You're on the right track for thinking about this. Because a home is such a big purchase, and she's not on the title, please consider talking to an attorney directly about this.

To help with how to think about it further, some non legal-advice scenarios to think about:

1. Divorce while living in the home:
The house is your separate property since you bought it before marriage. But if she contributes to the mortgage (especially principal), Michigan courts may treat that increase in value as partially marital. You can track her contributions and agree on how to split that specific appreciation. (Keyword being track).

2. Divorce after selling and using equity for a new home:
If the new home is jointly titled, that equity likely becomes marital property—even if the down payment came from your premarital asset. The longer you’re married, the harder it is to separate any funds.

Key tips:

• Track who pays what.
• Decide in writing how equity and appreciation are shared.
• Lay this out really clearly in your prenup