Yes, a cohabitation agreement is a great idea to spell out what belongs to who, how new purchases will be treated, how the living space will be allocated, etc.
It's much better to talk about everything and write it all down now to prevent misundertandings and disputes later.
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If you feel discussion is a good idea, then it is also a good idea to consult a family practioner who can point out areas that should be addressed and who will help you put your agreement into writing. This is especially important if you have significant separate property which you intend to keep separate, or you have children for whom you wish to provide.
this is similar to a pre marital agreement for those planning to marry. i have written similar agreements for those wishing to live together without marriage. we spell out who is financially responsible for what items, what property belongs to one, to the other, what happens to jointly purchased property upon separation.
i highly recommend this to you. in illinois, if you live together without marriage, neither of you have any real remedy to sort this all out upon separation without a contract. the ill. supreme court case on this is hewitt. after about 20 years living together and two children, upon separation, she got nothing as all was in his name. do not make the same mistake.
Why don't you just get married and enjoy the legal benefits that marriage provides to you? Scheduling a consultation with a family law practitioner can inform you about those benefits and why they might be important to you. You can still have a prenuptial agreement and talk with your partner about how you would like to resolve matters amicably in the event that you separate/divorce.
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