Some schools may want to know if you already have any skill and/or experience in original research.  Other programs just want a writing sample (not necessarily original research) to indicate your level of writing skill, on the reasonable expectation that they will be the one to train you in research.

So, without know anything more than you note above, I'd say you must submit a writing sample of excellent quality to at least demonstrate your ability to think, analyze, and clearly communicate.  Polish up to a high gloss one of your seminar papers, for example.  Or, if necessary, write something new-- a book review, or an opinion piece, or the like.  The PhD is about writing (in addition to research), and doing a lot of it.  If this stumps you and you can't produce something of high quality on demand (not necessarily an opus, but at least an essay on something of interest to you), you should reconsider the whole enterprise.

Also, and importantly: your admissions essay (very well written, too) is the place you have to explain your studio background, and also your recent and very compelling call to do research on X or Y, and how you can now think of no other career.

Good luck!

Thanks for taking the time to respond. And more importantly responding to the point. I have also looked at some of the MS programs being offered in UC berkeley, Gatech . Getting admitted to the MS programs seems to me like an easier option to slip into the phd program. I know it would be an additional year of studying/ research but then it would directly be geared towards the phd program of similar topic. Plus I would definitely have some 'research/ writing samples' at the end of it.What are your thoughts? Dont mind if Im sounding too vague. I have JUST started.

That's a sound strategy, and not uncommon. 

My first application to do a PhD was from finishing an MArch; I was offered entry into the MA program for a year in the way you describe.  I was already in my mid 30s with a lot of seminar papers and early research under my belt, so I opted for another program which admitted me directly to the doctoral program.  But if my circumstances had been like yours, I'd've done the 2nd masters route.  Plus, this gives you the indispensable benefit of trying out the whole thing beforehand.  That year can tell you a whole lot before you make the big decision to really go ahead with it.

Hmm..looks like I have to take the longer route. Anyways, do you know of any websites that I can find samples of "writing samples/ research papers" related to architecture? especially the ones that are submitted as part of application to pursue ms or phd?

Don't just go by one person's experience.  Ask around.  Inquire.  (There are several PhD students on here who I'd've expected to chime in by now.  Smokety?  Philip?  +i?  Javier?)

My most important piece of advice is this: find out all you can about the programs you're interested in, as well as the faculty members who work on the kinds of research you think you want to do.  This information can only be helpful to your quest.

As for writing samples: a seminar paper, almost regardless of topic (but well-researched and well-written, ideally) is the kind of sample most people submit.  You know already what that looks like, even if you haven't done one yourself. 

agree is good idea to ask philip or smokety since they are in process now and genuinely smart...

for what is worth i came from m.arch in canadian school and did phd at u of tokyo in japan on urban planning.  to be honest the main thing needed for me was the support of my advisor/professor.  he had additional requirements for phd students, for instance that we were all required to have worked professionally in offices for some years before applying to join his lab, and i guess more subtly we were also required to be at least tangentially interested in work that was related to his own project.

for writing i used articles that i prepared for magazines related to my student work as well as lectures i had presented for same.  in hindsight it was not standout work academically but the committee understood what i was capable of and also that i had some rudimentary analytic skills.  i presented same material for the interview.

since i was going into faculty that was made up of designers and practising architects and not historians this was ok.  a lot of the phd research there was based in fieldwork and practice as well so it was a different approach than more lit-based study.  we still had to write but it was to be backed up by original data collected in the field rather than based in searching through historical records or books. 

which is to say, it really matters where you are studying and who you will be studying with.  the kinds of papers you write should fit in with the kind of work you will be doing in the program.

I hadn't taken a look at this thread previously because I originally thought that it was one of those spam postings with links to "dissertation writing services!"

Anyways... Given that you're interested in the more technical PhD programs, I'm not sure how applicable any advice that I have to give might be since I'm in a more traditional History/Theory program. My gut feeling is that the writing sample will carry less weight for these types of programs, but I could be wrong. I would recommend getting in touch people at the programs that you find interesting. (Shameless plug: UPenn has several programs that you might find interesting, including an MS, the relatively new MEBD, and a technically-oriented PhD.) In Penn's case the History/Theory PhD program is headed by David Leatherbarrow while the Technology PhD program is headed by Ali Malkawi. I suspect that they look for very different things in applicants and application packages.

My first question for you would be, Why do you want to do a PhD? What will it get you that a one-year degree won't? Also, I know that some PhD programs tend to require that you already have a post-graduate, research-oriented degree before they will accept you. (From what I understand, Princeton is one of these.) With this in mind, this might be the right move. I would talk to the programs that you're interested in to find out if it would be possible to continue directly on from the MS program into the PhD program. 

As far as writing samples, I submitted a conference paper that I wrote/presented the year before I applied and 1-2 short seminar papers. All of them were loosely related to the topics that I was interested in exploring in my PhD research at the time. (My topic, like most of my colleagues, has shifted since entering the program.)

hey citizen, if you see this can you send me an email via the magic of archinect? i have a question about planning phd programs.

the big question is why do a PhD when you might be able to open the career path through a MS is Sustainable Design or similar. If you want to teach, what sort of program do you want to be part of as faculty? not all places require the Piled higher and Deeper terminal degree.

The UPenn programs that Philip mentioned is good, other places to look at pretty well known for leading the research in this area are MIT, USC (their MS), Cal Poly Pomona MS in Regenerative Studies, UMN MS in Sust Design, Berkeley,  University of Washington, RPI/CASE, and UBC to name a few. But there are significant differences - are you more interested in energy modeling/building simulation, or materials research, or life cycle assessment, or sustainable human behavior, or eco-cities, or living buildings, or the interface between buildings/landscapes, or sustainability rating systems, or the history of sustainable design, or adaptive re-use, or building systems/controls, or the economics of sustainability, or healthy buildings, or sustainability specs, or daylighting, or design tools, or commissioning, or water use, or shrinking cities, or vertical farming, or waste/recycling, or sustainable construction, or interactive buildings, or ???? Sustainability is a huge arena that is growing bigger every day.

You need to have a clear research agenda for a PhD, while a MS program allows you to refine your agenda first. You also need to have a clear idea of where you intend on applying the hard gained knowledge from graduate school after the fact.

Good luck!