporary design course, but that's just me.