Superfluous studies in architecture are not investments.  Don't go into massive debt for a glorified general arts degree(s) unless you fully understand the cost/repayment terms and how it will impact the next 10-20 years post graduation. 

hmmm that is a little unfortunate that he is pushing you to do another masters that is pricey, before applying for the phd. The reason he is doing this is because it is rather competitive to get a paid phd position and if you studied at the school you are applying to directly after your graduated your chances will be slim- no matter how well you did in your masters. The reason for this is because what phd programs are looking for is somebody who already managed to do things such as publish articles (by themselves), exhibitions or curate before applying to the phd programs, as a way to build up their academic profile outside of the institution they studied or went to.  So if you have your heart settled on Leuven and you are doing your masters there, you have two choices do another masters elsewhere or publish some articles, do some exhibitions etc. before applying. 

I am doing a fully paid phd and I only did an M.arch once. My mentor who helped me along the application process would refuse to let me do another masters at an ivy league school- he said I was beyond that. He offered to let me study for one year at the institution he is dean at for one year for free to help me out, in case any of my applications failed. Luckily that was not necessary. 

I guess every mentor is different but I would recommend finding a way to do the phd without having to go into any debt. Perhaps consider other phd programs not just Leuven, perhaps some in Scadinavia? Or Switzerland? Somewhere where they are fully funded. By the time you finish the phd you will be in your thirties and by then you need to also think about things in life outside of academia- like having children and/or owning a house. 

OP, I would honestly suggest that you take the advice of people in your network over this decision, rather than relying on the opinions of people in these forums. There are not that many credible individuals on here. If you don't know someone to take advise considering reaching out to people you admire via LinkedIn or mutual connections l. 

You sound like a very worthy individual and I am glad you are considering continuing your education. I personally don't think that you will be too old for more education at 25. You have all your career ahead to work in offices. Also, if you consider universities like Delft or German TUs, the cost won't be high at all. Given your academic achievements, it's quite probable that you can get a scholarship too. So, don't quit on your aspirations! Best of luck. 

Do you want the same degree in the same subject?

The fundamental question that appears to be unanswered is: What does the OP want to do with these degrees? I don't know how anyone could weigh in on this with any real confidence without an answer to that question. 

Thanks a lot to all for the comments, they do help me in one way or another. In the meantime, I am also trying to contact specialists for their insights.

Someone asked a good question above, about what this whole plan means to me and what I would do with these degrees. For me that is simpler, and it is something I can figure out on my own. I am definitely convinced that stopping my education next year after graduation is out of the question. I am very confident in continuing my education; especially in doing the phd. I want to get more specialized in a field of architecture (i have different options here that interest me), I would love to teach one day, and I plan to work on research for a part of my career. At the same time, I would not like to let go of designing. I am currently 22 so I believe I can do both research and design in my future.

What confuses me is the decision I should make this year (starting applications). I am struggling to realize the pros and cons of taking another master, and investing time and money in it.

I thought of looking for phds in other countries as well, but I have a feeling that it would be harder to find a good mentor, or to create connections in that university... I have a feeling I might be very wrong here haha, I didn’t do enough research to see how this goes.  

I am reading all comments with enthusiasm. Thanks to all! 

Answer does depend on what Uni/programme you go for, what your research project will be and if you are comfortable doing a PhD in English. 

In the US your studies are 5 years - basically they embed a Masters degree in your studies as the first years are really course work and then you sit for exams to progress.  US you generally have to have speak/read/write a 2nd language for the purpose of research when I assume you have at least French/flemish.  

In the UK you can do a PhD in 3 years but that generally assumes you either are coming with experience, a project that you have worked on or a masters degree where you have undertaken research methods and have gained some expertise in a canon of knowledge.  

PhDs often touch 2 disciplines to demonstrate originality.  I know TU Delft requires your thesis has to include six or more statements of new knowledge.  US and UK do have good funding for PhDs.  in UK there is a 3+1 Phd Model where you do a masters then onto the PhD.  Funding in the UK is often tied to funding councils and specific research grants tied to supervisors.  

Have a look at https://www.jobs.ac.uk/search/... or https://www.findaphd.com/fundi... 

Don’t put all your eggs in your teachers basket...

I combine research with design and a little bit of teaching now and then, don’t have a PhD and probably never will because the pay for that here is shit and I can’t afford it anymore. You don’t really need a second master to apply for a phd, why not first try to land the phd you’re looking for and if they turn you down you could always get a second master’s to try again, if your first master’s  doesn’t align with your preferred research agenda. Like you said, you’re just 22, plenty of time for everything...just make that time count.

It is an interesting/frustrating dilemma. The first answer from archinet seems from someone in a similar position to you and pretty informed as well. 

I did PhD in Japan which is entirely out of context for what you are going through. I can say that in Japan (and maybe in Europe as well?), the minimum requirement to teach is both a PhD and a license, preferably with a good amount of experience...so it is not odd at all to pursue a PhD, and not a waste of time either, especially if you are interested in doing more than a regular practice allows. Definitely do it when you are younger though, because it becomes harder to carry off later in life.

The need to go out of your school and come back is something I have heard a lot, and probably informs your professor's recommendation. Probably you have already talked with them about this, and honestly they will know better than anyone what it takes to make it through the process, so I would put some serious weight on their POV.

for what it is worth, with regards to doing PhD as well as being an architect, I can speak for myself and say it opens opportunities that otherwise will not come easily. It does not shut down access to practice, though it can make it harder without a bit of a hard head.

which is all a long-winded way of getting to the point that it depends on what you are planning for your career, and what kind of future you are aiming for. Debt is not a small thing, especially for an ivy. I'm kind of curious why your prof thinks this is a good idea...