Thank you for your questions:

Is it fair/reasonable to request a fair portion of the gained equity if I pay half of the mortgage and associated home maintenace costs? An example: if we split and he sells the house I could be entitled to a percentage of the equity (1% per month of Cohabitation). So if I lived there for 50 months (over four years) I could get 50% of the equity IF he sells the house.

Considering that the house has zero equity when you move in, that seems like a fair resolution. However, you are not protected in this scenario in the event he does NOT sell the house. Consider how you want to be compensated in that case.

If we get married, could I specify in the cohabitation agreement that the house is joint marital property (even though I won't be on the deed or on the mortgage)?

There are a couple problems here. The first is that once you marry, you may need to enter into a new agreement. The second problem is that, although you can agree between yourselves to the ownership designation, it has no bearing on title ownership.

If I move out and he does not sell the house, I would leave without claiming to be paid for my contributions. I figured if we're not engaged or married within 3 years I would move out anyways and am OK with not "making any money." I would not want us to break up and have him sell the house without giving me a fair share of what I've contributed.

If you are concerned about being compensated, you could insert a clause that he must refinance the mortgage and draw out your share of the equity, or if he is unable, that he MUST sell the property.

Another note, a cohabitation agreement should always be drafted by an experienced family law attorney. Agreements such as this are subject to scrutiny by the courts later on if either party makes it an issue, and you wouldn't want to rely on an agreement that is actually unenforceable.

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