i know there is a house(bjornaer?) that forbids itbut what of the rest?
Is your question "Can your magus move from one House to another?"
If so, the answer is yes, with some limitations.
As mentioned in the core book, 4 houses are True Lineages (Bonisagus, Guernicus, Mercere and Tremere). Their memebers are all magi that have been taught by magi already members of that house, theoretically all the way back to the Founder of the House.
4 Houses er Mystery Cults. They accept outsiders, with certain limitations. You have to be initiated into their Outer Mysteries, meaning you can't join if that's not an option (Bjornear, to magi who already have or have had familiars, Criamon if you're remotely sane, etc). It also means you have to drink the cool aid - meaning that you must accept their beliefs or philosophies. There's a reason the word 'Cults' are right there in the name.
The last 4 Houses (Flambeau, Jerbiton, Tytalus and Ex Misc) are Societies. They openly accept outsider, if you're willing to agree with them. Except the Tytalii, who tend to prefer if you disagree with them, but that's neither here nor there. You still have to be accepted into the House by the primus, but joining Ex Misc isn't going to be much of a challenge.
But since that's in the core book, you knew all that already, right?
However, if your question really is "Can my character be a member of more than one house?" the answer is... Yeah, apparently.
House Tytalus has been known to declare certain (usually troublesome) magi from other houses to be members of house Tytalus, without asking their permission first.
IIRC, the section of Societas which discusses House Tytalus 'adopting' new members states that this causes significant problems for the adopted mage, as no other House allows for dual membership (House Tytalus don't ask for the adoptee's permission either, they just declare them to be a Tytalean mage). Basically, House Tytalus are being dicks, in a truly shocking turn of events
Beside what Tellus say there also that house Bonisagus have the right to snatch away apprentices from other houses no matter if they are already introduced in the house secrets or not.
How this work from the rule point is not given in any text I read so fare.
And to be thorough...
A magus can be cast out of a House by the Primus. There is no hard and fast rule why you would be cast out and not outirght be a subject to a Wizard's March.
Once cast out, the magus is considered an Orbi (orphan) and has one year to seek membership into another house. Failure to succeed means the mage is considered an hedge wizard. With insider knowledge of the Order. With knowledge of Parma magica... Not only will he be killed on sight, he will be actively hunted to be killed, pater/mater expected to be leading the hunt.
Usually, this does not happen since, as Tellus mentionned, seeking membership of house Ex Miscellanea is not really a challenge. I would be even considering , unless Ex Miscellanea specifically forbids a mage to join their rank, that shaking hands with an Ex Miscellanea might be all that is required to join the band.
Basically, the Primus thinks you did something bad. It might be bad for the House, or bad for the Order. You have a year to find a new house after being declared Orbus, or risk being Marched. So, it might be used as a punishment for something that a Primus is pretty sure that the magus did, but can't prove. I can also imagine the Primus sending a letter to the Primus ex Miscellanea requesting that he not be allowed to join that House, even listing reasons why, but ultimately it's up to the Primus ex Misc. Sending a letter to the Prima or Primus Tytalus would be futile, they're just contrary enough to adopt the orbus magus...
House Criamon accepts A magus orbus to prevent him being without house. But I think I'd rather take my chances in House Tytalus, I'm not to keen on Criamon in this edition
If someone wants in, do you really think they will let just let them in?
Becoming a Tytalus is a 15 year endeavor. I seriously doubt that one could do the same in a single year, and being declared Orbus may not be enough...
If you want to join House Tytalus, you clearly don't belong!
Now, now, Ovarwa, subtlety is important, and House Tytalus recognizes this. If you can sneak in a sideways hint that you're joining to have an easier time screwing over other Tytalus magi, you'll probably get in with as few hitches as the current Primus is capable of restraining himself to.
You either are a Tytalus or not! One doesn't join the House, one declares oneself part of it... and then, time will tell.
Or you get declared a Tytalus. When you fight back against the declaration, you only prove it's true.
... Sometimes I wonder if I'm the only person on these forums who actually portrays House Tytalus as being so utterly unreasonable, rather than just making jokes about it.
That is an utterly unreasonable interpretation of how someone of House Tytalus should act.
So far there have been two Tytalus magi in my saga.
The first (an NPC) declared Wizards War on a PC he'd never met due to rumours of diabolism (the player wanted to drop the character without definitively killing them off, so it seemed like a good way to put him on the run).
The second is a player character who has played peacemaker twice now... because if everyone is fighting then the only way to disagree with everyone at once is to tell them not to fight!
Then again, my saga is set in a Theban Tribunal that frowns upon any house that begins with a T.
Tytalus don't have to be the mustache twirling villain anymore than house Tremere does. There are many forms of conflict that can be sought out and don't require the Tytalus being an a$$ about it. The Tytalus could be an excellent defender of a covenant, magically speaking or even politically speaking.
In our previous game, we had a Tytalus, who was power-hungry, driven, arrogant, egotistical, and generally aggravating, but also capable and intelligent. He was often the covenant leader, he was firm, he gave reasons (which were not always complete) for his actions, had public and private goals, and did a good job growing out manor base. Good model for how to go about executing his agenda.
To answer the other interpretation of the question; Yes, you can change a House, but you have to be extremely respected, a great leader with political backing, or answer a need of the House. Look at Quendalon of Merinita or Garus of Flambeau, both Primi.
I'd go more for needing all three of these things than needing just one, personally...
speaking of the houses, i understand they are different here that in mage ascension.
i haven't reached their chapter yet but i assume that the tremere are more than a cult of personality around lord tremere(or his successors) seeking to infiltrate and take over the world right?
Oh boy, I should have seen it coming.
Mage the Ascension was written based on older version of Ars magica, under White Wolf. Thus, the editorial line was trying to fit everything within the same world, Mage being the continuation of Ars magica world. Ars Magica was more grim and dark then, with almost a demon under every stone and the Order partially corrupted by power-hungry mages.
When Atlas took over Ars Magica, there was some heavy redcons, recentering the background on Mythic Europe, making it consistent with this single background and drop whatever legacy/baggages was in the way.
It includes cleansing of Tremere story of vampire/diabolisms. When Tremere's tried to overtake the Order they were stopped during the Sundering. Vampire/Diabolism stories were never true. That's only a few paragraphs. It leaves room for interpretation and background uncertainty: every GM can fit whatever suits the Saga he wants to tell. At this stage, it is very much everybody for himself as long as it makes a good story, nobody will be right or wrong.
So it is more or less impossible to fit within the same world Ars Magica 5th ed and Mage the Ascension. They are two different - IMHO - incompatible paradigms even with 8 centuries gap.
By the way, there is as much shadows and uncertainty regarding what was behind House Diedne destruction and the Schism war. Depending on your take, it is a boogeyman's tale to ward off magus to stray away for the Code, or it was a war between Latin and other traditions, or it was a set up by a few Houses (Guernicus, Tremere ?) who wanted to keep control of the Order and were threathen by the Diednes well organised structure, or they were really evil worshippers... your takes is as good as any other.