Do I have to leave the lab, quitting all experiments exactly when my three years are up? I know that only a small minority of students submit within three years, and in the fourth year I will be registered as 'thesis only' - to me, this means I should clear out of the lab never to return on the 30th September this year! Or is it the case that I can do a few scattered experiments to tidy things up, but not start anything substantial? I think I'll find it difficult to do this, the nature of my work is that I'm either properly doing experiments or not. I get the impression that the fourth year students who are still hanging around in their white coats are 'just' doing routine analysis of products etc, but could be wrong.
Are the rules clear-cut about this?

And how does it work for some humanities students who are 'thesis only' all along as it were? (not carrying out experimental work?) - please don't take offence at that anyone! if anyone can shed light on how humanities research and thesis writing works (what does the 3 year deadline mean to you?) I'd be glad to hear it.
Finally, I've also been told that it's up to the individual supervisor to decide whether you can stay in the lab or not - surely this isn't fair? Or is it allowed as long as you state in your thesis that 'this work carried out between mmyy and mmyy'?

After your 3 years are up you are not allowed to work in the lab unless you have paid the full uni fee (1 000 £ per semester) i.e. you lose your insurance for working in the lab (and your bench , etc.). You can then either go into continuation status (180£/term), which allows you to use limited office/library space and limited access to supervision or you arrange somethimg with you demartment and become a visiting scientist or something like that. If your supervisor is particularly nice he/she might pay the full fee or the continuation fee- IF. If not you do.
You should have a phd student handbook for your uni that states the exact terms & conditions and costs for your particulat uni.

I'm on continuation and paid the fee myself.

Thanks everyone for the replies - anonGerman, your reply was particularly helpful - I wasn't aware of that. So the university doesn't mind how long we stay (within reasonable limits) as long as we're paying? What do you think is the main motivation behind the three year limit then? I think it's something to do with external funding - the more students that are pushed through the more the money flows! I'll investigate the rules for my uni, and of course I know the situation depends on my supervisor, but at the minute I feel like some extra time in the lab wouldn't go amiss...especially since I'm not allowed to submit until May (uni regulations - only two submission dates a year, in September and May), rather than spending 7 and a bit months perfecting my thesis I could get a few more results...thanks.

May 2007 that is...

No, you can't go on forever, after the four years are over the uni gets a bit unpleasant because you are ruining their ratings (of successful students within the time frame).
Anyway- these new 4 year programs will end up in people doing their phd for 4,5 years because the standards/expectations will get higher.

Three year programs are not that bad, because you can still submit a thesis even if your data is not good enough for a publication- you just have to defend it well.

If your supervisor agrees it should be no problem to stay in the lab for a few more months. That's not that uncommon, but the experience in my lab has shown that a few more months often don't help that much if things aren't working anyway- just means you've got a longer chapter on things that went wrong.

The two submission dates seem a bit strange to me- check that out. For a phd there are normally no specific dates for submission- as long as your phd has taken longer than two years and shorter than 4 you should be able to submitt whenever you are done. You then have to waite 2-3 months for the viva (and pass) and then do the corrections. The actuall graduation ceremony, where Richard Attenbury (im my case) gives you a nice piece of paper and you wear a hat are twice a year (though you don't have to attend).

I agree with AnonGerman, having 2 fixed thesis submission dates per year seems highly irregular.

Okay, I checked the PhD regulations which say "To allow the thesis to be examined in time for graduation at a particular date, the deadlines" are May and September, so there is a subtle difference with what I had thought - IN TIME FOR GRADUATION AT A PARTICULAR DATE (summer and winter respectively) - but any time is okay... that's good to know.

Hi Sue, not sure about lab based work - I'm a social psychology PhD student. However my uni has recently changed its position to those who go over the 3 year period. One day we received an email from the research office saying that we were registered as full time students for a 4th year. Some of us had already paid a writing up fee (for acess to library, supervision, facilities etc) but this was refunded to us. Our respective departments paid the university full time fees for a year. As you can imagine, we all thought it was some kind of elaborate hoax, but further confirmation revealed that it was a number crunching excercise. It increases the number of research students. I'm not sure what happens after 4 years though - I think it's 4 years then you're out. However I think the uni may have shot itself in the foot, as already people are talking about a PhD in terms of having 4 years instead of 3.