Hi Sunydaz, I hope our day is going well. Adding to the previous answer...
Assuming "purchase" means adding you to the deed (and no funds were exchanged for the property received), your partner has gifted you a portion of the home that he purchased. In this case, a gift tax return may have been required for that tax year.
https://turbotax.intuit.com/tax-tips/estates/the-gift-tax/L1sFpFeXV
Your situation is referred to as "joint tenancy" or "tenancy in common". Usually you would have an agreement in place as to what percentage of the home is owned by each person. Your partner's basis would be the amount originally paid for the home (including certain closing cost) plus cost of improvements made to the home. But, because he has gifted you a portion of the home, his basis is determined by the agreement you have in place.
The percentage each person owns is decided by the ownership agreement made between you and your partner. Your cost basis would be a portion (determined by your percentage of ownership) of the original cost plus cost of improvements (not repairs) made to the home. 50% ownership equals 50% cost basis.
https://turbotax.intuit.com/tax-tips/home-ownership/real-estate-tax-tips-owning-property-as-a-tenanc...
