Many roleplaying games utilize character classes and character levels in order to create a sense of progression. House Games eschews that in favor of a sandbox approach. Character progression still exists, but it is up to the player to choose it's direction.
There are three primary means of advancement in House Games: 1) gaining and spending Experience Points, 2) roleplaying, both in game and through journals and side-games, and 3) gaining supernatural Powers. These are discussed here.
Experience Points are used to improve your character's base stats: Attributes, Abilities, and Willpower. The primary means of earning experience is through playing House Games. You'll get 2xp just for playing, and another 2xp if you manage to succeed in your objectives. Additionally, the GM will choose one player as the MVP each game, and that player will get a bonus 2xp.
There are a few other ways of earning "extra credit" experience, detailed on the Experience page on the wiki.
There are only a few things that characters may spend their experience on. Attributes and Abilities are increased by spending an amount of experience proportionate to your current rating in that Attribute or Ability. An Attribute costs an amount of experience equal to your current rating x4, and Abilities cost current rating x2. Going from rank 0 to rank 1 in an Ability obviously doesn't work with this system, so for that particular case, there is a flat cost of 3xp. Abilities and Attributes have a cap of 5.
Increasing Willpower costs a flat 4xp, the cost is not tied to your current rating. Willpower can be raised to 10.
In rare cases, players may purchase Merits, or "buy off" a Flaw. Doing either of these requires both the approval of your GM and appropriate roleplaying to justify your acquisition of the merit/loss of the flaw. You had better come up with a really good justification, as GMs will be hesitant to allow this.
The Importance of Roleplaying
Experience can only take you so far. In the world of House Games, a minimal amount of characterization has stats or systems associated with it. This is intentional: having a statistical representation of a characters morality, for instance, is not only a nightmare to properly system out, but it constrains characters in an unfun way and removes the tension from any complex moral choices that creative Game Masters might implement.
Your Cell may choose to implement some optional systems that address things which other Cells prefer to roleplay. There's nothing wrong with that; every Cell should feel free to play the game that they will most enjoy. But even in a more rules-oriented group - where characters all have Morality ratings, persuasion is a skill for which you roll dice, and you can roll Intuition to get a sense of whether you're on the right track - even there, proper roleplaying is essential to character growth, and those rules simply reduce some of the sandbox elements of House Games. Which is not necessarily a bad thing! Too much freedom can make you feel lost sometimes; think about it like the difference between ordering food at Mongolian barbecue or ordering food at In'n'Out. You have decisions to make either way, and if you're clever you might just find a hidden menu.
The bulk of storytelling in House Games does not lie with the GM. Rather, it is a group effort centered around the dynamic interplay between the varied adventurers in your play group. The GM tells a single, self-contained story, while your band of merry heroes goes on new adventures together every week.
In a number of important ways, House Games are like a semi-serialized TV show; in particular, it reflects the "Monster of the Week" format exemplified in early seasons of Buffy the Vampire Slayer. The Gang gets together and hangs out, and most weeks get sent on a new, weird, self-contained mission. The story is told through the characters, not the plot. Willow's growth into a powerful witch was only possible because Mrs Carpenter joined the gang on some adventures and shifted her path toward magic.
Because of this, the way that your character behaves in game is crucial to their development and story arc. Things can and will change, no matter how much planning you did at character creation. Growth in House Games is very organic. Not every event or interaction has to be life-altering, but keep on the lookout for major developments that might push you in a new direction.
Ultimately, though, your character's story is yours to tell, and to that end we encourage all players to explore their characters through creative writing. There are a number of short writing exercises which, in addition to providing an outlet for character growth and the ability to tell a more nuanced story than you might be able to in the heat of the moment in game, also grant bonus experience.
The most important of these is the Journal. Each character may write a journal entry (at least a half page long, but if you're inspired, go longer!) detailing the events of a game from their characters perspective. Journals may also be done for Side Games, or to talk about events occurring during a Downtime (though they provide less xp than the Game Journal).
Additionally, players are encouraged to fill out the Questionnaire, a list of very personal questions that can grant insight into who your character is and where they want to be.